Two-, three- or four-wire connection with the Pt100
A resistance thermometer measures a resistance. The connecting cable has one too — and depending on the wiring it ends up in the reading fully, partly or not at all.
That is why the wiring type appears in the order key directly after the measuring element. With longer cables it matters more than the tolerance class.
Two-wire
The lead resistance enters the measurement in full.
Three-wire
The lead resistance is compensated — as long as all cores are of equal length and equal cross-section.
Four-wire
Current and voltage path separated, the lead resistance drops out completely.
R = resistance of one core. Marked red is the share of the lead that ends up in the reading.
Two-wire
Two cores, the lead resistance enters the measurement in full. Rough rule of thumb: 1 Ω of lead resistance falsifies a Pt100 by around 2.6 °C. Only sensible for short cables — or where a head-mounted transmitter sits in the connection head and the signal is passed on as 4…20 mA already.
Three-wire
Three cores, the lead resistance is compensated by calculation. This is the industry standard, and for the vast majority of measuring points the right choice. Prerequisite: all three cores have the same cross-section and the same length.
Four-wire
Four cores, current and voltage routed separately. The lead resistance drops out completely, regardless of length and cross-section. Needed for reference measurements, in the calibration laboratory, with very long cables and wherever 1/10 DIN B is specified — a tighter tolerance class in a two-wire connection is money thrown away.
Comparison
| Two-wire | Three-wire | Four-wire | |
|---|---|---|---|
| Cores | 2 | 3 | 4 |
| Lead resistance | enters in full | compensated | drops out |
| Typical use | short cable, transmitter in the head | industry standard | reference, laboratory, 1/10 DIN B |
| Duplex element | 4 cores | 6 cores | 8 cores |
Terminal block in the connection head, top view. Numbering as embossed on the block, core colours to DIN EN 60751.
Duplex measuring elements and core colours
Colour code to DIN EN 60751: red for the measuring resistor, white for the free terminal depending on the wiring. A second element is marked black and yellow. In a form B connection head a duplex Pt100 in four-wire connection fits on the eight-pole block only.
What the cable costs in a two-wire connection: with 0.22 mm² it is more than 40 K after 100 m already.
What else costs accuracy
- Route measuring cables at least 0.5 m away from power cables; twisted and shielded cables reduce interference.
- Immerse deep enough: 6 to 8 times the thermowell diameter in liquids, 10 to 15 times in gases.
- Terminals in the connection head take conductor cross-sections up to 2.5 mm².
Unsure with a long cable?
Tell us the cable length, cross-section and required accuracy — we will work out for you whether three-wire is enough.
Two-wire
Three-wire
Four-wire
Comparison
Duplex elements and core colours
What else costs accuracy
All resistance thermometers
Tolerance classes Pt100
Head-mounted transmitters
Connecting cables
Question about the design?
Tell us the medium, temperature and installation situation — you will have a proposal within one working day.