P.A. HILTO HIL TON N LTD. LTD.
EXPERIMENTAL OPERATING AND MAINTENANCE MAINTENANCE MANUAL
PRESSURE PRESSURE MEASUREM MEA SUREMENT ENT BENCH B ENCH F110 F110/E/1/001 APR 11
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(i)
POLICY STATEME STA TEMENT NT After Af ter Sales Servi Ser vi ce We, P.A. Hilton Ltd., attach considerable importance in being able to retain the confidence and goodwill of our clients in offering an effective effective after sales service. Every effort is made to answer answer clients correspondence promptly and to provide a rapid follow up of spares and replacement parts by maintaining comprehensive stocks of components usually available ex-stock. Should our clients encounter any difficulty in operating or maintaining a Hilton product we would ask that as a first step they contact the Hilton representative in their country or, in the absence of a local representative, write direct to P.A. Hilton Ltd. In the extreme case a problem may arise in the operation of equipment which could seriously disrupt a teaching or research schedule. In such circumstances rapid advice from the manufacturers manufacturers is desirable and we wish our clients to know that Hiltons' will accept from them a transfer charge telephone call from anywhere in the world. We ask our clients to treat this service as an emergency service only and to use it sparingly and wisely. Please do be aware of the time differences that may exist and, before making a telephone call, make notes of the problem you wish wish to describe. English is a preferred language. Our telephone number is "Romsey (01794) 388382" and the telephone is normally manned between 0800 and 1700 hrs GMT every day. Advance notice of an impending telephone call by Fax would be appreciated. Each product manufactured by P.A. Hilton Ltd., is tested under operating conditions in our permanent installations before despatch. Visitors to Horsebridge Horsebridge Mill are encouraged to operate and evaluate our equipment with initial guidance from a Hilton engineer.
EDUCATION AND TRAINING EQUIPMENT Declaration of Conformity: 2006/42/EC Directives (where applicable) 2006/95/EC 2004/108/EC
We declare that the following unit complies with the above EEC directives: F110 Pressure Measurement Bench
The use of the apparatus outside the classroom, laboratory, study area or similar such place invalidates conformity with the protection requirements of the Electromagnetic Compatibility Directive (2004/108/EC) and could lead to local prosecution. For and on behalf of P.A. HILTON LIMITED
Technical Director
P.A. HILTON LIMITED Horsebridge Mill, King's Somborne, Stockbridge, Hampshire, SO20 6PX, England. Tel No. Fax No. E-mail:
National (01794) 388382 International +44 1794 388382 +44 1794 388129
[email protected]
(iii) INDEX
Schematic Diagram SYMBOLS AND UNITS SCHEMATIC DIAGRAM NOTATION INSTALLATION AND COMMISSIONING Connecting the Pressure Gauges and Manometers INTRODUCTION DESCRIPTION USEFUL DATA OPERATION MAINTENANCE Cleaning Draining Pressure Gauges EXPERIMENTAL CAPABILITIES Suggested Experimental Procedures 1. Investigation Of Manometer Pressure Measurement Methods. 2.
Investigation of Pressures Above and Below Atmospheric Pressure a) A Simple U Tube Manometer and Pressures ABOVE AND BELOW Atmospheric Pressure. b) An Inclined Tube Manometer and Pressures ABOVE AND BELOW Atmospheric Pressure
Page 1 2 3 3 6 7 8 9 10 11 11 11 11 12 13
19
25 3
Examination of the Difference Between Absolute and Gauge Pressure.
31
1 Schematic Diagram Front View
Diagram Key 1. Positive Pressure Gauge 2. Compound Pressure Gauge 3. Inclined Manometer Tube 4. Vertical Tube 5. U Tube Manometer 6. Compound Pressure Gauge Connector 7. Vertical Tube Connector “INCLINED TUBE 1” 8. Vertical Manometer Connector “U TUBE 1” 9. Vertical Manometer Connector “U TUBE 2” 10. Inclined Manometer Tube Connector “INCLINED TUBE 2”
11. Positive Pressure Gauge Connector
2 SYMBOLS AND UNITS
Symbol
Quantity
Fundamental Unit
A
Area
m2
g
Acceleration due to gravity
m/s2
h
Height
p
Pressure
m N/m2
(Pascal),
Bar or mBar Note The pressure units used in this manual are N/m2 as these are made up from fundamental units (Newtons and m2) however the “Pascal” is the derived SI unit for pressure.
