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TEMPERATURE MEASUREMENT

Temperature measurement technologies: how temperature sensors work and which one to choose

Temperature is the most commonly measured quantity in a process plant, and the right instrument depends on what you need: an accurate value, a very high temperature, a robust sensor for a vibrating machine, a signal that travels far, or simply a contact that trips at a limit. This page covers the four technologies we supply — RTDs, thermocouples, temperature transmitters and temperature switches — how each one works, where it fits and where it does not, and how to choose.

The four temperature measurement technologies

For each one: the principle in a sketch, where it fits and where it does not, and how many instruments of that technology are in our catalogue.

Pt100 resistance thermometers (RTDs)

A thin platinum element, usually Pt100 (100 Ω at 0 °C), whose electrical resistance rises almost in a straight line with temperature. The instrument passes a small current through it and turns the resistance into temperature.

Where it fits
Accurate, stable measurement from about −200 to +600 °C: process liquids, food and pharmaceutical lines, heat exchangers, energy metering.
Where it does not
Temperatures above about 600 °C, and points with strong vibration, where a wire-wound element can fail.
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Thermocouples

Two wires of different metals are joined at the tip. When the tip is hotter than the other end, the pair produces a small voltage — a few millivolts — that depends on the temperature difference; the instrument also measures the cold end and works out the temperature at the tip.

Where it fits
Very high temperatures (furnaces, kilns, flue gas, up to about 1,700 °C depending on the type), vibration, and fast response with a thin sheath.
Where it does not
Applications that need high accuracy or long-term stability, and long direct cable runs, where the millivolt signal needs a matching extension cable.
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Temperature transmitters

An electronic module, in the sensor head or on a DIN rail, that reads the RTD or thermocouple and sends a 4–20 mA signal — often with HART or a fieldbus — to the control system, over the same two wires that power it.

Where it fits
Long cable runs, electrically noisy plants, hazardous areas (intrinsically safe versions) and remote diagnostics.
Where it does not
It does not measure on its own: it always needs a sensor, and its accuracy is that of the transmitter plus the sensor.
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Temperature switches

A switch that opens or closes an electrical contact when the temperature passes a set point. Mechanical versions use a filled bulb and capillary or a bimetal; electronic versions use a sensor, usually an RTD, and a relay output.

Where it fits
Alarms, interlocks and shutdown on overtemperature — independently of the control system, with no programming.
Where it does not
Wherever you need to see or record the temperature itself: a switch only says above or below the set point.
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At a glance

The typical properties of the four technologies in one table, and their typical temperature range in one chart.

Temperature measurement technologies compared: what they give, typical range, typical accuracy, output signal, response and main limitation
TechnologyGivesTypical rangeTypical accuracyOutput signalResponseMain limitation
RTDs (Pt100)Continuous value−200…+600 °C±0.15 °C (class A) to ±0.3 °C (class B) at 0 °CResistance (Ω), 2-, 3- or 4-wireModerateMore sensitive to vibration; rarely used above 600 °C
ThermocouplesContinuous value−200…+1700 °C±1.5 °C (class 1) to ±2.5 °C (class 2), type KMillivolts; extension cable to the transmitterFastLower accuracy; drift at high temperatures
Temperature transmittersContinuous value−200…+1700 °C *±0.1% of span (transmitter), plus the sensor4–20 mA, HART, Profibus PAAs the sensorNeeds loop power; one more component in the loop
Temperature switchesOn/off at a set point−50…+400 °CRepeatability, typically ±1% of rangeContact (SPDT/DPDT)Slow to moderate (mechanical)Alarm or trip only — no continuous value

Typical ranges, not a model commitment. RTD accuracy to IEC 60751 at 0 °C; thermocouple accuracy to IEC 60584, by type and class; for transmitters (*), the range is that of the sensor connected to them. We confirm the figures of each instrument for your application.

Typical temperature range by technology

Typical temperature range of each technology, in °C. For transmitters, the range of the sensor connected to them.
  • RTDs (Pt100)−200…+600 °C
  • Thermocouples−200…+1700 °C
  • Temperature transmitters−200…+1700 °C *
  • Temperature switches−50…+400 °C

Application guide: which technology for which job

Each technology exists to solve a particular problem. Start from yours — the temperature, the environment, what the signal is for — and see which technology we would suggest, which one to avoid, and why.

