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TRIBHUVAN UNIVERSITY INSTITUE OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING PULCHOWK CAMPUS
Experiment Setup for Low Pressure Measurement (Assignment on EG851EE Instrumentation )
Submitted by: Santosh Sharma
Submitted to: Prof. Dr. R. R. Staphit Department of Mechanical Engineering
(066MSR5 15)
Date: Dec. 6, 2010
0
Measurement of Low Pressure using McLeod Gauge. A. Objective of Experiment Intellectual Skills:
Able to understand different pressure measuring instruments. Able to understand working principle of McLeod gauge and analyze it from generalized measurement system. Able to compare different pressure measuring instruments. Able to select appropriate pressure measuring instrument for different situations.
Motor skills
Ability to measure low pressure by using McLeod gauge. Ability to observe and record the reading.
To be familiar with pressure measuring device.
B. Condition for your experiment When low pressure measurement is required. (less than atmospheric pressure) C. Instrumentation 1) McLeod Gauge 2) Low pressure System 3) Any other instrument available in lab to measure low pressure .
Specification : Pressure measurement tutor using McLeod gauge. Range : 0.01micron to 50 mm of Hg. Bulb, Reservoir, capillary should be made of transparent material. Provision of cut off and different valves at proper places. All Joints must be leak-proof..
D. Planning of experiment McLeod gauges operate by taking in a sample volume of gas from a vacuum chamber, and then compressing it by tilting, and infilling with mercury. The pressure in this smaller volume is then measured by a mercury manometer, and, by knowing the compression ratio, the pressure of the original vacuum can be determined. It should be noted that McLeod Gauge is an absolute measuring device.
1
E. Theory
A slug of mercury moving in a tube is used to isolate a volume of gas at the pressure to be measured. The gas in the volume is then compressed by a known amount, and the final pressure is obtained with a manometer. Boyle's law is then used to find the initial pressure from the final pressure, and the initial and final volumes. This is a relatively slow and laborious process, and a liquid nitrogen cold trap must be used to prevent the vapor pressure of mercury from perturbing the pressure measurements. However, the readings of the McLeod gauge are absolute, and it is used to calibrate other gauges, such as ionization gauges.
Figure 1 Schematic Diagram of McLeod Gauge
2
F.
Stepwise procedure:
Locate the position in the system of which pressure has to be measured.
Connect unknown pressure source to the instrument at point 'P' as shown in figure 1.
Mercury level is adjusted to fill the volume represented by darker shading (Under these conditions, unknown pressure fills the bulb B and capillary C. Volume of unknown pressure is volume of bulb and capillary.)
Mercury is then forced out of reservoir D up into the bulb and reference column E.
Continue the forcing of mercury till it reaches to cutoff point F. (A known volume of gas is trapped in the bulb and capillary).
G.
Mercury level is then further raised until it reaches a zero reference point in E.
Volume remaining in the capillary is read directly from the scale.
The difference in the height of two columns is the measure of trapped pressure.
Initial pressure is then calculated by using equation P 1 = P2V2 /V1 (Boyle's Law)
Carry out the same procedure for other readings.
Observations:
Volume of bulb= ------------------cm
3
Volume of capillary = ------------------cm
3
Initial volume = Volume of bulb + volume of capillary = ----------------- cm Area of tube, At = ------------------ cm
2
3
3
Observation Table:
H.
S.No.
Difference in height of
Final Volume
Initial Pressure in cm of Hg
two columns (h) cm
V2 = At × h cm
3
P1 = P2V2 /V1
Result:
I.
The pressure measured by McLeod gauge is ------------cm of Hg. J.
Possible errors in measurement
Accidental release of the mercury into the test chamber.
The gauge is also easily contaminated.
Deviation from Boyle’s Law due to the non-ideal nature of gas.
The presence of condensable vapors, or absorption or desorption effects on the walls may easily produce systematic errors. Use of clod trap in the tube connecting the McLeod gauge to the gauge to be calibrated may lead to gain of pressure.