3 SCHEMATIC DIAGRAM NOTATION Please refer to the schematic diagram on page 1 To assist in identifying all of the components there is an annotated schematic diagrams on page 1. Each relevant component has a number identifier that is the same in both diagrams. In order to simplify component identification in the text the relevant number is placed alongside the component name which is also in bold text. For example on page 1 positive pressure gauge would be identified in text as positive pressure gauge(1). This convention is used throughout the manual. INSTALLATION AND COMMISSIONING
Remove the unit from the packing case and stand it on a flat and level bench or table . Do not destroy any packing materials until the packing list has been checked. Examine the unit for damage in transit - if any is found, the insurers should be notified immediately. The Pressure Measurement Bench F110 has a number of optional accessories. These are:- a Deadweight Tester F110A, a Pressure Transducer and Digital Display F110B and a Data Acquisition Option FC110A. Each of these options has a dedicated manual detailing its installation, operation and maintenance, together with suggested experimental procedures and example results. The Pressure Measurement Bench F110 is normally shipped without fluid in the manometers. It is first necessary to fill the manometers before the unit can be used. The manometers are normally filled with coloured water. A small bottle of dye is supplied in the accessories for the unit. This is conventional food colouring and when exhausted any locally sourced food dye may be used. Take a clean vessel and fill with approximately 0.5 litres of water and mix a small amount of dye until a suitable colour density has been achieved.
A large bore hypodermic syringe is supplied and this may be used to fill the manometer tubes in a controlled manner. It is also used at a later stage for pressurisation of the system and must not be discarded.
4
Draw a volume of fluid from the vessel and place this to one side. Cut a short length( about 100mm) of the black hose supplied and connect this to one of the vertical manometer connectors(8 or 9). Then take the pre-filled syringe and use this to part fill the manometer. DO NOT simply inject the full syringe into the tube.
SLOWLY inject some liquid into the manometer and observe the level in the tubes.
The syringe may be used to inject and remove fluid while the fluid is in the upper region of the tubes.
5
It is necessary to approximately fill both tubes to approximately half height.
Once the tubes are approximately as shown above then disconnect the syringe from the manometer. The procedure for filling the inclined tube is identical and the levels need to be similar when the tubes have been opened to the atmosphere .
6 Connecting the Pressure Gauges and Manometers The vertical manometer (5) , inclined tube manometer(3) , compound pressure gauge(2) and positive pressure gauge(1) can be interconnected in a variety of ways using the T connectors and black plastic tubes supplied.
The syringe must be emptied of liquid and connected to the manometer and pressure gauge using the black hose and T connectors provided.
The tubes connect to the coupling on the panel(6,7,8,9,10,11) as shown above. To allow for the majority of situations cut two lengths of black hose to a length equal to the width of the base of the unit. Connect one to the positive pressure gauge(1) connector(11) bottom right. Connect the other to the syringe. Cut 3 lengths approximately 300mm long and connect these to the vertical manometer connector(9) “U TUBE 1”, . vertical tube connector(7) “INCLINED TUBE 1”and to the Differential pressure gauge connector (6)
Above is an example of the inter-connections that can be made. The unit is now ready for use.
7 INTRODUCTION Please refer to the schematic diagram on page 1
The measurement of pressure, is one of the fundamental requirements of many engineering applications. There are several different ways of measuring pressure and the purpose of the Hilton, F110 Pressure Measurement Bench and its options, is to allow students to investigate many of these. One of the simplest, and most accurate methods, of measuring low pressures is the use of a vertical or inclined tube manometer. The basic F110 Pressure Measurement Bench consists of one U tube manometer(5), a combined vertical and inclined tube manometer(3,4), a differential pressure gauge(6) and a positive pressure gauge(1). Both of the manometers rely upon measurement of the difference in height of a column of liquid of known density. The unit allows students to undertake these measurements in a fundamental way in order to understand the procedure and the mechanics of how the manometer operates. The two pressure gauges fitted both bourdon tube devices. One illustrates differential and/or positive, negative pressures and the other positive or gauge pressure relative to atmosphere. The optional Deadweight Tester F110A allows students to calibrate a Bourdon tube gauge in order to understand the fundamental fact that pressure is simply force per unit area. In order to understand the mechanics of the Bourdon tube gauge, the gauge on the deadweight tester has a clear front panel, allowing the mechanism to be clearly seen operating. The optional Pressure Transducer and Digital display F110B combines with the Deadweight Tester F110A to allow students to investigate, the output of a pressure transducer in terms of voltage. Here, a voltage signal is used to represent pressure and students calibrate this, together with the Bourdon gauge. A third option, the Data Acquisition Option FC110A enables students to record the signal from the pressure transducer on a computer and analyse the results using most standard spreadsheets.
