Current Loops
Current loop signals (i.e. 0-1 mA DC, 4-20 mA, 10-50 mA etc) ride on the supply or signal voltage supplied by a power supply. This can be a separate device, such as the back of panel mounting Deltron 112A, which will convert a standard 120VAC or 240VAC line input into a stable 24V DC output @ 1.2 amps (1200 milliamps). Or the power supply might come from a smart digital panel meter, such as the Red Lion PAXP0000. The meter is powered by 120/240 and outputs a 24 VDC @ 50 milliamps. The Deltron example could power up to 60 separate 4-20 mA signals (1200 milliamps divided by 20 milliamps = 60); the Red Lion however only two. Sizing should really be based on at least 25 milliamps per device. The power supply's 24 Volt DC output will be reduced by the voltage drop of each instrument in the series connection of the loop, and by the length and gauge of the wire utilized. Care must be taken that the voltage is not dropped below the minimum operating range of the instruments used in the loop. The signal voltage in the loop is the difference between the voltage at one terminal (referenced to ground) and another terminal (see diagram above). Fortunately, most Digital (about 20 ohms) and Analog Instruments (about 3 ohms) have a very low resistance. Therefore, their utilization of voltage in the series circuit is usually minimal. One exception is if a receiver, such as a chart recorder or other such device, is really a 1-5 VDC device, and takes the 4-20 mA signal only through a 250 ohm resistor mounted on the signal input terminals of the device. According to Ohm's Law, the maximum drop for such a device is 5 VDC 5 VDC (250 ohms x 20 milliamps or .02 amps = 5) Ohm's Law: Voltage = Current x Resistance or E=I x R Current =Voltage Divided by Resistance or I=(E/R) Also fortunately, the length and gauge of instrument wire utilized has also usually a minimal effect on the voltage drop. Let us take a common situation with a single sensor and readout, and a distance between the two of 1000 feet: A common instrument signal wire is 22 Gauge. The resistance of this wire is only 0.0165 per foot (each leg must be considered). A thousand foot run from the sensor to the power supply would provide the following resistance: 1000 x 2 x 0.0165 = 33 ohms.
By Ohm's Law, Current times Resistance (I x R) = Voltage (E). 20 milliamps (.02 amps) x 33 ohms = 0.66 volts. If you have a 24 volt DC power supply, you would still have 23.34 volts left over. Even 10,000 feet would only take 6.6 volts away. Caution: If you remove an instrument from the ioop, the entire ioop goes down. Another consideration is the prevention of GROUND LOOPS. LOOPS. Please click on the underlined for a discussion on this topic.
RESISTANCE TEMPERATURE DETECTORS INTRODUCTION A resistance temperature detector (commonly called RTD, resistance bulb, etc.) assembly consists of (1) an element (2) a support or bobbin for the element (3) a protection tube or sheath (4) connecting wires which extend from the element to the termination end (5) a means of securing the connecting wires to the termination end, and (6) a means of connecting it to the resistance-measuring equipment.
Resistance temperature detectors (R.T.D.) or resistance thermometry is based on a well-known principle that most metals increase in resistivity when their temperature is increased, and on cooling to the original temperature, will return to the original resistivity. The resistance-temperature curves of pure metals, e.g., platinum and nickel, over definite spans makes them ideal materials for the elements in resistance thermometers. Laboratory resistance temperature detectors of pure platinum, fully annealed and strain free, have been chosen as the International Standard of Temperature Measurement from liquid oxygen [(LO2)-182.97 deg C] to the melting point of antimony [(sb) + 630.5 deg C]. Range -250 to +500 deg C for platinum. Temperature coefficient: .003915 ohms/ohm/ deg C and .00385 ohms/ohm/ deg C. (0.00385 or din standard has been adopted as the world and USA standard.) Pure nickel has been widely used as a temperature-sensitive element over the range of -700 deg C to +3000 deg C principally because of its low cost and high temperature coefficient of resistivity.
