Knowledge Center · Temperature transmitter guide

Temperature transmitter guide — head-mounted and rail-mounted

A temperature transmitter converts the signal from a temperature sensor — the resistance of a Pt100 or the voltage of a thermocouple — into a 4–20 mA current signal that travels long cable runs without losing accuracy. Head-mounted transmitters sit inside the sensor's connection head; rail-mounted transmitters sit on a DIN rail in the control cabinet.

Head-mounted transmitters — catalogue page (PDF) Rail-mounted transmitters — catalogue page (PDF)

What a transmitter does

A Pt100 presents its measurement as a small change in resistance, a thermocouple as a voltage of a few millivolts. Both are delicate signals: they pick up electrical noise, and every metre of cable adds resistance or calls for special conductor materials.

A transmitter converts the measurement into a 4–20 mA current signal as early as possible. The scale is simple: 4 mA is the bottom of the calibrated range, 20 mA is the top. Because the same current flows through the entire loop, the signal arrives with the value it left with — cable length and lead resistance drop out of the equation, and ordinary copper cable carries it. Most temperature transmitters are loop-powered: the same two wires that carry the signal also power the electronics.

Head-mounted

A head-mounted transmitter is fitted inside the sensor's connection head, directly on top of the insert. The delicate sensor signal travels centimetres before it becomes 4–20 mA — the shortest possible run — and everything from the head to the instrument is ordinary two-wire copper cable. One sensor, one transmitter, one cable: the fewest components in the loop. The arrangement is drawn in the connection diagrams on the Pt100 wiring page.

Rail-mounted

A rail-mounted transmitter clicks onto a DIN rail in the control cabinet, and the sensor's own signal runs the whole way to it. For a Pt100 that means a 3- or 4-wire connection to compensate for the cable; for a thermocouple it means extension wires or compensating cables in the matching alloys — the thermocouple colour codes article covers them. The return is serviceability: every transmitter in the plant sits side by side in the cabinet, channels are added rail by rail, and configuration and service happen in one place — with no need to open connection heads out in the process.

HART communication

HART is digital communication carried on top of the 4–20 mA signal. The analogue current remains the measurement; HART adds configuration, diagnostics and documentation on the same two wires, so a transmitter can be re-ranged, checked and documented from the control room. The programme includes transmitters with HART 5 and HART 7 — the head-mounted 5337A temperature transmitter, for example, communicates with HART 7.

Hazardous areas

For measuring points in classified areas, several transmitters in the programme carry hazardous-area approvals such as ATEX — the 5331D EEX is an example. The approval belongs to the transmitter; see the temperature transmitter range for the variants and their certificates.

Choosing between them

Head-mounted is the choice when the priority is signal integrity with the fewest components: the conversion happens at the measuring point, and everything downstream is standard cable. Rail-mounted is the choice when the priority is central service and scale: many loops in one cabinet, configured and maintained without touching the field. Both deliver the same 4–20 mA — the question is where the conversion should live. Talk to us about your installation — the right transmitter follows from the measuring points and the plant around them.

Frequently asked questions

What is a 4–20 mA temperature transmitter?

A device that converts a temperature sensor's signal — Pt100 resistance or thermocouple voltage — into a 4–20 mA current signal: 4 mA at the bottom of the calibrated range, 20 mA at the top. The current signal holds its value over long cable runs and is carried by ordinary copper cable.

Why 4 mA and not 0 mA?

Because a live zero separates "measuring low" from "dead". With the range starting at 4 mA, a reading of 0 mA can only mean a fault — a broken cable or a lost loop — and is detected immediately. The 4 mA base current also powers a loop-powered transmitter's own electronics.

Head-mounted or rail-mounted?

Head-mounted converts the signal at the measuring point with the fewest components in the loop. Rail-mounted gathers every transmitter in the cabinet, where they are configured and serviced centrally. Signal quality favours converting early; serviceability and channel count favour the rail.

Why use a transmitter instead of wiring the sensor directly?

Three reasons: the 4–20 mA signal is insensitive to noise and lead resistance over long runs; it is carried by standard copper cable instead of extension or compensating cable; and many transmitters report a sensor break by driving the current out of range, so a fault is seen immediately.

What is HART?

Digital communication on top of the 4–20 mA current signal. The analogue value remains the measurement; HART adds remote configuration, diagnostics and documentation on the same two wires.

When is HART worth it?

When transmitters are re-ranged, checked or documented after installation. With HART, that happens from the control room over the existing wiring; with plain analogue transmitters, each change is a visit to the device with a programming tool.

Are ATEX-approved transmitters available?

Yes — several transmitters in the programme are ATEX approved for hazardous areas, such as the 5331D EEX. See the temperature transmitter range for details.

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