8 DESCRIPTION
Please refer to the schematic diagram on page 1. The Hilton Pressure Measurement Bench F110 ,base unit consists of a panel on which are mounted two simple manometers and two Bourdon tube pressure gauges. The basic F110 Pressure Measurement Bench consists of one U tube manometer(5), a combined vertical and inclined tube manometer(3,4), a differential pressure gauge(6) and a positive pressure gauge(1). Both of the manometers rely upon measurement of the difference in height of a column of liquid of known density. The unit allows students to undertake these measurements in a fundamental way in order to understand the procedure and the mechanics of how the manometer operates. The two pressure gauges fitted both bourdon tube devices. One illustrates differential and/or positive, negative pressures and the other positive or gauge pressure relative to atmosphere The U tube manometer(5) has two connection points(8,9) towards the top of the panel. These are labelled on the panel as U tube 1, the left-hand tube(9) and U tube 2, the right hand tube(8). Students may connect to either or both tubes as required, using the hose provided. The inclined tube manometer is arranged as one vertical tube (4), and one inclined tube(3). Again students had made connect to either side of the manometer. The vertical side of the inclined tube manometer has a connection point labelled inclined tube 1 and shown as vertical tube connector (7) on the schematic diagram. The inclined tube of the inclined tube manometer has a connection point on the panel labelled asinclined tube 2 and shown on the schematic diagram as inclined manometer tube connector (10). The compound pressure gauge (2) has a connection point (6) on the front panel. The compound pressure gauge may be used to measure pressures above atmospheric and below. The positive pressure gauge (1) also has a connection point (11) on the front panel. The positive pressure gauge can only be used to measure pressures that are above atmospheric pressure. The other options available in the pressure measurement range are described in detail in their own installation operation and maintenance manuals.
9 USEFUL DATA
Density of Pure Water = 1000 kg/m3 Pressure acting on a column of liquid P gh Where
= Density of the liquid in the tube (kg/m3)
g = Acceleration due to gravity ( m/s2) h = Difference in the height of the column of liquid (m) P = Pressure N/m2 or “Pascal” Note The pressure units used in this manual are N/m2 as these are made up from fundamental units (Newtons and m2) however the “Pascal” is the derived SI unit for pressure.
10 OPERATION
It is assumed that the Pressure Measurement Bench F110 has being installed and commissioned as details on page 3. The manometers and pressure gauges, may be used in any combination in order to illustrate the different methods of pressure measurement. It is assumed that tubes have been connected as details on page 6. Any device that will give a low pressure (in the range of approximately 500 mm water gauge) can be used together with the unit. However, for ease of operation the syringe supplied, may be used to provide a constant low pressure either above or below atmosphere for use with the unit. It is important that the pressure gauges are not connected in isolation to the syringe as it is possible to over pressurise the gauges and permanently damage them. It is recommended that unless specifically instructed, when pressure gauges are used, they should also be connected to a manometer using one of the T connectors provided.
In each of the suggested experimental procedures, contains instructions for four-manometer and pressure gauge connection. However any permutation may be undertaken, that is within the range of the instruments.
11 MAINTENANCE Cleaning The panel and tubes may be cleaned with a damp cloth and a little soap if necessary. Do not use any solvent or aggressive cleaner, particularly on the clear plastic tubes. Draining If it is necessary to drain the tubes, this can be easily achieved by inverting the panel with the help of an assistant and simply blowing through one of the tubes connected to one side of each manometer. The liquid will vent through the other tube. It may be necessary to do this more than once as the liquid may collect in the other tube initially and effectively the user will be trying to “blow” the contents of the tube upwards. Pressure Gauges The pressure gauges, do not contain any user serviceable parts. No attempt should be made to dismantle the pressure gauges.
12 EXPERIMENTAL CAPABILITIES
1.
Investigation Of Manometer Pressure Measurement Methods.
2.
Investigation of Pressures Above and Below Atmospheric Pressure a) A Simple U Tube Manometer and Pressures ABOVE AND BELOW Atmospheric Pressure. b) An Inclined Tube Manometer and Pressures ABOVE AND BELOW Atmospheric Pressure
3.
Examination of the Difference Between Absolute and Gauge Pressure.
13 1.
INVESTIGATION OF MANOMETER PRESSURE MEASUREMENT METHODS
Procedure It is assumed that the Pressure Measurement Bench F110 has being installed and commissioned as details on page 3.
Before connecting any of the manometers students should first observe the height of the liquid in both of the manometer tubes at rest.
The height of the liquid should always be measured relative to the bottom of the meniscus as shown below.
It should be explained to the students that the meniscus is there due to the surface tension of the water. Different fluids used in manometers can have different degrees of meniscus, and particularly in the inclined tubes of inclined tube manometers. The liquid metal mercury, which is sometimes still found in manometers, has a meniscus that curves in the opposite direction to water and other fluids. In the case of mercury the measurement is taken to the TOP of the meniscus as shown below.
