I/EC
Instrumentation
Modern Trends in Resistance Thermometry Modern metallurgical techniques and products have increased service life of thermocouples by P. H. Stirling and Henry Ho, Canadian Industries Limited
M IODERN
PROCESS CONTROLS o f t e n
call for a combination of high precision, long term stability, fast response, linearity, and high sensitivity in temperature measuring elements. Neither the thermocouple nor the resistance thermometer can satisfy all these requirements. The thermocouple gained its popularity because it could be connected to galvanometers or moving coil millivoltmeters to give a direct reading of temperature. Development of the recording potentiometer and improvements of the stability of the early thermocouples gave them sufficient precision for most applications. For the last three decades it has held the preferred place in industrial temperature measurement. This picture is now changing to more complex control systems. Thermocouples
In comparison with resistance thermometers, the thermocouple has the advantage of the fastest response, possesses good long term stability, and has a precision of 0 . 1 % up to 1200° C. and about 0.5% from 1500° to 2300° C. It may be used at temperatures over 1000° C. higher than is practical for resistance thermometer. High temperature thermocouples are fairly linear at temperatures above 500° C. with the exception of molybdenum-tungsten. A disadvantage is that lower temperature thermocouples are less linear. In the region of 50° to 150° C. many show a marked curvature which is often inconvenient. Another inconvenience is the necessity of providing a stable reference junction for accurate measurements. Semiconductor thermocouples using various intermetallic compounds vs. graphite have given electromotive forces of several hundred times greater than normal thermocouples but these suffer from the disadvantage of great fragility and extreme sensitivity to contamination. Thev
are also difficult to prepare in a reproducible manner. Active development of high temperature materials such as carborundum semiconductors is under way. An intriguing possibility might be the fabrication of a robust and inert high temperature thermocouple from dissimilarly doped carborundum rods. The annoying effects of corrosion and deterioration of thermocouples have been minimized if not altogether eliminated, in some cases, by use of coaxial metallic protective sheathing and the refractory oxide powder insulation, Ceramo, first developed by the Thermoelectric Co. A recent wrinkle is the inclusion of two titanium scavenger wires in the insulating powder along with Chromel-Alumel thermocouple wires. The titanium wires allow the Chromel-Alumel thermocouple to be cycled in the 800° to 1050° C. range, under oxidizing conditions, using Inconel or stainless sheathing, without loss of calibration or deterioration of the thermocouple.
C a r e must b e exercised in the thermal design o f the zero heat flux p r o b e and the associated control circuitry t o a v o i d undue heating or cooling o f the p r i m a r y sensing element
Any residual oxygen from the insulation or oxygen diffusing through the sheathing combines preferentially with the titanium. The Ceramo construction is now also available in ultrahigh temperature materials. Tungsten-rhenium thermocouples with beryllia insulation and tantalum or molybdenum sheathing are available. Temperatures of 2200° C. can be measured in a hydrogen or an inert atmosphere or in vacuum. Introduction of noble metal thermocouples usable up to 800° C. in corrosive conditions, such as Pallador, palladium-palladium-gold (Johnson, Mathey, and Mallory), with an electromotive force close to that of Chromel-Alumel, and the extension of the useful thermocouple range from 1500° to 2300° C. with tungsten-rhenium and iridium— iridium-rhenium thermocouples are signs of a healthy development program in thermoelectricity. Care must be taken when using thermocouples to eliminate stray electromotive forces. When working with any of the sheathed thermocouples, it is important to flame anneal any sharp bends to remove installation strains; otherwise a slow shift in calibration will be found in service as the strains slowly anneal out. This procedure is more important in the smaller sizes of wires. With 0.004-inch thermocouple wires shifts of several degrees have been observed due to this cause on fairly low temperature service, 200-220° C , with iron-constantan thermocouples. When joining thermocouple extension wires, it is better to scrape them clean and twist together and subject them to mechanical pressure preferably under a spring washer. This gives results similar to the best welds but with greater reproducibility. Terminal blocks located in regions of high thermal gradients should be mounted on thick conducting blocks and covered to minimize temperature differences. V O L . 52, N O . 7
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INSTRUMENTATION Resistance Thermometers
The resistance thermometer in its early form was bulky, fragile, and slow in response. Many engineers still think of it in these terms. The ordinary metallic resistance ther mometer is roughly fifty to several hundred timesslower in response than the fastest industrial thermocouple. Small semiconductor resistance ele ments with relatively fast responses are now available which have time constants five to ten times larger than the best thermocouples. This deficiency is compensated by other advantages of the resistance ther mometer. It is capable of extreme precision, 0.01% up to 1100° C. It lias a sensitivity some several hundred times greater than a thermo couple. A platinum resistance thermometer can give 4 mv. per ° C. whereas only 10 μν. per ° C. or so is obtained from a platinum thermo couple. Resistances of copper and nickel alloys can be combined to give an accurate linear temperature characteristic and this can be impor tant where output is to be used for on-line computing. The high sen sitivity easily allows control to with in ± 0.05° C. and this can often be significant in the control of evapo rators and similar equipment. Perhaps the most important result of the large signal output is that it decreases sensitivity of the measure ment to electrical pickup and noise. This is particularly useful in solid state amplification and switching of temperature measurement signals. It is thus possible to switch larger output signals from resistance ther mometers electronically without prior amplification. This simplifies the design of suitable data-logging or on-line computing equipment. Semiconductor resistance ele ments are available in several forms, thermistors, doped germanium or silicon elements, or doped intermetallic compounds. These have all been used in recent applications. The thermistor, an oxide material often in the form of a glass-coated bead with platinum alloy leads, has a negative resistance coefficient which is roughly ten times that of a metal. It follows the semiconductor tem perature law [R2 = Riexp 6 ( 1 / Ά — l / 7 ' ι ) ] . Doped semiconductor ma terials can have either positive or negative temperature coefficients de 60 A
