Selecting the right temperature sensors for each application requires a close examination of the requirements.



Imagine you are an instrumentation engineer looking at the prints for a new plant, and you see the many locations marked out for temperature transducers. You wonder, what exactly has been specified to fill each space? Thermocouples, because of the lower cost and familiarity, or RTDs, for the high accuracy and linearity?

Temperature transducer selection affects many aspects of the design and installation of the equipment in the plant such as:
  • The type of wire that needs to be run.
  • The type of instrument that will be in the control room on the other end of that wire.
  • Whether there will be local junction boxes with terminal strips or transmitters. And, if so, what type of transmitter is required.
  • Whether any special piping considerations need to be made to install or protect the sensor or to provide the required response time.
These are just some of the details, and the actual sensor selection and design has not even been discussed yet. Looking at the big picture can help narrow down the choices in a logical way. Look first at survival of the sensor and then the finer points such as any performance requirements.

Temperature Range

The International Temperature Scale (ITS-90) defines temperatures between 13.8003 K (-438°F [-259°C]) and 1234.93 K (1,763°F [962°C]) by use of platinum resistance thermometers (PRTs) calibrated at specified sets of fixed points. While this is fine in a laboratory, you are not likely to find an industrial-grade RTD that will cover this entire range adequately. Note that the standard says PRTs, which is the plural. One PRT will not cover the entire range adequately in a laboratory situation either, which means several must be used.

ASTM defines the platinum RTD for use over the range -328 to 1,202°F (-200 to 650°C). This is a good guideline to follow even though IEC extends the upper limit to 1,562°F (850°C). Industrial-grade platinum RTDs can be manufactured for use to 1,562°F, but it is not an easy task. Also, you may find that standard warranties are not valid for this type of service. Fortunately, it is estimated that well over 90 percent of all contact temperature measurements made in industry are below 1,202°F (650°C).

Thermocouples also have temperature limitations based on which type is specified and on what gauge of wire it is constructed. A typical 0.25" OD, single, mineral-insulated thermocouple has 16 AWG wire within it. ASTM E-608 recommends the following temperature limits for such base-metal thermocouples:
  • Type T, which can be used to 698°F (370°C).
  • Type J, which can be used to 1,328°F (720°C).
  • Type E, which can be used to 1,508°F (820°C).
  • Type K, which can be used to 2,102°F (1,150°C).
Keep in mind that although ASTM states these thermocouples can be used at these temperatures, it does not mean that they will last forever. The higher the application temperature, the sooner they will begin to drift and the shorter the life. ASTM does not publish any guidelines on drift or life expectancy due to the many variables that may affect thermocouple performance.

Above these temperatures, you must consider precious metal thermocouples: platinum- rhodium alloys or tungsten-rhenium alloys. These tend to get expensive, but when measuring temperature above about 1,832°F (1,000°C), the life expectancy is much longer, and they are not as prone to drift.

ASTM E-230 lists suggested upper temperature limits for platinum-rhodium Types R and S thermocouples as 2,700°F (1,480°C), and for Type B as 3,100°F (1,700°C). These values are for protected 24 AWG construction. Tungsten-rhenium thermocouples have been used for measurement of temperatures as high as 4,172°F (2,300°C).

It is difficult to quantify repeatability for both RTDs and thermocouples due to the tremendous effect that the application has on the results.

Accuracy

After temperature range, which essentially helps with the decision about whether or not a particular sensor can be considered for an application, the accuracy of various types should be evaluated.

A standard ASTM Grade B RTD will provide true accuracy (as compared to the published R vs. T tables) of ±0.45°F at 32°F (±0.25°C at 0°C). Due to variations in temperature coefficient, this same Grade B thermometer may only provide temperature readings within 5.4°F at 1,202°F (3.0°C at 650°C). More accurate Grade A RTDs are available at an additional cost but should perform within 2.23°F at 1,202°F (1.24°C at 650°C). It is important to note however, that when ASTM Grade A or IEC Class A accuracy is desired, the temperature range of use must, by definition, be specified. Some manufacturers might generically state Class A accuracy, misleading some to believe that those sensors perform within Class A accuracy across their entire temperature range. Without calibration and verification at a second point representative of the application temperature, this is rarely the case.

By comparison, the most common base-metal thermocouples, Types J and K, will provide accuracy of 3.96°F (2.2°C) or 0.75 percent (whichever is greater) when supplied in standard accuracy. That could be as much as ±8.775°F at 1,202°F (±4.875°C at 650°C) -- considerably larger than even the Grade B RTD. This is true across the range, up to the RTDs’ upper limit of 1,202°F (650°C), for Grade B RTDs vs. standard tolerance thermocouples, as well as Grade A RTDs vs. thermocouples selected for special tolerance limits. Simply stated, if accuracy is important and all other conditions permit it, select an RTD over a thermocouple.

Repeatability and Stability

Repeatability and stability are not easy to quantify for RTDs or thermocouples due to the tremendous effect that the application has on the results. For instance, ASTM E-230 Part 6, Table 1, Note 3 states: “Caution: Users should be aware that certain characteristics of thermocouple materials, including the EMF vs. temperature relationship, may change with time in use; consequently, test results and performance obtained at time of manufacture may not necessarily apply throughout an extended period of use. Tolerances given in this table apply only to new wire as delivered to the user and do not allow for changes in characteristics with use. The magnitude of such changes will depend on such factors as wire size, temperature, time of exposure and environment.”