As mercury is a cumulative poison, its regular use in manometers is becoming rarer. However students may come across mercury in manometers in their working lives and should be aware of the difference in operation. Students should also be made aware of the poisonous nature of mercury . There is no mercury present in the Hilton Pressure Measurement Bench F110
14 For the U tube manometer in the photograph below the rest height of both tubes of liquid(when observed at the base of the meniscus) is 292mm (left hand tube) 292mm (right hand tube). Under these conditions, the pressure, acting on the top of both columns of liquid is equal, and is that of the atmosphere.
It is recommended that students also take the opportunity to look at the level of the liquid in the inclined tube manometer.
As may be seen, the liquid is at the same horizontal level due to the effect of gravity.
15
Take the syringe and pull out the plunger so that it is about half way out of the tube.
Connect the tube from the syringe to a T and use this to connect the syringe to both the positive pressure gauge (1), and the vertical manometer connector(9), U tube 1. Take great care and slowly push in the plunger, a short distance and observe the U tube manometer(5) and the positive pressure gauge (1). It will be seen that the pressure gauge needle moves and that the column of liquid in the U tube manometer rises on the U Tube 2 side and falls on the U Tube 1 side. Note that the syringe is connected to the U Tube 1 side . Adjust the plunger until the pressure gauge shows approximately 10 mBar. Record the height of the liquid in U Tube 1(left side) and U Tube 2 (right side) and the pressure gauge reading. Push in the plunger further until the pressure gauge shows approximately 20 mBar. Again record the height of the liquid in U Tube 1(left side) and U Tube 2 (right side) and the pressure gauge reading. Continue pushing in the plunger in stages until the maximum reading on the pressure gauge is shown(60mBar) Do not exceed the maximum indicated value or the pressure gauge will be damaged . Typical results are shown below. Sample No 1 2 3 4 5 6 7
U Tube 1 mm 292 342 393 445 495 550 570
U Tube 2 mm 292 238 192 140 91 36 17
h mm
(U tube 1 – U tube 2) 0 104 201 305 404 514 553
Pressure mBar 0 10 20 30 40 50 54
Gauge
16 Calculations The difference in height between the U Tube 1 and U Tube 2 is determined by subtracting the smaller value from the larger. In sample No 2 in the table h
342 238
104 mm
Applied pressure from syringe
342mm 104mm 238mm
From the useful data on page 9 the pressure lifting the column of liquid above the present datum
P gh Where
= Density of the liquid in the tube (kg/m3)
g = Acceleration due to gravity ( 9.81 m/s2) h = Difference in the height of the column of liquid (m) P = Pressure N/m2 or “Pascal” Note that it is important that the units of measurement are consistent. The density of the water in the tube (from page 9) = 1000 kg/m3 Hence
P gh 1000 9.81 0.104 1020.24 N
Note that the pressure gauge is calibrated in mBar or
/ m2 1
1000
of a Bar
Now
1 Bar = 1 105 N/m 2 Hence 1 mBar =
1 105 1000
100
N/m 2
N/m 2
17
Hence
1020.24 N / m 2
1020.24
mBar 100 =10.2mBar
Referring to the table on page 15 the corresponding pressure indicated by the pressure gauge was 10 mBar Repeating the calculations for the other sample data gives the following. Sample No 1 2 3 4 5 6 7
U Tube 1 mm 292 342 393 445 495 550 570
U Tube 2 mm
h mm
P gh
(U tube 1 – U tube 2)
mBar 0.0 10.2 19.7 29.9 39.6 50.4 54.2
292 238 192 140 91 36 17
0 104 201 305 404 514 553
Pressure Gauge mBar 0 10 20 30 40 50 54
The difference between the pressure gauge and the manometer readings may be accounted for by a number of factors. Potential Errors in the Manometer Realistically observing the scale used it would be reasonable to assume that it is possible to read the height to within an accuracy of 1to 2mm worst case. Using the same calculation this represents a pressure of
P gh 1000 9.81 0.002 1.962 N
/ m2
In terms of the same units as the pressure gauge
1.962 N / m 2
1.962
mBar 100 =0.01962mBar
This is much smaller than the differences between the pressure gauge and the values calculated from the manometer. For the manometer the only other potential errors are in a) The density of the water b) The measuring scale c) Local gravity “g” None of the above factors are likely to affect the results by a large margin. Potential Errors in the Pressure Gauge a) The pressure gauge is a mechanical device that will have friction and stick characteristics. To observe this make a small change to the pressure by carefully pressing or pulling on the syringe. Observe the pressure gauge needle movement and then gently tap the gauge dial. The needle will be seen to move either up or down depending upon the direction of the change in pressure. This is the effect of friction. At the same time note that the manometer does not require any ”tapping”. b) The pressure gauge is calibrated by the manufacturer against a master gauge. The expected degree of accuracy will depend upon the gauge construction and fundamentally this dictates its cost. Higher accuracy gauges will cost significantly more than industrial quality gauges. The advantage of a pressure gauge is that it can be calibrated in the required units and calculations are not required for an immediate reading. c) The pressure gauge will ultimately wear and its accuracy will be affected certainly if it is over pressurised. In the same situation the manometer will simply spill its liquid. It can be refilled.