ADVANTAGES
Absolute Measurement Measurement - Resistance thermometers, unlike thermocouples, do not require a reference point. No ice baths or compensation circuits. High Output Output - With an output of 50 to 200 times that of a thermocouple, resistance thermometers permit the use of simpler indication and control instruments. No amplifiers are needed and the resulting system is less expensive and more reliable. Greatest Accuracy Accuracy - The pre-eminent position of the resistance thermometer as a precision temperature measuring instrument is demonstrated by its selection to define the International Temperature scale from -260 deg C to +660 deg C. The main reasons for its selection are: 1) the exceptional stability and 2) the repeatability of the resistance thermometer. USES
Resistance Thermometers can be used for a wide variety of industrial applications. A high electrical output can be obtained by using the RTD with many types of simple resistance bridges. This high output can then be fed directly into recorders, temperature controllers, transmitters, or digital readouts which can be calibrated to read very precise increments of temperature over wide dynamic ranges. RTD's can also be read out on precision laboratory bridges and digital ohmmeters. BASIC INSTRUMENTATION A simple Wheatstone bridge circuit with a reasonable high impedance detector is recommended for reading out RTD probes. If the detector impedance is assumed infinite Eo Rx Rs -- = ------ - -----E Rx - R Rs + R where where R = Ratio Ratio Arm Arms s Rx = Probe resistance (at temperature x) Rs = Balancing arm (equal to Rx at lowest temperature temperature which may be variable for zero set.)
Such a bridge is non-linear, when the probe undergoes any reasonable temperature excursion. In the case of platinum wire, the ratio arms (R) should be as large as possible (at least 10 times Rs) to minimize bridge non-linearity. To protect the probe and minimize the errors due to self-heating, an operating current of 1 MA is recommended. This current can be controlled by choice of R or L. To measure temperature difference, two identical probes can be used in adjacent arms of the bridge (second one replaces Rs). In this case, provisions for zero setting (if desired) should be moved to one of the R arms. CONSIDERATIONS FOR RTD SELECTION
1. How the point point of measureme measurement nt can be made. Whether Whether in a small area, area, which would necessitate a tip sensitive, or a large area which would make a stem sensitive more desirable. 2. The The O.D O.D.. of the the tub tube. e. 3. The temperature temperature and/or temperature temperature range of the media media to be measured measured accuracy excellent at room temperature. 4. What length length of immersion immersion would be required required for your application. application. 5. How the R.T.D. R.T.D. is to be inserted, inserted, and how best it can can be supported supported or mounted. mounted. 6. If pressure pressure or vacuum vacuum has to to be maintained, maintained, then then the R.T.D. R.T.D. has to to be supplied supplied with either a compression fitting, fixed fitting, head with connector, or a thermowell. TYPICAL APPLICATIONS
PRECISION PROCESS TEMPERATURE CONTROL • • • •
Textile Chemical Food Brewing
AUTOMATIC TEMPERATURE CONTROL • • • • • •
Test Chambers Oven Temperature Plastic Extruders Injection Molders Solder Pots Bearing Temperature
READILY AVAILABLE RTD INSTRUMENTS: • • • • • • • •
Digital Temperature Indicators 12-Inch Round Chart Recorders Branom Steam Control Systems and Multipoint Rtd Indicators Crompton and Jewell Rtd Analog Meters and Setpoint Controllers Red Lion digital Indicators and Controllers R.I.S. Transmitters and Trips and 36 Point Alarm Monitors Rustrak Miniature Recorders West Rtd Controllers: On-Off, Hi-Lo Limit, or PID GLOSSARY OF TERMS
RTD - Denotes resistance temperature detector, a device which provides a useable change in resistance to a specified temperature change. SENSING ELEMENT - The electrical portion of an RTD (Resistance winding) in which the change originates. CALIBRATION ACCURACY (INTERCHANGEABILITY) - The conformance of the RTD's measured output to a standard calibration curve calibrated by a governmental standards agency such as NBS or calibrated on equipment directly traceable to NBS. REPEATABILITY - The ability of the RTD to reproduce consecutive readings when the same temperature is applied to it consecutively under the same conditions, and in the same direction.
