Sensors, Actuators

Temperature measurement with RTDs, thermocouples

Process sensing: The most common process sensor measurement is temperature and resistance temperature detectors (RTDs), and thermocouples are the most widely used sensors for industrial temperature measurements. See 11 summary tips for temperature sensor selection.
By Gary Prentiss December 11, 2018
Courtesy: Moore Industries
In addition to various RTD and thermocouple sensors, different sensor construction types also are available: standard solid sheathed elements (top and middle) or a flexible trim-to-length sensor with a shorter sensor capsule (below), designed for quicker response times and better resiliency to vibration, such as the Worm from Moore Industries. Courtesy: Moore Industries

In addition to various RTD and thermocouple sensors, different sensor construction types also are available: standard solid sheathed elements (top and middle) or a flexible trim-to-length sensor with a shorter sensor capsule (below), designed for quicker response times and better resiliency to vibration, such as the Worm from Moore Industries. Courtesy: Moore Industries

Temperature measurements comprise the largest segment of all process measurements and their accuracy and reliability can often have a significant impact on the efficient operation and safety of a facility. Selecting the most appropriate sensor type can improve the accuracy, repeatability, and stability of temperature measurements and can decrease operating and maintenance costs.

Resistance temperature detectors (RTDs) and thermocouples handle 90% or more of the temperature monitoring within industrial facilities, which means specific details on each detector can help with informed decisions regarding selecting the best sensor for various applications.

RTDs at-a-glance

Temperature ranges: RTD are recommended for measurements from -200 to 850°C (-328 to 1,562°F). When purchasing a new sensor, inform the supplier of the operating range for the sensor to encourage use of the best materials and manufacturing techniques for that operating range.

Operation: RTDs operate on the principle that the electrical resistance of their metal elements increases as temperature increases.

Construction: Common resistor materials are Platinum (Pt), Nickel (Ni) and Copper (Cu). Because platinum is more stable, more linear and covers wider temperature ranges, it has become the industry standard today. While nickel and copper may be found in existing builds, most new installations will use platinum throughout.

High purity platinum is often used to manufacture an RTD sensing element in either a wire-wound design (platinum wire is wound around a substrate spool) or thin film design (pure platinum is deposited onto a ceramic substrate). Modern RTDs can be used at higher temperatures because the substrate materials used today are stable at elevated temperatures.

Suggestion: use thin-film sensors from -40 to 850°C (-40 to 1562°F) and wire-wound sensors when dropping to temperatures near -200°C (-328°F).

RTDs with 2, 3 or 4 wires: RTDs can be built with 2-wire, 3-wire and 4-wire construction.

Suggestion: use RTDs instead of thermocouples (T/C) whenever possible for superior accuracy, repeatability, and stability.

An RTD only functions properly when the element is insulated/isolated from the protective sheath surrounding it. Typical insulating materials are Magnesium oxide (MgO) or Alumina oxide (Al2O3). Should the insulation break down due to moisture and contamination, the RTD must be replaced. As an RTD must be insulated, using a measuring circuit that is not isolated could provide cost savings.

In instances when a close coupled temperature transmitter is not used with an RTD, the RTD is connected to the measuring circuit by copper wire.

Things to keep in mind when preparing to select an RTD include:

  • The sensor’s name indicates its resistance at 0°C (32°F). Example: 100Ω Pt RTD measures 100Ω at 0°C; 500Ω Pt RTD measures 500Ω at 0°C, etc.
  • Modern measuring circuits use a constant current source to generate excitation current.
  • High impedance voltage measurements factor in to RTD performance. (High impedance means there is no current flow through the voltmeter and its leads.)
  • Resistance is calculated using Ohms Law: V = IR or R = V/I

RTD sensor accuracy

Table 1: RTD accuracy values

It is best to use RTDs over T/Cs when possible. The best RTDs are built to the IEC 60751 standard which calls for accuracy values as shown in table 1 below.

Premium/special-grade RTD sensors: When RTDs are aged by the manufacturer, it minimizes drift once they get into the field. RTDs that are temperature cycled for 1000 hours at 0° and 600°C and will maintain higher accuracy for 5+ years. Typically, only Class A sensors are thermally aged.

Premium-grade T/C wire helps with thermocouple measurements; upgrading to Class A RTD sensors also helps by cutting uncertainty in half.

Thermocouples at-a-glance

Thermocouple (T/C) technology is based on the Seebeck effect wherein two dissimilar metals fused together at both ends will generate an electric current when one junction is at a different temperature than the other.