Application guide for temperature measurement: for each application, the technology we suggest and why, and the one to avoid and why
ApplicationWe suggestWhyAvoidWhy not
Very high temperatures: furnaces, kilns, flue gas, above about 600 °CDepending on the type, a thermocouple reaches about 1,200 °C (types K and N), 1,600 °C (types R and S) and 1,700 °C (type B).A platinum element is rarely used above 600 °C: IEC 60751 stops at 850 °C, and near the top of its range the element drifts.
High accuracy and long-term stability: food, pharmaceuticals, heat exchangers, energy meteringA class A Pt100 is within ±0.15 °C at 0 °C and stays stable for years; a transmitter carries the reading to the control system without losing accuracy.Even a class 1 type K thermocouple allows ±1.5 °C, and thermocouples drift with time at high temperatures.
Strong vibration or shock: engines, compressors, rotating machineryA welded junction inside a mineral-insulated sheath has nothing fragile to break.A wire-wound platinum element is more sensitive to vibration; if you need RTD accuracy, ask for a vibration-resistant design.
Only an alarm or a trip at a set temperature: overtemperature protection, interlocksThe contact switches directly, with no PLC and no programming, and works independently of the control system; versions with SIL 2 certification exist, such as the 12 Series. — If you also need to see or record the temperature, use a sensor with a transmitter instead — or both.
Signal over a long cable to a PLC or DCS, with diagnosticsA 4–20 mA signal is not affected by the resistance of the cable and resists electrical noise; with HART, as on the D72, the control room can also read diagnostics.Wired directly over a long distance, the millivolt signal of a thermocouple picks up noise and the resistance of 2-wire RTD leads adds to the reading.
Small spaces, small pipes and surfaces, or a fast responseA thermocouple can be made with a very thin sheath, and a thin sheath responds quickly.A bulb-and-capillary switch needs room for its bulb, fully immersed in the medium.
Hazardous areas (ATEX)Intrinsically safe transmitters are available — the TT-2000, for example, with ATEX and IECEx Ex ia. — Any technology can be used with the right certification and installation; we check it for your zone.
Hygienic processes: food, beverages, pharmaceuticalsAccurate, stable and available with hygienic connections — the Klay Pt100, TT-2000 and TT-4000 with EHEDG and 3-A connections. — What matters most is the hygienic process connection and the materials, not only the technology.

Thermowells: protecting the sensor

A thermowell is a closed tube, machined from bar stock or made from pipe, that is fitted into the process and receives the sensor. It keeps the pressure, the flow, corrosion and abrasion away from the sensor, and lets you remove the sensor for inspection or replacement without stopping the process or opening the line.

The price is a slower response: the heat has to pass through the wall of the thermowell before it reaches the sensor. A thermowell in fast flow must also be checked against vibration from the vortices that form behind it — the usual method is the calculation of ASME PTC 19.3, which Delta Mobrey offers with its thermowells.

  • Immersion: the tip should reach well into the flow — a common rule of thumb is about a third of the pipe diameter, and at least ten times the diameter of the sensor.
  • Material: stainless steel for most applications, special alloys or a PTFE cover for corrosive media.
  • Connection: flanged, threaded or welded, to suit the line and the pressure rating.
  • The sensor should touch the bottom of the bore, so that the heat reaches it quickly.

How to choose a temperature sensor

The right technology follows from the application. These are the criteria we look at:

  1. Temperature range

    Up to about 600 °C, an RTD is usually the first choice; above that, a thermocouple of the right type.

  2. Accuracy and stability

    Pt100 classes A and B are defined by IEC 60751, thermocouple classes by IEC 60584. For control and quality, the stability over years matters as much as the accuracy on day one.

  3. Measurement or switch

    Do you need a value to see, record and control with — or only a contact at a limit? A switch is the simplest answer to the second.

  4. Signal and distance

    For long cables and a control system, a transmitter with 4–20 mA, HART or a fieldbus. For a short run, the sensor can be wired directly.

  5. Response time

    A thin sheath responds faster; a thermowell protects the sensor but slows the response.

  6. Environment

    Vibration, hazardous areas, hygiene requirements and corrosive media determine the construction, the certification and the materials.

Temperature measurement technologies: frequently asked questions

Need help choosing a temperature sensor?

Tell us about the medium and the operating conditions, and our engineers will recommend the right technology and the specific instrument.