Temperature sensors for building automation: RTD, NTC or 0–10 V

Temperature is the most common measurement in a building, from water circuits to duct, room and outdoor air. Choosing temperature sensors for building automation is mostly a choice between Pt100 and Pt1000 resistance thermometers, NTC thermistors and transmitters with a 0–10 V or 4–20 mA output. That choice decides which input module you need, how far the sensor can be from it and how accurate the reading will be. Below we compare the options, calculate the effect of cable resistance and list which ICP DAS input modules read each sensor type.

The sensor types

Pt100 and Pt1000

Platinum resistance thermometers (RTDs) follow a standard curve defined in IEC 60751. A Pt100 has 100 Ω at 0 °C and a Pt1000 has 1000 Ω. With the standard coefficient, resistance rises by about 0.385 Ω per kelvin for a Pt100 and 3.85 Ω per kelvin for a Pt1000. Sensors from different manufacturers are interchangeable within their tolerance class, which makes RTDs the usual choice for water circuits and energy calculations.

NTC thermistors

The resistance of an NTC thermistor falls as temperature rises, and the change per kelvin is large, so lead resistance hardly matters. The curve is non-linear and differs between types: a 10 kΩ NTC from one manufacturer does not necessarily match a 10 kΩ NTC from another. The input module must use the exact curve, or readings can be several degrees off at the ends of the range.

0–10 V and 4–20 mA transmitters

A transmitter combines a sensor element with electronics that convert temperature into a standard signal over a fixed range, for example 0–50 °C. It needs a supply, usually 24 V, but any analog input can read it. Voltage signals are sensitive to voltage drop and interference on long cables. A 4–20 mA loop is more robust over distance, and a current below 4 mA shows a broken wire.

2-wire, 3-wire and 4-wire RTD connections

With a 2-wire connection the input measures the sensor plus the resistance of both conductors. A 3-wire connection adds a third conductor so that the module can measure the lead resistance and subtract it, assuming equal conductors. A 4-wire connection separates the measuring current from the voltage sensing and removes the lead resistance almost completely.

Worked example: a copper conductor of 0.5 mm² has about 0.035 Ω per metre at 20 °C. In a 2-wire circuit both conductors add to the reading:

A 2-wire Pt100 is therefore only acceptable with very short leads. A 2-wire Pt1000 is often good enough for room and duct sensors. For long runs, and for water temperatures that feed energy calculations, use 3-wire or 4-wire connections. Lead resistance also varies with cable temperature, so a fixed offset correction is never exact.

Accuracy: sensor, cable and input module

The total error is the sum of the sensor tolerance, the wiring effect and the input module. IEC 60751 class B allows ±(0.3 + 0.005·|t|) °C, which is ±0.4 °C at 20 °C, and class A allows ±(0.15 + 0.002·|t|) °C. Input module accuracy is usually stated as a percentage of the full-scale range (FSR), so convert it using the temperature span of the selected input type. Use twisted, shielded cable, keep sensor cables away from mains and motor cables, and earth the shield at one end only.

Comparison of temperature sensors for building automation

Which ICP DAS input modules read which sensors

RS-485 modules with Modbus RTU

• M-7015-G: 6 RTD inputs for Pt100, Pt1000, Ni120, Cu50, Cu100 and Cu1000, configurable per channel, with open wire detection.
• M-7015P-G: the same 6 inputs with lead resistance elimination for 3-wire connections, designed for long-distance RTD measurement.
• M-7033-G: 3 RTD inputs for Pt100, Pt1000 and Ni120 in 2-, 3- or 4-wire connection, with lead resistance elimination in 3- and 4-wire mode.
• M-7005-G: 8 thermistor inputs with predefined types and user-defined types entered as Steinhart coefficients, plus 6 alarm outputs.
• tM-TH8: 8 isolated thermistor inputs, also with user-defined types.
• M-7017R-G: 8 differential inputs for voltage up to ±10 V and for current; current inputs need an external 125 Ω resistor.

Ethernet modules with Modbus TCP

• ET-7015 and PET-7015: 7 RTD inputs for Pt100, Pt1000, Ni120, Cu100 and Cu1000 in 2- or 3-wire connection, with 3-wire lead resistance elimination. PET-7015 adds PoE to IEEE 802.3af.
• PET-7215: the same RTD inputs with a 2-port Ethernet switch for daisy-chain wiring.
• PET-7005: 8 thermistor inputs with user-defined types and 4 digital outputs.
• PET-7017: 8 jumper-selectable voltage or current inputs for transmitters.

Other options

If the BMS uses BACnet/IP, the BNET-2310M reads 0–10 V transmitters directly on its ±10 V analog inputs. If a controller only has voltage or current inputs, the SG-3013-G signal conditioner converts one Pt100, Pt1000 or Ni120 sensor into 0–10 V or 4–20 mA. For room climate, the DL-10 reports temperature (typically ±0.4 °C), humidity and dew point over Modbus RTU.

Checklist before you choose

• Sensor type and exact curve: Pt100 or Pt1000 coefficient, NTC type or its Steinhart coefficients
• Connection type (2-, 3- or 4-wire) and the longest cable run
• Required accuracy at the operating point, including the sensor tolerance class
• Points per location and the available network: RS-485, Ethernet or PoE
• Protocol on the BMS side: Modbus RTU, Modbus TCP or BACnet

Send your sensor list and cable lengths to our technical sales team for a module proposal.