Thermocouple vs RTD: choosing a temperature sensor for testing

Temperature is the most common measurement on a test bench, and the first decision is the sensor type. Thermocouple vs RTD is a choice between range and robustness on one side and accuracy and stability on the other. This article compares the two sensor types, explains the wiring details behind most measurement errors, and lists the ICP DAS modules that read each type.

How the two sensors work

A thermocouple is a junction of two different metals. It produces a small voltage that depends on the temperature difference between the measuring junction and the point where the thermocouple wires meet copper, the cold junction. A type K thermocouple gives roughly 41 µV per °C, so the input must resolve microvolts and reject noise.

An RTD (resistance temperature detector) is a resistor whose value rises with temperature. A Pt100 has 100 Ω at 0 °C and changes by about 0.385 Ω per °C; a Pt1000 has 1000 Ω at 0 °C. The input module drives a small current through the sensor and measures the voltage across it.

Thermocouple vs RTD at a glance

Choose an RTD when you need accuracy and stability between about −200 and +600 °C. Choose a thermocouple for higher temperatures, small or fast probes, and harsh mechanical conditions.

Cold junction compensation

A thermocouple measures a difference. To get the absolute temperature, the module measures the temperature at its own terminals and adds the equivalent thermocouple voltage. This cold junction compensation (CJC) is only as good as the temperature match between the terminals and the CJC sensor, so mount modules away from heat sources and draughts. Run thermocouple or matching extension cable all the way to the input terminals; a copper section in between creates new junctions at an unknown temperature.

Among ICP DAS inputs, the ET-2218HS-16 and USB-4018HS compensate automatically on every channel. The M-7018-16-G and the SG-3011 conditioner provide an adjustable CJC offset for fine calibration. For PCI-1800U boards, the DB-8225 terminal board has a cold junction circuit on analog input channel 1.

Lead resistance: 2-, 3- and 4-wire RTD connections

In a 2-wire connection, the lead resistance adds directly to the sensor resistance.

Worked example: a Pt100 on 10 m of 0.5 mm² copper cable sees about 0.34 Ω per conductor, or 0.69 Ω in the loop. Divided by 0.385 Ω/°C, the reading is about 1.8 °C too high. The same cable with a Pt1000 gives an error of about 0.18 °C, because the sensor changes ten times as much per degree.

A 3-wire connection uses a third lead to measure and subtract the lead resistance, assuming all leads are equal, and is the usual choice for test-bench wiring. A 4-wire connection carries the excitation current on one pair and measures the voltage on the other, which removes lead resistance from the result; use it for calibration and reference measurements.

The M-7015P-G, ET-2215H, ET-7015 and ET-7215 provide 3-wire lead resistance elimination. The M-7033-G accepts 2-, 3- and 4-wire RTDs, with lead resistance elimination for 3- and 4-wire connections.

Response time and sample rate

Sensor response time depends mostly on construction and thermal contact with the test object. Thin, exposed thermocouple junctions react fastest; a sheathed probe of either type reacts more slowly as its diameter grows.

The input module must also sample fast enough. Standard multi-channel modules share their sample rate across channels: the M-7018-G samples 10 Hz in total over 8 channels and the M-7015-G 12 Hz over 6, which suits slow thermal tests. For fast transients, the ET-2218HS-16 and USB-4018HS sample thermocouples at 100 Hz per channel, and the ET-2215H samples RTDs at 90 Hz per channel in fast mode, with ±0.1 % of FSR accuracy against ±0.05 % in normal mode at 1.5 Hz. The SG-3011H thermocouple conditioner has a switch-selectable response time of 0.5 ms or 100 ms.

Which ICP DAS modules fit

Thermocouple inputs

• M-7018-G (8 channels) and M-7018-16-G (16 channels): RS-485 modules with Modbus RTU and DCON, differential inputs for types J, K, T, E, R, S, B, N and C, plus voltage and current ranges. The M-7018R-G adds 240 Vrms overvoltage protection and open thermocouple detection.
• M-7019R-G: universal 8-channel input for thermocouples (including types L, M and L DIN43710), voltage and current, with an individual range per channel.
• ET-2218HS-16: 16-channel Ethernet module with standard thermocouple connectors, 100 Hz per channel and automatic CJC.
• USB-4018HS and USB-4018HS-16: 8 or 16 channels on USB, bus powered, ±0.5 °C accuracy and 100 Hz per channel.

RTD inputs

• M-7015-G: 6-channel RS-485 module for Pt100, Pt1000, Ni120, Cu50, Cu100 and Cu1000 with open wire detection; the M-7015P-G adds 3-wire lead resistance elimination. The M-7033-G has 3 channels for 2-, 3- or 4-wire RTDs.
• ET-2215H and ET-2215H-16: 8 or 16 high-speed RTD channels on Ethernet, 90 Hz per channel.
• ET-7015 and ET-7215: 7-channel Ethernet RTD modules designed for long-distance RTD wiring.

Signal conditioners

For DAQ boards or PLCs with voltage or current inputs, the SG-3011 (thermocouple) and SG-3013 (RTD) convert one sensor to a voltage or current output, with 1000 VDC three-way isolation.

Send your sensor types, temperature ranges, channel count and required sample rate to our technical sales team, and we will suggest suitable modules.