STABILITY - The ability of an RTD to retain its repeatability (and other specified performance characteristics) for a relatively long period of time. SELF-HEATING - internal heating resulting from electrical energy dissipated within the resistance sensor. This is usually specified in watts or millivolts/ deg C. This is determined by the amount of power it takes to raise the output of the sensor 10C under certain conditions such as air, water or oil flowing at a specified velocity. RESPONSE TIME - The length of time required for the output of an RTD to respond to 63.2% of a step change in temperature. This is usually specified in air, oil or water flowing at a specified velocity. MAXIMUM SAFE CURRENT - The maximum current recommended to be applied to a particular RTD to prevent burn out or open circuiting. This is determined by the sensor wire diameter and the configuration. INSULATION RESISTANCE - The resistance measured between specified in-insulated portions of an RTD (such as between sensing element and outer case) when a specified DC voltage is applied. PRACTICAL PRECAUTIONS
1. Use shielding shielding and twisted-pa twisted-pair ir wire, avoid avoid stress and steep steep gradients, gradients, use large large extension wire. Use 3 wire or 4 wire cable. 2. Due to its constructio construction, n, the RTD RTD is somewhat somewhat more fragile than than a thermocoup thermocouple le and some care should be taken to protect it 3. A current is and must must be passed passed througout througout the the RTD to provide provide a voltage that that can be measured. This current causes joule (I2R) heating within the RTD. This selfheating does appear as a temperature error. To reduce self heating errors, use the minimum current possible, and use the largest rtd you can that will still give you the response you need. A typical value for self-heating error is 1/2 deg c per milliwatt in free air. If you immerse the RTD in a liquid, or any other thermally conductive medium, you will dissipate the self-heating aspect to a negligible error. MOST COMMON RTD TYPES
1. 1/8 inch x 2 inch encapsulat encapsulated ed 100 ohm ohm platinum platinum (.00385) (.00385) RTD for surface mounting. 2. 1/4 inch od od x any practical practical length length ss sheath sheath 100 ohm ohm plt RTD RTD (.00385)--availa (.00385)--available ble with 1/8 inch, 1/4 inch or 1/2 inch brass or ss fitting or with standard wells and
with aluminum or cast iron heads, or high temperature plastic (450 or 850 deg f) heads.
THERMOWELL CONVERSION KIT THE ADAPTER SET CONSISTS OF: 1. An adapter nut. 2. A metal liner.
SELECTION OF BIMETAL THERMOMETER: 1. Measure well depth by instering a pencil or
small diamter rod into well until it reaches the bottom (Figure 1). 1) . 2. Using thumb as index, measure distance from end of rod to index point (Figure 2). 2) . 3. Refer to Selection Table (below) to select proper thermometer stem length. Thermometer stem length must match well depth as indicated on the Selection Table. INSTALLATION OF ADAPTER AND THERMOMETER 1. Drop or push push metal metal liner liner into well. 2. Thread Thread adapte adapterr nut into into well well and tighten tighten.. 3. Install Install Reotem Reotemp p Bimetal Bimetal Therm Thermome ometer ter into into well. (Note: a small amount of graphite and grease or other heat transfer compound on the lower 2" of stem and liner will improve response time.) Well Depth Depth in in Inches Inches Bimetal Stem Length 3 - 3-1/4
4"
5 - 5-1/4
6"
7 - 7-1/4
8"
THERMOWELLS (DRY WELLS) A. Usage & Disadvantages The use of thermowells (also known as dry wells) is common in industrial applications involving the need to remove a temperature sensor from a tank or line without shutting down the system. Thermowells, however, have some basic disadvantages which must also be taken into consideration for any temperature application: 1. Transmission Time: The added mass, and the type of material has a slowing effect upon how quickly the actual temperature reading will show up on the indicator (brass has fastest transmission time). 2. Fit: Proper, very tight fit is essential, as air gaps create insulation, and therefore inaccurate readings. Unfortunately, there are few standards in the industry for Remote Reading Gas and vapor Filled Thermometers, with every brand, every style and every range with a different diameter and length and connection. Bulb lengths and dimensions, internal & external thread requirements etc must be carefully measured for the specially ordered well, so that the internal diameters and lengths and connections match the sensing bulb. Heat transfer compounds should be used whenever an absolutely tight fit is not possible; an inexpensive compound consists of a paste containing 1/3 water and 2/3rds magnesium hydroxide (available from us, or from chemical suppliers). Thermocouples and RTD's can also come in any size and shape; a common size, however, is 1/4" OD, and with a 1/2" NPT Spring Loaded Male Fitting, these can fit into inexpensive and commonly found bimetal thermometer wells, which have a .260 bore. 3. "Lagging" thermowells take into account the insulation, pipe fittings, or walls etc. through which a sensor might have to pass.
