Wire a PT100 directly to an RTD input when the cable is short and the receiver has a proper PT100 direct input with 3-wire or 4-wire lead compensation.
Use a PT100 to 4-20mA transmitter, usually a head mounted temperature transmitter inside the sensor’s connection head, when the run is long, the area is electrically noisy, or the receiver only has 4-20 mA inputs.
The transmitter converts the small resistance change into a current that cable resistance does not affect, at the cost of one more device to buy, configure and calibrate.
Who this is for: plant, utility and maintenance engineers adding RTD temperature monitoring to tanks, bearings, ovens, cold rooms or process lines, and integrators choosing between RTD and 4-20 mA input cards.
What a PT100 gives you, and why wiring matters
A PT100 is a platinum resistance thermometer with 100 ohms at 0 °C. Its resistance versus temperature relationship and tolerance classes are defined in IEC 60751:2022 (scope and edition). The common -200 °C to +850 °C range and the 0.003851 per °C coefficient are described in TI’s RTD measurement guide SBAA275A. In round numbers that is about 0.385 ohm per °C near room temperature.
That small slope is the whole problem. Every ohm of copper lead resistance that gets counted as sensor resistance reads as roughly 2.6 °C of error. TI’s A Basic Guide to RTD Measurements works through how 2-wire, 3-wire and 4-wire connections handle that lead resistance.
Is PT1000 better for long cables?
A PT1000 has ten times the resistance and ten times the slope, so the same lead resistance causes about one tenth of the error in a 2-wire connection. It still needs a receiver that supports PT1000 and it does not remove noise pickup on long runs.
PT100 3 wire vs 4 wire (and 2 wire)
| Connection | How leads are handled | Lead error | Typical use |
|---|---|---|---|
| 2-wire | Both lead resistances add to the reading | Highest, grows with length | Very short leads, or a head transmitter mounted at the sensor |
| 3-wire | Receiver measures and subtracts lead resistance, assuming leads are matched | Small if leads are equal | The common industrial default |
| 4-wire | Separate current and sense pairs, sense leads carry almost no current | Effectively removed | Lab, calibration, high-accuracy points |
TI’s guide notes that 3-wire compensation assumes matched lead resistances, and that 4-wire connections remove lead resistance error because the sense leads carry no excitation current. In practice, a 3-wire run with mismatched terminals, corroded joints or mixed cable cores loses part of its compensation.
Can I run a 3-wire PT100 on a 2-core plus shield cable?
No, the shield is not a matched conductor. Use three matched cores so the compensation works as intended.
How long can a direct PT100 run be?
There is no single number. It depends on the cable resistance, whether leads stay matched, the noise environment and the receiver’s input design. Many receiver datasheets state a maximum lead resistance; work back from that to cable length. If the run is long and passes VFDs or heavy power cables, a transmitter at the sensor is the safer choice.
PT100 to 4-20mA transmitter: what changes
A head mounted transmitter sits in the sensor’s connection head (or a DIN rail transmitter sits in a nearby junction box) and does the RTD measurement right next to the element. The cable back to the panel then carries a temperature transmitter 4-20mA signal over two wires. Loop current is the same throughout the loop, so cable resistance does not change the reading as long as the loop supply has enough voltage, as explained in TI’s basics of 4-20 mA current loop transmitters. A broken wire drops the loop to 0 mA, below the 4 mA live zero, so the receiver can flag it.
| Point | Direct RTD input | Head mounted 4-20 mA transmitter |
|---|---|---|
| Cable | 3 or 4 matched cores per point | 2 cores per point |
| Cable length sensitivity | Lead resistance and matching matter | Mainly loop voltage budget |
| Noise | Millivolt level signal, more exposed | Current signal, less affected |
| Receiver needed | RTD input card or module | Any 4-20 mA input |
| Wire break detection | Depends on receiver | Below 4 mA, visible on any input |
| Extra items | None | Transmitter, configuration, loop supply |
| Calibration | Sensor plus receiver | Sensor plus transmitter plus receiver |
| Range | Set in the receiver | Set in the transmitter (e.g. 0 to 150 °C) |
Does the transmitter make the measurement more accurate?
Not automatically. It removes long-cable lead error and reduces noise, but it adds its own accuracy and drift figures, plus the receiver’s 4-20 mA input error. Add the sensor class, transmitter accuracy and input accuracy together when you compare.
Do I still need 3-wire between the PT100 and a head transmitter?
Inside the head the leads are short, so 2-wire error is small, but most head transmitters accept 3-wire or 4-wire and it costs nothing to use it. Follow the transmitter’s wiring diagram.
Cost and maintenance view
A direct RTD input saves the transmitter cost and one configuration step, which suits short runs inside a panel or skid. A transmitter adds hardware cost per point but uses cheaper 2-core cable and any 4-20 mA input, and the 4-20 mA signal is easy to check with a loop calibrator.
On sites with many existing 4-20 mA inputs and long runs, the transmitter route is usually less trouble over the life of the plant. For sensors bonded to earthed metal in noisy areas, choose an isolated transmitter.
Checklist
- Note cable length and route for each point, and whether it passes VFD or power cables.
- Check what the receiver accepts: PT100 direct, PT1000, or 4-20 mA only.
- For direct wiring, use 3 or 4 matched cores and check the receiver’s maximum lead resistance.
- For transmitters, set the range to fit the process with headroom, and check the loop voltage budget.
- Decide whether the transmitter needs isolation.
- Specify the sensor tolerance class to IEC 60751.
- Plan calibration of sensor and loop together.
Where Embedos fits
Embedos Edge accepts 4-20 mA and 0-10 V DC analogue inputs on devices in the platform (for example Edison or Maxwell). The confirmed path for PT100 points today is: PT100, head mounted or DIN rail 4-20 mA transmitter, then an Edge analogue input scaled to °C. Edge then sends readings over MQTT or HTTPS, keeps data with data buffering during network outages, gives remote visibility of device health and connection status, and can raise threshold alerts by SMS, email or call.
Where temperature already sits in a controller or meter, Edge can read it over RS-485 Modbus RTU or Modbus TCP instead.
The Turing series is the temperature-specialist (Thermocouple / RTD expansion). Can read RTD or Thermocouple Sensor Inputs directly, log data, and transmit data over MQTT, HTTP with or without TLS, over 4G Sim Card or Wi – Fi, SMS, email or call, directly.
Next step. Drop a Mail!
The Embedos Turing (ET) series is the temperature-specialist.
Thermocouple / RTD Data Logger / Gateway.
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