18 In conclusion the manometer provides a highly accurate and low cost method of measuring pressures that are above atmospheric pressure. Unless high density liquids such as mercury are used there is a limit to the practical range of pressures that can be measured due to the length of tubes required and the practical nature of measuring these heights.
19 2
INVESTIGATION OF PRESSURES ABOVE AND BELOW ATMOSPHERIC PRESSURE
a) A Simple U Tube Manometer and Pressures ABOVE AND BELOW Atmospheric Pressure Procedure It is assumed that the Pressure Measurement Bench F110 has being installed and commissioned as details on page 3.
Before connecting any of the manometers students should first observe the height of the liquid in both of the manometer tubes at rest.
The height of the liquid should always be measured relative to the bottom of the meniscus as shown below.
It should be explained to the students that the meniscus is there due to the surface tension of the water. Different fluids used in manometers can have different degrees of meniscus, and particularly in the inclined tubes of inclined tube manometers. The liquid metal mercury, which is sometimes still found in manometers, has a meniscus that curves in the opposite direction to water and other fluids. In the case of mercury the measurement is taken to the TOP of the meniscus as shown below.
As mercury is a cumulative poison, its regular use in manometers is becoming rarer. However students may come across mercury in manometers in their working lives and should be aware of the difference in operation. Students should also be made aware of the poisonous nature of mercury . There is no mercury present in the Hilton Pressure Measurement Bench F110
20 For the U tube manometer in the photograph below the rest height of both tubes of liquid(when observed at the base of the meniscus) is 292mm (left hand tube) 292mm (right hand tube). Under these conditions, the pressure, acting on the top of both columns of liquid is equal, and is that of the atmosphere.
It is recommended that students also take the opportunity to look at the level of the liquid in the inclined tube manometer.
As may be seen, the liquid is at the same horizontal level due to the effect of gravity.
21
Take the syringe and push the plunger fully home. Connect the tube from the syringe to a T and use this to connect the syringe to both the compound pressure gauge(2), and the vertical manometer connector(9), U tube 1. Take great care and pull OUT the plunger, a short distance and observe the U tube manometer(5) and the compound pressure gauge (2). It will be seen that the pressure gauge needle moves INTO THE NEGATIVE (-) REGION and that the column of liquid in the U tube manometer rises on the U Tube 1 side and falls on the U Tube 2 side. Note that the syringe is connected to the U Tube 1 side . It should be explained to the students at this point that the syringe is reducing the pressure in the U Tube 1 side(left column) of the manometer below that of atmosphere. The atmospheric pressure on the column of liquid in the U Tube 2 (right ) side of the manometer is pushing down on the liquid and forcing it UP in the U tube 1 (left) side tube. Hence the U tube manometer is showing the Difference between the pressure in the left tube and that of the atmosphere. It is a differential pressure measuring device.
Adjust the plunger until the pressure gauge shows approximately -10 mBar. Record the height of the liquid in U Tube 1(left side) and U Tube 2 (right side) and the pressure gauge reading. Pull out the plunger further until the pressure gauge shows approximately -20 mBar. Again record the height of the liquid in U Tube 1(left side) and U Tube 2 (right side) and the pressure gauge reading. Continue pulling the plunger in stages and recording the heights and pressure until the maximum reading on the pressure gauge is shown (-50mBar) Do not exceed the maximum indicated value, or the pressure gauge will be damaged . Now reverse the procedure . Disconnect the syringe and pull out the plunger before re-connecting. Now PUSH the syringe plunger IN until the compound pressure gauge returns to approximately +10mBar. Note that the liquid in the manometer tubes has now reversed in movement and the liquid in the U Tube 1 (Left) falls while the liquid in the U Tube 2 rises. It should be explained to the students at this point that the syringe is increasing the pressure in the U Tube 1 side(left column) of the manometer above that of atmosphere. The atmospheric pressure on the column of liquid in the U Tube 2 (right ) side of the manometer is pushing down on the liquid BUT the pressure in the syringe is ABOVE that of atmosphere and is forcing the liquid UP in the U Tube 2 (right hand tube) . Hence the U tube manometer is showing the Difference between the pressure in the left tube and that of the atmosphere. It is still a differential pressure measuring device.