B. Installation Considerations
The most common method of installing a well is to purchase and install a "tee" from a plumbing supply house and use a standard threaded well; ASA 150#, 300# and 600# Flanged Wells, Van Stone Wells, and Socket Weld types are also readily available. All threaded wells are made in easily welded or brazed materials. This is important for installations requiring sealing; the pipe thread provides the mechanical strength, while the brazing or welding provides the seal. The object is to measure the temperature of the medium, so the insertion should be to the point in the pipe where the measurement is desired, usually in the middle of the pipe. However, the sensing portion and range of the instrument will often determine the minimum insertion length of the well. The "U" dimension of a well is the insertion length of the sensing bulb (the distance from the tip of the internal bottom of the well to the first thread or other connection means) should be entirely immersed in medium being measured. A properly installed element will project into the liquid an amount equal to its sensitive length plus at least one inch. In air or gas, the element should be immersed its sensitive length plus at least three inches. Some low range bi-metal thermometers, for example, are not available without at least a 4" length stem. Normally, bi-metal thermometers have a sensitive length of 2.5"; RTD's usually have a sensitive length of 1" or so; thermocouples have sensitive lengths of 1/4" or so; grounded thermocouples are tip sensitive, and have a faster response to temperature changes than ungrounded types (but ungrounded thermocouples help prevent current loops and induced voltages that often destroy the thermocouple millivolt signal). Industrial liquid-in-glass thermometers come standard with either a 2" stem (Submarine Thermometers) or more commonly, 3 1/2" stems (Standard Industrial and Retort); sometimes 6", 8", 9" 10" and up to 48" types can be found. Careful measurement of the "U" dimension is necessary for a correct well fit.
C. Velocity Rating Factor Tapered shank wells provide greater stiffness for the same sensitivity. The higher strength-to-weight ratio gives these wells a higher natural frequency than the equivalent length straight shank well, thus permitting operation at higher fluid velocity. Another consideration might be materials of construction; some wells made of stainless steel, for example, may take higher temperatures, pressures and velocities than a brass one. Fluid, flowing by the well, forms a turbulent wake (the "von Karmen" trail) with a frequency based upon the diameter of the well and the velocity of the fluid. If the wake frequency equals the natural frequency of the well, the well will literally shake itself to pieces and break of f from the piping. Velocity tables are available from us for most types of standard wells, materials, pressures and temperatures. For simplicity sake,
brass is rated at 350 0F, steel and stainless @ 1000 0F, monel @ 900 0F service. Slightly higher velocities might by possible at lower temperatures. Typical ratings for straight stepped thermowells in maximum fluid velocity feet per second: 1/4" OD Stem "U" Dimension Dimension | Material Material of Constructio Construction n | FPS 2.5"
4.5"
Brass C.S. 304 & 316 Brass C.S. 304 & 316
| 207 | 290 | 300 | 75 | 105 | 109
D. Materials of Construction To prevent electrolysis, the well should ideally be constructed of the same material as the piping. Another consideration is the corrosive conditions the well will face, as well as strength necessary to face these conditions. Wells are often cut from bar stock in brass, monel, 304 and 316 SS, or other special grades of stainless steel, inconel, hastelloy B & C, Nickel, and Titanium. The least expensive are steel and brass constructions as well as 304 and 316 SS. For high temperature thermocouple use, 302 SS sheaths, silicon carbide or porcelain sheaths are common well solutions.
THERMOCOUPLES INTRODUCTION The basic theory of thermocouples dates back to 1821 when T.J. Seebeck discovered that a current is induced into a closed circuit of two dissimilar metals by heating one of the two junctions. And, as long as the temperature differences exists between the two junctions, current will continue flowing through the circuit.
While the theory is nearly 150 years old, incorrect application of thermocouples still affects today's sophisticated industrial processes. In any temperature control system, the heart of that system is the temperature sensing device -- in this case, thermocouple. Without proper application or understanding of basic thermocouple circuits, even the most complicated system cannot function. In his discovery, Seebeck also concluded that any two metals can be used. However, the magnitude and direction of the generated current are functions of the magnitude of the temperature difference between the junctions and the thermal properties of the metals used in the circuit. Therefore, not every combination of metals is acceptable for thermocouple usage.
THERMOELECTRIC CHARACTERISTICS A thermocouple should have thermoelectric characteristics such that the electromotive force (emf) produced per degree of temperature change is sufficient to be detected by standard measuring instruments. The device must also be capable of withstanding temperature extremes for prolonged periods, rapid temperature changes, and corrosive atmospheres while exhibiting reproducibility and a high degree of accuracy. The Instrument Society of America (ISA) has established a type of code and limits-oferror specifications for thermocouple wire, shown below:
Table 1: Thermocouple Wire Specifications Specifications
ISA LIMITS OF ERROR DESCRIPTION
ISA TYPE
TEMPERATURE RANGE
STANDARD
SPECIAL
Copper/Constantan
T
-300 deg. F to -75 deg. F
--
+/- 1%
-150 deg. F to -75 deg. F
+/- 2%
+/- 1%
-75 deg. F to +200 deg. F
+/- 1.5%
+/- 3/4 deg F
0 deg. F to +530 deg. F
+/-4 deg F
+/- 2 deg F
+530 deg F to +1400 deg F
+/- 0.75%
+/- 0.375%
+32 deg. F to +600 deg. F
+/- 3 deg F
--
+600 deg. F to +1600 deg. F
+/- 0.5%
--
0 deg. F to +530 deg. F
+/- 4 deg F
+/- 2 deg F
+530 530 deg. eg. F to +23 +2300 deg. F
+/- 0.75 0.75% %
+/- 0.3 0.375%
Iron/Constantan
Chromel/Constantan
Chromel/Alumel
J
E
K
Platinum/Platinum (+10% Rhodium)
S
0 deg. F to +1000 deg. F
+/- 5 deg F
--
Platinum/Platinum (+13% Rhodium)
R
+1000 deg. F to +2700 deg. F
+/- 0.5%
--
Six thermocouples are covered by this system: •
Copper vs. Constantan (ISA Type T) (Nickel Alloy) Superior for use at sub zero temperatures. Withstands corrosion well and is recommended for temperatures within the range of -300 deg F to +700 deg F.
