Temperature sensor probes (Pt100 RTD assemblies)

01

IEC 60751 Class AA, A or B, and why 4-wire connection matters

Endress+Hauser's technical information for its TMT162R RTD assembly lists IEC 60751 tolerances for wire-wound Pt100 elements: Class AA (formerly 1/3 Class B) ±(0.10 + 0.0017|t|) °C from 0 to 250 °C, Class A ±(0.15 + 0.002|t|) °C and Class B ±(0.3 + 0.005|t|) °C from −200 to +600 °C, where |t| is the absolute temperature value in °C. It notes that 4-wire connection makes the measurement independent of cable properties and is presumed for Class A and 1/3 DIN B accuracy. [1]

Editorial calculation: at 100 °C those classes allow ±0.27 °C (AA), ±0.35 °C (A) and ±0.8 °C (B); at 300 °C, Class A allows ±0.75 °C and Class B ±1.8 °C. The figures are for the element; installation and the transmitter add their own errors.

The TST90 datasheet shows the effect of 2-wire connection: lead resistance to compensate of 0.06 Ω per metre of each conductor for the straight-tip insert (0.12 Ω/m for the reduced tip). Editorial calculation: 10 m of 2-wire connection on the straight tip adds about 2 × 10 × 0.06 = 1.2 Ω in series with the sensor unless compensated, which is why 3- or 4-wire connection is preferred for accuracy. [2]

Give the supplier the measuring range, required accuracy (tolerance class) at your operating temperature, sensor element and number of elements, wiring (2, 3 or 4 wire), response time needed, immersion and insertion lengths, sheath diameter and material, thermowell and process connection, process pressure, flow and vibration, head or cable, transmitter output, ingress protection and hazardous-area approvals.

02

Check response time and immersion as installed

Endress+Hauser quotes response times in water flowing at 0.4 m/s per IEC 60751. Its TST40N assembly reaches 90% of a step (t90) in 8 s with a 6 mm sheath and 6 s with 3 mm. Its TST90 assembly, with a 6 mm insert inside a thermowell, needs t90 = 72 s with a straight-tip thermowell but 20.5 s with a reduced tip (t50 = 24 s and 7.5 s). The TMT162R assembly without thermowell and transmitter gives t90 = 8 s for a 10 K step. [3] [2] [1]

The TST90 and TST40N datasheets state that for correct measurement the immersion length must be 20 times the thermowell or insert diameter, to avoid errors from heat conducted away through the process connection; shorter lengths need external insulation of the connection, neck and head. The TMT162R information adds that in pipes with a small cross-section the sensor tip should reach or extend slightly past the pipe centreline, and that thermowell load limits depend on pressure, flow velocity, density, temperature and immersion. [2] [3] [1]

Editorial calculation: a 9 mm thermowell needs about 20 × 9 = 180 mm immersion by that rule, and a 6 mm insert used without a thermowell about 120 mm; in a small pipe that may require an elbow or tilted installation.

03

Limit self-heating from the measuring current

The current that reads an RTD also heats it. A 2025 study by Otomański, Pawłowski and Szlachta (Polish universities of technology, Sensors) measured the thermal resistance between sensor and surroundings as the slope of indicated temperature against heating power, in still air. They recommend choosing the measuring current so that self-heating does not exceed 25% of the sensor's declared tolerance, and they note that the external thermal resistance depends on heat transfer between the sheath and its medium, so it should be found in situ. [4]

Editorial calculation: Class A allows ±0.15 °C at 0 °C, so the 25% rule leaves 0.0375 °C for self-heating; with the steel-cased Pt100's 0.061 K/mW in still air, that permits about 0.61 mW, or about 2.5 mA through 100 Ω. [1] [4] Editorial note: the same sensor in flowing liquid has a lower external thermal resistance than in still air, so ask the supplier for the self-heating coefficient in your medium and the transmitter's excitation current.

04

Pt100 process assemblies against NTC and wireless probes

The Endress+Hauser data on this page describe Pt100 assemblies to IEC 60751 tolerance classes [1]; the linked makers also offer NTC thermistor probes and Zigbee wireless sensors with external probes, which are specified by thermistor curve or by the radio device rather than by an IEC 60751 class. Ask which element is inside before comparing accuracy.

For each probe ask for the sensing element and its tolerance class or curve, the response time and its test conditions, insulation resistance, and the sheath material and diameter.

05

Element, mineral-insulated insert, sheath, thermowell and head

06

Range, class, t90, immersion and insulation values for the order

Response time in this table is only comparable when the test medium is stated; the Endress+Hauser figures are for water flowing at 0.4 m/s. [2] [3]

07

Send the process, accuracy and installation data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Process: [range; medium; pressure; flow; vibration]
Accuracy: [class at operating temperature; calibration]
Sensor: [Pt100 or other; number; wiring]
Response: [t90 needed]
Installation: [pipe size; immersion; thermowell; connection]
Head: [terminal; transmitter; output; IP; Ex approval]

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