Continue pushing the plunger in stages until the maximum reading on the pressure gauge is shown (+50mBar) Do not exceed the maximum indicated value or the pressure gauge will be damaged .
22
Typical results are shown below. Sample No 1 2 3 4 5 6 7 8 9 10 11 12 13 14
U Tube 1 mm 292 241 194 141 89 35 292 341 390 440 491 545 292
U Tube 2 mm 292 337 388 441 491 545 292 243 194 145 94 40 292
h mm
(U tube 1 – U tube 2) 0 -96 -194 -300 -402 -510 0 98 196 295 397 505 0
Pressure mBar
Gauge
0 -10 -20 -30 -40 -50 0 10 20 30 40 50 0
Calculations As the manometer is a differential pressure measuring device it is not always immediately apparent when undertaking the calculations what “sense” the pressure being measured is occurring(i.e positive or negative).
All that is apparent is that the pressure in the tube containing the LOWER column of liquid is higher than the pressure in the tube containing the HIGHER column. This pressure is higher
This pressure is lower
This represents the Pressure DIFFERENCE
In the case of the pressures being measured in the first part of the experiment(below atmospheric pressure) the manometer will appear as shown overleaf.
23
For the pressures below atmospheric pressure the arrangement is as shown below. Atmospheric Pressure
The Pressure BELOW Atmospheric in the Syringe
For convenience it is helpful to calculate the height difference in the columns as a negative number to represent the difference as a negative pressure relative to atmospheric pressure. Hence for sample No 2 in the table on page 15 (U tube 1 – U tube 2) h
241 337
96mm
From the useful data on page 9 the pressure lifting the column of liquid above the present datum
P gh Where
= Density of the liquid in the tube (kg/m3)
g = Acceleration due to gravity ( 9.81 m/s2) h = Difference in the height of the column of liquid (m) P = Pressure N/m2 or “Pascal” Note that it is important that the units of measurement are consistent. The density of the water in the tube (from page 9) = 1000 kg/m3 Hence
P gh 1000 9.81 ( 0.096) 941.76 N
Note that the pressure gauge is calibrated in mBar or
/ m2 1 1000
of a Bar
Now
1 Bar = 1 105 N/m 2 Hence 1 mBar =
1 105 1000
100
N/m
N/m 2
2
24 Hence 941.76 N
/ m2
941.76
100 =-9.4mBar
mBar
Referring to the table on page 15 the corresponding pressure indicated by the pressure gauge was -10 mBar Repeating the calculations for the other sample data gives the following. Note that the sense of the calculations (U tube 2 – U tube 1) are the same through the full extent of the data. Sample No 1 2 3 4 5 6 7 8 9 10 11 12 13 14
U Tube 1 mm 292 241 194 141 89 35 292 341 390 440 491 545 292
U Tube 2 mm 292 337 388 441 491 545 292 243 194 145 94 40 292
h mm
(U tube 1 – U tube 2) 0 -96 -194 -300 -402 -510 0 98 196 295 397 505 0
Pressure Gauge mBar 0 -10 -20 -30 -40 -50 0 10 20 30 40 50 0
P gh mBar
0.0 -9.4 -19.0 -29.4 -39.4 -50.0 0.0 9.6 19.2 28.9 38.9 49.5 0.0
The potential errors in both the manometer and the pressure gauge are similar to those on page 17 for the positive (only) pressures. However the manometer is still going to be the more accurate measuring device. There are other ways of increasing the resolution of the manometer and this is to use an inclined tube.
25 2
INVESTIGATION OF PRESSURES ABOVE AND BELOW ATMOSPHERIC PRESSURE b) An Inclined Tube Manometer and Pressures ABOVE AND BELOW Atmospheric Pressure
The inclined manometer tube(3) on the unit allows students to investigate the increased resolution that an inclined tube manometer can give. The method of operation is identical to the U tube manometer(5) but the single inclined tube (inclined at 35º from the vertical) allows students to compare this with operation of the conventional U tube manometer. Procedure It is assumed that the Pressure Measurement Bench F110 has being installed and commissioned as details on page 3.
Before connecting any of the manometers students should first observe the height of the liquid in both of the manometer tubes at rest.
If students have already carried out the first experiments on the U tube manometer ,the fact that the liquid rests in the tube at a level condition should already have already been observed. In the case of the following example the levels were:Vertical Tube(4) 291mm Inclined tube(3) 354mm Note that as both tubes are used and both zero points recorded, it is of no consequence if the unit is not absolutely level. In some manometers a large volume reservoir takes the place of the vertical tube. In this type of manometer establishing the unit on a level bench is essential. This will be examined in a later section of this manual.
26 The inclined tube is used to provide an amplification of the reading and a greater resolution when examining small pressure changes.