•
Iron vs. Constantan (ISA Type J) Above 1000 deg F, the rate of oxidation of the iron wire increases rapidly and the thermocouple should be enclosed in a protection tube of suitable material. Protected Iron vs. Constantan thermocouples are recommended for temperatures up to 1600 deg F.
•
Chromel vs. Alumel (ISA Type K) (Nickel Chromium Alloy) or (Nickel Manganese, Aluminum, Silicon)
These are trade names of Hoskins Manufacturing Company. Chromel vs. Alumel thermocouples have excellent characteristics when supplied with protection tubes up to 2200 deg F. •
Chromel vs. Constantan (ISA Type E) Although only in limited use in industrial applications, this type thermocouple has the highest emf output of any standardized metallic type. They may be used in oxidizing, inert or reducing atmospheres to 1600 deg F and at sub-zero temperatures they are not subject to corrosion. Indications are that the future will see more consideration given to this combination.
•
Platinum vs. Platinum-Rhodium Platinum vs. 90% Platinum + 10% Rhodium (ISA Type S) and Platinum vs. 87% Platinum + 13% Rhodium (ISA Type R) are used for temperatures up to approximately 3100 deg F, depending upon the atmosphere. Both types should always be provided with a high temperature ceramic protection tube.
Various other combinations of materials are used for thermocouples throughout industry but with far less frequency than the six basic types. Combinations of platinum and rhodium with various percentages of each have been regularly available for some years. Iridium vs. Iridium with 40, 50 or 60% Rhodium thermocouples have found acceptance in some high temperature applications up to 3600 deg F. Proper thermocouple selection is primarily determined by the temperature range in which its use is intended. Other factors, such as atmosphere, abrasion, vibration, and location will determine the type, size, and configuration of the complete assembly which includes protection tubes and mounting facilities.
LOCATION Proper location of the thermocouple is probably the most important factor in obtaining accurate temperature control. Thermocouples should be in a position to have a definite temperature relationship to the heat source and workload. A good 'rule of thumb' in locating thermocouples is to place them between the workload and heat source. The thermocouple should be located 1/3 the distance from the heat source and 2/3 the distance to the workload.
If a thermocouple is located too close to the heaters, a long warm-up time will result. The thermocouple will sense the heat before it reaches the workload, and this means rapid on/off action of the controller. In effect, the controller is controlling the heater and not the workload. In rare cases, voltage will be induced into the thermocouple circuit at high temperatures when located too near the heaters.
When a thermocouple is located too close to the workload, there is a substantial delay in sensing the proper control point and the result is overshooting the temperature. In most cases, it is better to be too close to the heaters than the workload as once a temperature point is passed, it becomes difficult to cool the workload unless a forced cooling system is used. Two thermocouples connected in parallel could be used, one
located near the heaters and the other near the workload. Both will balance these two factors and provide closer control.
Another consideration in location is when locating a thermocouple in a thermocouple well. If it is not bottomed correctly, located at the bottom of the well, the thermocouple will be reading the air temperature around it and not the temperature of the workload.
COMPENSATION The compensation method used by all millivoltmeter manufacturers is to attach a bimetallic spiral to the top hairspring of the coil suspension system. This spiral is selected according to the range of the instrument and will deflect the indicating pointer correspondingly with changes in ambient temperature. Once ambient is set mechanically, using a zero adjust screw, it is not necessary to change the setting during the operation of the instrument. In solid state instruments, the compensation is achieved electronically by placing a temperature sensor, such as a thermistor or RTD, at the cold junction to monitor its temperature. The signal from this sensor is used to compensate for variations in cold junction temperature. The automatic compensation for ambient temperature is sufficient in most industrial applications. However, in laboratory experiments or critical control situations, when maximum accuracy is desired, one of two cold junction compensation methods are used. One method is to place the cold junction in an agitated ice bath, shown below. The instrument will then be set at 32 deg F (0 deg C), which is the temperature of the ice bath.