Temperature ranges: There are various combinations of dissimilar metals that are used to construct T/Cs. The finished products are referred to as the T/C type. For each type, mV vs temperature tables exist and are included in this reference manual (all mV vs temperature tables are created with the T/C cold junction at 0°C (32°F)).

Operation: A T/C sensor has two junctions. The measurement junction (sometimes called the hot junction) is where the two metals connect. The reference junction (also called the cold junction) is the open circuit end that connects to the measuring circuit.

When a temperature difference exists between the hot and cold junctions, an mV signal is generated that is proportional to the temperature difference. The mV value increases with the rising temperature. The relationship between the mV and temperature is non-linear.

In a real world T/C measurement, the measuring circuit will likely be any temperature but 0°C (32°F). The measuring circuit must measure the temperature of the cold junction and reference that temperature back to 0°C (32°F). This electrical compensation is called cold junction compensation (or reference junction compensation). Most T/C measuring circuits perform this operation.

Construction: T/C junctions can be built with the hot junction grounded to the external sheath or ungrounded (insulated from the sheath). A grounded T/C will respond more quickly but the T/C is then in contact with the process voltage. For this reason it is important that the measuring circuit be isolated to block the formation of a ground loop and resulting measurement error.

Within a temperature assembly, the T/C is usually embedded in magnesium oxide (MgO) and a metal sheath. Then it’s inserted into a thermowell or protection tube. This helps protect the sensor from environmental contamination. Even an ungrounded T/C will eventually go to ground when the MgO becomes contaminated with moisture and salts.

Suggestion: Measure the T/C with an isolated measuring circuit.

Thermocouple sensor accuracy: It is best to use thermocouple sensors that are built to the ASTM E230 standard, which governs thermocouple accuracy for types E, J, K, and T.

Thermocouple reference tables are provided in ASTM E230/E230M-12 Standard Specification and Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples.

Premium/special grade thermocouple wire

Thermocouples can be constructed with premium or special grade wire which reduced uncertainty by half. The premium/special designation essentially indicates that this wire has a higher purity alloy mix.

Table 2: Premium or special grade wire reduces thermocouple uncertainty

Suggestion: If an application requires T/Cs instead of RTDs, use a premium grade T/C; the cost difference is negligible and premium wire provides greater stability.

Wire contamination is a consistent problem with thermocouples. Accuracy chart values assume wire has not been contaminated by the chemicals in the process or environment. As contamination occurs, error generally increases to a point necessitating sensor replacement.

Sensor trimming for high accuracy

After considering the sensing element, consider the application. If it demands the best accuracy possible, a temperature measurement system with bath calibration. A Class A RTD sensor is calibrated in a bath to calibrate it to the transmitter or remote input/output (I/O) measuring device. This process eliminates the final “as-built” offset error that exists in every sensor. The sensor should include a traceable calibration report from the National Institute of Standards and Technology (NIST) that indicates the combined sensor and temperature transmitter uncertainty is typically better than ±0.01°F.

Temperature sensor selection: 11 tips

To optimize measurement performance and minimize long-term maintenance expenses, use the following tips as a practical guide for temperature sensor selection.

  1. Use an RTD when measuring in ranges between -40° and 850°C (-40° and 1,562°F)
  2. For temperatures as low as -200°C (-328°F), use a wire wound RTD
  3. Best practice is to use 4-wire and Class A RTDs
  4. Make sure the sensors are temperature cycled and “aged” for long term stability
  5. When applying RTDs below 0° and above 600°C, you want to know the process conditions in order to optimize the build: Temperature range, cycling, pressure, flow, media, vibration and surrounding environmental conditions (chemicals/atmosphere)
  6. When highest accuracy is needed, use sensor trimming
  7. If using 3-wire RTDs with long wire runs, and you cannot convert over to 4-wire RTDs, replace the 3-wire RTDs with 1000Ω Platinum RTDs
  8. If monitoring temperatures above 850°C (1562°F), use thermocouples
  9. If using thermocouples, use premium grade thermocouples and extension wire
  10. If using long thermocouple extension wire, be sure it is noise protected
  11. Replace contaminated TC extension wire with remote I/O.

Gary Prentice is vice president, sales, Moore Industries. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media, mhoske@cfemedia.com.

KEYWORDS: Temperature sensor selection, RTD, thermocouple

  • Measure temperature with RTDs rather than T/Cs.
  • Premium wire T/Cs should be used, if T/Cs are needed.
  • 11 tips help with temperature sensor selection.

CONSIDER THIS

Is it better to spend nominally more for a better sensor or replace them more often?

ONLINE extra

More details on temperature sensors and measurement accuracy is available in the free Moore Industries Temperature Reference Guide.

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Gary Prentiss
Author Bio: Vice president, sales, Moore Industries