The inclined tube operates as follows:A
B dL dI Zero Datum dv h 55°
35°
Initially with no pressure applied the liquid will sit on the Zero datum line. If a pressure is applied at A and the pressure at A is greater than at B then the liquid in the tubes will move as shown. The liquid in the vertical tube will fall by an amount dv. The liquid in the inclined tube will rise by an amount dL. The VERTICAL increase in the height of the liquid in the inclined tube is dI. Hence the difference in height between the vertical and inclined tubes is h The length dL is greater than dI and if this is measured using a mm scale it means that there is a greater resolution. In order to calculate the vertical height dI from the inclined length dL the sine rule may be used. If the inclined tube is at 35° from the vertical then the angle between the horizontal and the tube will be 90-35 = 55° Hence
dI
o
dL Sin55
The overall vertical height change will be o
h dv dL Sin55
Procedure To undertake investigation of this proceed as follows. First record the rest condition of the liquid in the vertical and inclined tubes. In this example:Vertical Tube(4) 291mm Inclined tube(3) 354mm
27 Take the syringe and pull out the plunger . Connect the tube from the syringe to a T and use this to connect the syringe to both the compound pressure gauge(2), and the vertical tube connector(7) (labelled Inclined tube 1).
Now PUSH the syringe plunger IN until the compound pressure gauge (2) turns to approximately +10mBar. Note that the liquid in the vertical manometer tube falls and the liquid in the inclined tube(3) rises. It should be explained to the students at this point that the syringe is increasing the pressure in the vertical (left column) of the inclined tube manometer above that of atmosphere. The atmospheric pressure on the column of liquid in the Inclined tube 2 (right ) side of the manometer is pushing down on the liquid BUT the pressure in the syringe is ABOVE that of atmosphere and is forcing the liquid UP in the Inclined Tube 2 (right hand tube) . Hence the U tube manometer is showing the Difference between the pressure in the left tube and that of the atmosphere. It is still a differential pressure measuring device. Record the new height of the liquid in both the vertical and inclined tubes. Push the syringe in further and repeat the measurements when the compound pressure gauge shows approximately +20mBar.
Continue pushing the plunger in stages until the maximum reading on the pressure gauge is shown (+50mBar) Do not exceed the maximum indicated value or the pressure gauge will be damaged . Disconnect the syringe from the tube and allow the manometer and pressure gauge to return to the zero datum. Now push in the plunger on the syringe and re-connect to the tube. This time draw out the plunger until the compound pressure gauge(2) shows approximately -10mBar. Again record the new height of the liquid in both the vertical and inclined tubes. It should be explained to the students at this point that the syringe is reducing the pressure in the Vertical (left column) of the inclined tube manometer below that of atmosphere. The atmospheric pressure on the column of liquid in the Inclined Tube 2 (right ) side of the manometer is pushing down on the liquid and forcing it UP in the vertical tube 1 (left) side tube. Hence the inclined tube manometer is showing the Difference between the pressure in the left tube and that of the atmosphere. It is a differential pressure measuring device.
Pull out the syringe further and repeat the measurements when the compound pressure gauge shows approximately -20mBar.
28
Continue pulling out the plunger in stages until the maximum reading on the pressure gauge is shown (-50mBar) Do not exceed the maximum indicated value or the pressure gauge will be damaged . Typical results are shown below. Sample No
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Inclined Tube 1 mm 291 347 401 455 509 567 291 232 180 123 67 8
Inclined Tube 1 Zero mm 291 291 291 291 291 291 291 291 291 291 291 291
Vertical Difference mm
Pressure Gauge mBar
0 56 110 164 218 276 0 -59 -111 -168 -224 -283 0
Inclined tube mm
Inclined tube zero mm
Inclined difference mm
354 301 245 191 137 80 354 408 466 522 579 636
354 354 354 354 354 354 354 354 354 354 354 354
0 53 109 163 217 274 0 -54 -112 -168 -225 -282
0 -10 -20 -30 -40 -50 0 10 20 30 40 50 0
Note that the two grey lines of data show the zero pressure condition. Calculations.
A
B dL dI Zero Datum dv
h 55°
35°
Considering the diagram. For sample No 2 in the table.
dv 347 291
56mm
29
Similarly for the inclined tube
dL 354 301
53mm
The vertical displacement on the inclined tube may be calculated from
dI
o
dL Sin55
53 Sin55o
43.4mm
Hence the total difference in vertical height of the liquid between the vertical and inclined tube is
h dv dI
56 43.4
99.4mm
From the useful data on page 9 Pressure acting on a column of liquid P gh Where
= Density of the liquid in the tube (kg/m3)
g = Acceleration due to gravity ( m/s2) h = Difference in the height of the column of liquid (m) P = Pressure N/m2 or “Pascal” Hence
P gh 1000 9.81 0.0994
975.1 N / m 2 1
Note that the pressure gauge is calibrated in mBar or
1000
of a Bar
Now
1 Bar = 1 105 N/m 2 Hence 1 mBar =
1 10
5
1000
100
N/m 2
N/m 2
Hence
975.1 N / m 2
975.1
mBar 100 =9.751mBar
This compares with 10mBar on the compound pressure gauge.