In the other method, the cold junction is situated in a precisely controlled temperature above ambient, as shown below. In this case, ambient compensation is not necessary. The mechanical zero adjustment is set at the cold junction temperature being maintained. The normal temperature being maintained is 150 to 200 degrees F at the cold junction.
In normal applications, if the cold junction is located too close to the heat source, conduction and radiation heating will cause inaccurate readings. Errors will also occur when using copper wire or the wrong thermocouple lead wire. When copper wire is used, the cold junction in effect remains at the thermocouple connector block instead of the instrument. This will cause the instrument to read low in most cases unless the cold junction and instrument are known to have the same ambient temperature. There is one application where cold junction compensation is not a factor. When two thermocouples are connected in series opposing, as shown below, a millivoltage is produced which is the difference in millivolts between the temperature at both thermocouples. As the difference in degrees between the two thermocouples is being measured, cold junction compensation is not necessary.
Each millivolt measuring instrument is calibrated for both the type of thermocouple being used and the length and gauge of the lead wire. The thermocouple lead wire is in effect in series with the thermocouple wire and the meter movement. Using wrong thermocouple lead wire can be avoided by simply following the color-coding used by all manufacturers (Table 2, below). A solid state controller can be used with up to 100 ohms of external resistance without having to be recalibrated.
Table 2: Calibration symbols and color codes for thermocouple and extension wire Type
ISA Symbol
Positive Polarity-Color Conductor (+) Code
Negative (-)
Thermocouple
J -- Iron
+
White (Magnetic)
Constantan
-
Red Brown
Extension
JX -- Iron
+
White
Constantan
-
Red Black
Thermocouple
T -Copper
+
Blue
Constantan
-
Red Brown
Extension
TX -Copper
+
Blue
Constantan
-
Red
Thermocouple
E -Chromel
+
Tan
Constantan
-
Red Brown
Extension
EX -Chromel
+
Tan
Constantan
-
Red Brown
Thermocouple
K -chromel
+
Yellow
Alumel (Magnetic)
-
Re Red Yellow
Extension
KK -Chromel
+
Yellow
Alumel
-
Red Brown
Thermocouple
S -- PT 10% RH
+
--
Platinum
-
--
--
Thermocouple
R -- PT 13% RH
+
--
Platinum
-
--
--
Extension
SX --
+
Black
Alloy 11
-
Overall
Blue
Red Green
Copper When a millivoltmeter is calibrated, a series resistance (commonly called a calibrating spool) is used between the moving element coil of the instrument and the thermocouple tip. The resistance of the wire must be determined and used in the calibration of the instrument. If the resistance of the thermocouple wire and extension wire is higher than the instrument is calibrated for, the temperature readings will be low and if the resistance is lower, the temperature readings will be high. Where the thermocouple and extension wire are a significant portion of the circuit, then we must also consider the resistance change of the thermocouple wire at elevated temperatures. It may be necessary to calibrate instruments at the operating temperature. As an example: 5 feet of .020 dia. platinum vs. platinum 10% Rhodium thermocouple wire, would have a resistance of 2.3 ohms. At 2500 deg F, the resistance would be 2.3 x 3.5 ohms, or 8.5 ohms. A millivoltmeter with a sensitivity of 10 ohms per volt would have an error of approximately 4% at 2500 deg F. The effects of temperature on the thermocouple and thermocouple extension wire are shown in Table 3. To illustrate the effects of incorrect lead length calibration on the millivoltmeters, we have charted the errors that can result for various ranges and thermocouples by deviating in resistance from the calibrated lead length. Table 4 is based upon 10 ohms per millivolt sensitivity instrument. Instruments with less sensitivity would show greater errors. A meter with a 5 ohm per millivolt sensitivity would have errors twice as great.