pending on type, and these are of the same order of magnitude as the thermistor. They are sensitive to contaminants and the leads are often fragile. Thermistors are generally used up to 300° C. and a new ther mistor material is available from Fenwall Electronic which is usable up to 800° C. This has a slightly lower temperature coefficient than ordinary thermistors. Rapid im provements in semiconductor manu facturing procedures over the last few years have led to the production of thermistor temperature probes which are interchangeable and which possess good stability. It is now possible to obtain thermistors com mercially which are very closely matched over short temperature spans ( ± 1 0 ° C.) and which do not differ greatly over large spans and these arc ideal for narrow span displays. The thermistor's nonlinearity is not excessive over such short spans and the 40 mv. per ° C. from a 4-volt equal arm bridge makes it attractive for this application. Long term stability of properly aged thermistors is much greater than commonly recognized and much unfavorable criticism has re sulted from misunderstanding of the need for such aging. Similarly the stability of many high temperature thermocouples and metallic resist ance thermometers has also been unfairly criticized. Many experi menters have not realized the ne cessity of aging them for a sufficient period, at a higher temperature than the operating temperature of the device in order to stabilize their electrical characteristics. High-resistance resistance ther mometers enable one to measure tem peratures of rotating equipment, such as kilns, through slip-rings with little loss of accuracy and arc greatly superior to thermocouples. Thermal Gradient Compensated Probes
When attempting to measure tem peratures to better than 0.25° C , the old platitude that "The ther mocouple only measures the tem perature of the thermocouple"' should not be forgotten. If wells or pockets are used, metallic contact should be ensured by spring loading. Even if a "bare" temperature probe is used the temperature can be considerably in
INDUSTRIAL AND ENGINEERING CHEMISTRY
error because of conduction clown the probe and this heat flux along the axis of the probe. The ideal condition of zero axial thermal flux cannot be realized with a single temperature measurement point and an auxiliary sensing ele ment is needed to monitor the temperature gradient. Addition of a subsidiary heating or cooling ele ment gives zero heat flux axially. The heat source or sink is controlled from the temperature differential between the two temperature ele ments and is automatically adjusted until the elements give identical readings, eliminating the effects of the external environment. This simple idea has not been fully exploited. In cases where the tem perature measurement is the critical control parameter for a process, the slight extra complications of a zero flux temperature probe will often pay off extremely rapidly. Temperature Displays
There is a trend toward control ling processes by means of cxpandedscalc indicating instruments with offset-zeros. There is a real psycho logical advantage in displaying the deviations in a magnified fashion to plant operators. Dramatic improve ments have been achieved by merely changing the display to an offset-zero narrow-band instrument and retain ing the same control equipment. This is more readily accomplished with a resistance thermometer than with a thermocouple because of the greater stability of a comparison resistance compared to the stability of a thermocouple reference junction. Stability of the reference junction is often the limiting factor but there are now available precision thermo stats, 0.1° C. or better, for this pur pose. This extra complication is avoided with resistance thermometers and as the cost of a servo-driven resistance bridge is not very dif ferent from the cost of a potentiometric recorder with an offset po tential source the economic factors are no longer as heavily weighed in favor of the thermcouple. More sophisticated quality control using either the cumulative errorsignal or the integral of the square of the error signal is now being used in some processes. A two-pen recorder is often used, with the derived error
signal displayed on one pen and the control point, which may be cascaded to another control signal, on the other. The larger signal levels of the resistance thermometer allow the signal to be manipulated by electronic means. Some complicated temperature control devices which combine thermocouple signals and resistance thermometer signals have been used to indicate temperature differences amid violent process fluctuations. Thermocouple signals are used to compute a statistical correction for the slower resistance thermometer indications and this is added to the resistance thermometer signal to give the control signal. An indicated temperature difference close to the true average can be obtained this way giving stable control. It is also possible to display the derived "true temperature" from the approach curve of a slow transducer. This involves setting up the inverse transfer function of the transducer in a dynamic analog form. This may be possible if an over-all derived heat balance control is used which involves an on-line analog computer. Similarly the derived curve can be computed from the inverse relationship by means of an on-line digital computer used for process control. There is a trend toward more complex control systems using online process control computers or derived displays for accurate quality control. Greater reliability promised by solid state switching devices with no moving mechanical parts has inspired many development programs in this field and the resistance thermometer is preferred by such system designers beause of its high output. Resistance thermometers will not displace thermocouples completely or vice versa. Both have their place but the emphasis is changing and the low cost of present day miniature platinum resistance bulbs of fast response usable up to 1100° C. will speed their wider application.
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Engineering and product excellence is a tradition at Weksler, and the proof is in the reliable performance of over a million Weksler Instruments in every part of the world. The Weksler Specification Bulletin covers a complete line of instruments . . . industrial thermometers, dial thermometers, recording thermometers, recording hygrometers, bi-metal thermometers, laboratory thermometers and hydrometers. Write for your copy. vWËKSLËfJ J INSTRUMENTS
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly.
Dial Thermometers in 3V2", 4'/a", 6", 8V4" dial sizes.
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Indicating and Recording Instruments for Temperature. Pressure and Humidity Circle No. 72 on Readers' Service Card VOL. 52, NO. 7
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