For platinum RTDs, ASTM-1137 Part 9 requires the stability of the unit to remain within the specific accuracy grade (i.e., Grade B) for a four-week test. IEC 60751 has similar test criteria and also requires the unit to remain in tolerance after being exposed to its maximum temperature limit for a period of 1,000 hours. Thermocouples typically are not expected to perform within stability/repeatability limits as strict as these. RTDs, therefore, offer a certain degree of “set it and forget it” that is not possible with thermocouples. All temperature sensors should be calibrated periodically to verify their continuing performance, but RTDs generally lend themselves to longer calibration intervals and less frequent replacement due to drift.

Vibration

This is one area where thermocouples may have a slight advantage. Due to the sheer size of the wires used in thermocouple construction, they tend to stand up to high vibration better than most RTDs.

Remember that 0.25" OD mineral-insulated thermocouple previously mentioned. It had 16 AWG conductors within it, and these conductors are used to form the thermocouple junction. By contrast, a wire-wound RTD element may have lead wires of approximately 26 to 30 AWG, which are attached to a very fine platinum wire used to wind the sensor itself. The wire in these windings is generally in the range of 15 to 35 micron (that is about 0.00059 to 0.00138") in diameter and is consequently very fragile. High vibration has been known to cause problems in some wire-wound resistance elements that are not fully supported. Failures may be in the form of open circuits, noisy signals or intermittent high readings.

Fully supported wire-wound and thin-film RTDs tend to fare somewhat better than the semi-supported types. But keep in mind that the element leads are still only 26 to 30 AWG and therefore are relatively susceptible to breakage induced by continued high vibration. Also, special care must be exerted by the RTD manufacturer to properly package these elements for the rugged environment in which they will be used.

Thermocouples with ungrounded junctions offer heavier wires, which are fully supported, and generally tend to better survive the rigors of high vibration.

RTDs generally lend themselves to longer calibration levels and less frequent replacement due to drift. However, all temperature sensors should be calibrated periodically to verify their continuing performance.

Response Time

This is another area where thermocouples excel over RTDs, and it is a simple matter of physics to understand why.

Contact temperature sensors do not indicate the temperature of the area around them; instead, they indicate their own temperature along their own sensitive area. In order for any contact temperature sensor to indicate the temperature of the material in which it is in contact, the sensor first must come to thermal equilibrium with that environment. (This article will not discuss the theoretical aspect that the two never actually attain the same temperature; instead, for purposes of the discussion, accept that after some time, the two are approximately at thermal equilibrium.)

The most basic thermocouple is merely a junction of the two dissimilar metal wires. These could be simply a beaded or butt-welded junction, which turn out to be nearly the same diameter as the thermocouple wire itself. To indicate the surrounding temperature, the junction must be at that temperature. That junction might only be 0.010" in diameter (for a 30 AWG wire thermocouple), or smaller if finer wires are used.

RTDs are constructed as either a length of fine platinum wire wound around or within the former, or a layer of platinum deposited upon a substrate. In all cases, there is an area of platinum (the sensitive portion of the RTD) in contact with this inert, insulating former, and both are physically larger than a weld junction, generally speaking. Both the platinum and the insulator have thermal mass that must come to equilibrium with the surroundings before the sensor can give an accurate reading. Because there is generally more thermal mass involved than with the thermocouple junction, the thermocouple will respond faster when put in a similar environment.

That statement is true only when seeking extremely fast response times for each type of sensors, and working with bare resistance elements and exposed thermocouple junctions. If both sensors are encapsulated within metal sheaths and the thermocouple junction is isolated from the sheath (as an RTD circuit always is), then response times will be quite similar. For industrial installations where thermowells are used, there is virtually no difference in response time of either type.

Sensitivity

With regard to sensitivity, the RTDs are, very simply, superior. As an example, consider a platinum 100 Ω RTD with a 0.00385 temperature coefficient (ASTM and IEC Standard). From 32 to 212°F (0 to 100°C), its resistance changes from 100.00 Ω to 138.50 Ω (or 138.51 Ω), a difference of 38.5 Ω. If the sample application had been using 1 mA sensing current, which quite typically is used to avoid self-heating effects, Ohm’s Law (V=iR) says that you would see a difference of 38.5 mV over this range. By comparison, a Type E thermocouple, which provides the highest sensitivity of all recognized thermocouples, will show only a change of 6.317 mV. This is only about one-sixth of the sensitivity of the RTD. If your environment might provide electrical interference, the thermocouple will be at least six times more susceptible to it. And this is when using a Type E; other types have sensitivity as low as 0.33 microvolt/°C.

If a higher sensitivity is needed, a platinum 500 Ω RTD can provide five times the sensitivity of the platinum 100 Ω, or a platinum 1,000 Ω, to give you 385 Ω over that 180°F (100°C) range. These sensors just need to be paired with instruments that will accept a platinum 500 Ω or 1,000 Ω input.  

Life Expectancy

All of the factors previously mention about application parameters in the areas of temperature range and stability also apply to life expectancy: It all depends on the details. However, a few generalizations can be made. It is widely accepted that thermocouples are in a constant state of degradation and need to be checked and replaced periodically. By contrast, platinum RTDs may last indefinitely if the environment does not deteriorate them.

Each application must be judged on its own as to which technology is better suited. However, if an application requires a temperature transducer and you are not quite sure how to fill the space, run down the requirements and apply the concepts described. The answer may surprise you: Perhaps a thermocouple will meet the requirements, or maybe an RTD will be better suited for the situation.

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