Table 3: Thermocouple resistance change with temperature Multiplying factor for various temps; both wires same gauge 200 de deg F 400 de deg F 800 deg F 1600 de deg F 2500 de deg F Iron-Constantan
1.02
1.05
1.11
1.22
----
Chromel Alumel
1.05
1.14
1.30
1.62
2.01
Chromel-Constantan
1.13
1.33
1.7
2.5
----
Plat. 10% RH - Platinum
1.13
1.34
1.83
2.67
3.50
Plat. 13% RH - Platinum
1.13
1.33
1.80
2.60
3.40
Table 4: Deviation in Ohms from calibrated lead length
1 -- 0-2000 deg F C/A (4.02% at 20 Ohms) 2 -- 0-1200 deg F I/C, 0-1600 deg F C/A (4.97% at 20 Ohms) 3 -- 0-800 deg F I/C, 0-600 deg F C/A (7.65% at 20 Ohms) 4 -- 0-500 deg F CU/C, 0-2200 deg F PLT/PLT + 13% RH 0-2400 deg F PLT/PLT + 10% RH (14.32% at 20 Ohms) 5 -- 0-300 deg F I/C, o-350 deg F CU/C (22.15% at 20 Ohms)
THERMOCOUPLE CONNECTION There are two common errors in connection thermocouple circuits. One is to connect the extension lead wire completely reversed. In this case, you would receive a low reading because the reversal causes the emf generated at the connection of the thermocouple and extension lead wire to be subtracted from the emf generated by the thermocouple. A more obvious error is to completely reverse the thermocouple. The instrument in this case will read downscale with an increase in temperature.
Some control instruments feature 'thermocouple break protection' which means that in the event of an open or broken thermocouple, a small voltage is applied to the instrument which will cause it to read full scale and turn off the external circuit. Thus, in the event the thermocouple breaks because of a mechanical shock or vibration or if it is over-exposed to extremely high temperature and deterioration sets in, an unattended process will not overheat because of the loss of control. Another consideration in the thermocouple use is that the leads wires should never run in the same conduit with electrical lines. This may induce currents in the thermocouple wire, resulting in instrument errors and poor control. However, if this cannot be avoided, or if the induced currents are being picked up at the thermocouple itself, then one side of the thermocouple lead wire should be grounded through a 1.0 microfared paper capacitor at one of the thermocouple terminals in the instrument. In emergencies, a direct ground will sometimes work as well.
Occasionally, because of atmospheric conditions, corroding may occur on connections which cause a loss of the millivolt signal. Or, a poor connection between the lead wire and thermocouple could cause loss of signal. The gauge size of the wire used in thermocouples is again dependent upon the application. Usually, when longer life is required, for the higher temperature ranges, the larger size wires are chosen. When sensitivity is the prime concern, the smaller sizes should be used.
GLOSSARY OF TERMS CALIBRATE - General: to determine the indication or output of a measuring device with respect to that of a standard. CALIBRATE - Thermocouple: to determine the emf developed by a thermocouple with respect to temperature established by a standard. CALIBRATION POINT - General: a specific value, established by a standard, at which the indication or output of a measuring device is determined. CALIBRATION POINT - Thermocouple: a temperature, established by a standard, at which the emf developed by a thermocouple is determined. CELSIUS - The designation of the degree on the International Practical Temperature Scale. Also used for the name of the Scale, as "Celsius temperature scale." Formerly (prior to 1948) called "centigrade." CENTIGRADE - The designation of the degree on the International Temperature Scale prior to 1948. (See Celsius) COAXIAL THERMOCOUPLE ELEMENT - A thermocouple element consisting of a thermoelement in wire form, within a thermoelement in tube form with the two thermoelements insulated form each other and from the tube except at the measuring junction. CONNECTION HEAD - A housing enclosing a terminal block for an electrical temperature-sensing device and usually provided with threaded openings for attachment to a protecting tube and for attachment of conduit. ELECTROMOTIVE FORCE - (emf) The electrical potential difference which produces or tends to produce an electric current. EXTENSION WIRE - A pair of wires having such temperature-emf characteristics relative to the thermocouple with which the wires are intended to be used that, when
properly connected to the thermocouple, the reference junction is transferred to the other end of the wires. FAHRENHEIT - The designation of the degree and the temperature scale used commonly in public life and engineering circles in English-speaking countries. Related to the International Practical Temperature Scale by means of the equation: • •
Degrees Fahrenheit = (Degrees Celsius x 1.8) + 32 Degrees Celsius = (Degrees Fahrenheit - 32) / 1.8 FREEZING POINT - The fixed point between the solid and liquid phases of a material when approached from the liquid phase under a pressure of one standard atmosphere (101325 N/m squared). For a pure material, this is also the melting point. ICE POINT - The fixed point between ice and air-saturated water under a pressure of one standard atmosphere (101325 N/m squared). This temperature is 0 deg C on the International Practical Temperature Scale. KELVIN - The designation of the thermodynamic temperature scale and the degree on this scale. This kelvin scale was defined by the Tenth General Conference on Weights and Measures in 1954 by assigning the temperature of 273.16 degrees Kelvin to the triple point of water. Also the degree on the International Practical Kelvin Temperature Scale. MELTING POINT - The fixed point between the solid and liquid phases of a material when approached from the solid phase under a pressure of one standard atmosphere (101325 N/m squared). For a pure material, this is also the freezing point. PROTECTING TUBE - A tube designed to enclose a temperature-sensing device and protect it from the deleterious effects of the environment. It may provide for attachment to a connection head, but is not primarily designed for pressure-tight attachment to a vessel. RANGE - The region between the limits within which a quantity is measured. It is expressed by stating the lower and upper range-values. REFERENCE JUNCTION - That junction of a thermocouple which is at a known temperature. REFERENCE POINT - (liquid-in-glass thermometer) A temperature at which a thermometer is checked for changes in bulb volume.