30
Repeating the calculations in exactly the same way for all of the data in the table on page 28 gives the following results. Sample No
1 2 3 4 5 6 7 8 9 10 11 12 13 14 Sample No 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Inclined Tube 1 mm 291 347 401 455 509 567 291 232 180 123 67 8
Vertical dv mm 0 56 110 164 218 276 0 -59 -111 -168 -224 -283 0
Inclined Tube 1 Zero mm 291 291 291 291 291 291 291 291 291 291 291 291
Inclined Tube dL mm 0 53 109 163 217 274 0 -54 -112 -168 -225 -282
Vertical Difference mm 0 56 110 164 218 276 0 -59 -111 -168 -224 -283 0
Inclined Tube dI mm 0.00 43.42 89.29 133.52 177.76 224.45 0.00 -44.23 -91.75 -137.62 -184.31 -231.00
Pressure Gauge mBar 0 -10 -20 -30 -40 -50 0 10 20 30 40 50 0
Inclined tube h mm 0.00 99.42 199.29 297.52 395.76 500.45 0.00 -103.23 -202.75 -305.62 -408.31 -514.00 0
Inclined tube mm
Inclined tube zero mm
Inclined difference mm
354 301 245 191 137 80 354 408 466 522 579 636
354 354 354 354 354 354 354 354 354 354 354 354
0 53 109 163 217 274 0 -54 -112 -168 -225 -282
Pressure Gauge mBar 0 -10 -20 -30 -40 -50 0 10 20 30 40 50 0
P gh mBar
0.00 9.75 19.55 29.19 38.82 49.09 0.00 -10.13 -19.89 -29.98 -40.06 -50.42
As may be seen the calculations result in manometer pressure readings that are very close to the compound pressure gauge. However for the reasons outlined in the U tube experiments the manometer is more likely to be closer to the ACTUAL pressure difference .
31 3. EXAMINATION OF THE DIFFERENCE BETWEEN ABSOLUTE AND GAUGE PRESSURE.
It has been established that the manometers can indicate differential pressures. That is the DIFFERENCE in pressure that exists between one side of the manometer and the other. All that is apparent is that the pressure in the tube containing the LOWER column of liquid is higher than the pressure in the tube containing the HIGHER column. This pressure is higher
This pressure is lower
This represents the Pressure DIFFERENCE
Hence if one side of the manometer is OPEN to atmosphere then the pressure being indicated is relative to atmosphere and is known as GAUGE pressure. This is fundamentally because most (not all) dial type pressure gauges indicate pressures relative to the local atmosphere. A typical bourdon tube pressure gauge operates as shown below. The sealed C shaped tube “a” has the applied pressure inside. This causes it to try and straighten out (open up) in the direction of arrow “b”. This pulls on the tie bar and causes the lever to rotate in the direction of arrow “c”. The segment of gear teeth can a be seen at the end of this lever and on the shaft of the spindle of the dial needle are corresponding gear teeth. Hence this causes the needle to move in the c direction of arrow “e”. e b The pressure gauge(depending upon calibration and mechanical arrangement will indicate pressures above(or below) atmosphere. If a pressure gauge is fitted with an evacuated capsule or an evacuated gauge body then the pressure it indicates can be absolute pressure or relative to absolute vacuum. If the local atmospheric pressure is known then the pressure recorded by a gauge or manometer may be expressed in absolute terms by simply adding this to the local atmospheric pressure.
32 The following is a description of a device. It is NOT a suggested experiment as mercury is a cumulative poison and should not be used in an un controlled manner. If a CLOSED glass tube of approximately 900mm length is filled with mercury and then inverted into a reservoir of mercury the column of mercury in the closed tube will fall from the closed end of the as shown below.
Atmospheric pressure Height of mercury
As the tube is closed at one end and this space was formerly occupied by the mercury the space above will contain a vacuum. The atmospheric pressure is pressing on the open surface of the mercury in the same manner as the U tube manometer. Hence the height of the mercury in the tube h represents the atmospheric pressure RELATIVE to a vacuum.
P gh 1000 13600 h
Where in this case is the density of mercury. This is a typical description of a mercury BAROMETER. There are many varieties of barometer but the mercury tube barometer is one of the simplest and most accurate.