REFRACTORY METAL THERMOCOUPLE - A thermocouple whose thermoelements have melting points above that of 60 percent platinum, 40 percent rhodium, 1935 deg C (3515 deg F) RESISTANCE, INSULATION - (sheathed thermocouple wire) The measured resistance between wires or between wires and sheath multiplied by the length of the wire expressed in megohms (or ohms) per foot (or meter) of length. (NOTE: The resistance varies inversely with the length.) SEEBECK COEFFICIENT - the rate of change of thermal emf with temperature at a given temperature. Normally expressed as emf per unit of temperature. synonymous with thermoelectric power. SEEBECK EMF - The net emf set up in a thermocouple under condition of zero current. It represents the algebraic sum of the Peltier and Thompson emf. Synonymous with thermal emf. SHEATHED THERMOCOUPLE - A thermocouple having its thermoelements, and sometimes its measuring junction, embedded in ceramic insulation compacted within a metal protecting tube. SHEATHED THERMOCOUPLE MATERIAL - One or more pairs of thermoelements (without measuring junction(s)) embedded in ceramic insulation compacted within a metal protecting tube. THERMOCOUPLE - Two dissimilar thermoelements so joined as to produce a thermal emf when the junctions are at different temperatures. THERMOCOUPLE ASSEMBLY - An assembly consisting of a thermocouple element and one or more associated parts such as terminal block, connection head, and protecting tube. THERMOCOUPLE ELEMENT - A pair of bare or insulated thermoelements joined at one end to form a measuring junction and intended for use as a thermocouple or as part of a thermocouple assembly. THERMOCOUPLE (TYPE E, B, J, K, R, S, OR T) - A thermocouple having an emf-temperature relationship corresponding to the appropriate letter-designated table in ASTM Standard E 230, Temperature Electromotive Force (EMF) Tables for Thermocouples, within the limits of error specified in that Standard. THERMOPILE - A number of thermocouples connected in series, arranged so that alternate junctions are at the reference temperature and at the measured temperature, to increase the output for a given temperature difference between reference and measuring junctions.
THERMOWELL - A closed end reentrant tube designed for the insertion of a temperature-sensing element, and provided with means for pressure-tight attachment to a vessel. WORKING STANDARD THERMOCOUPLE - A thermocouple that has had its temperature-emf relationship determined by reference to a secondary standard of temperature.
Typical 4-20 mA Loop
Notes:
1. Loop impedance impedance must be considered, considered, to prevent prevent overloadin overloading g the power power supply. supply. 2. If any one instrument instrument is disconnected, disconnected, the complete control loop loop will fail fail to operate operate 3. 20 AWG shielded shielded twisted pair cable cable is recommended recommended.. The twisted twisted pair pair configuration configuration tends to distribute any electrical noise in a common mode; it is on both wires and tends to cancel itself out. 4. Wiring needs needs to be run run away form form sources of of EMI, such such as 120V 120V or larger power power lines lines or equipment that generates EMI. 5. When using using a shiel shield, d, some some simple simple rules rules apply: apply: a. A shield should be grounded grounded on on one end only only by using using the green wire wire from the the power source and not the negative (-) lead of the sensor connector or the negative (-) of any of the power supplies. b. Think of the shield shield as a flexible conduit in the wiring scheme scheme and and remember remember that metal conduits are always at earth ground. Attach it to ground at one end only to prevent it from becoming a conductor and creating a ground hazard. c. The recommended recommended point of grounding grounding is at the instrument, instrument, since itit is difficult difficult finding a good ground near a D/P or next to an Electronic Sensor Control Box.
