Product type
A temperature transmitter converts the signal from a temperature sensor into a standard output that the rest of a control system can read. It exists because neither sensor family produces a signal a control system can take directly: a resistance element returns a small resistance that depends on the lead length as well as on the temperature, and a thermocouple returns a few tens of microvolts that depend on the temperature of the terminals as well as on the temperature being measured.
A temperature transmitter converts the signal from a temperature sensor into a standard output that the rest of a control system can read. It exists because neither sensor family produces a signal a control system can take directly: a resistance element returns a small resistance that depends on the lead length as well as on the temperature, and a thermocouple returns a few tens of microvolts that depend on the temperature of the terminals as well as on the temperature being measured.
Transmitters are fitted in two places for two related reasons. The first is electrical: converting the signal at the sensor, before it travels, removes the interference that a microvolt or a milliohm signal picks up on a machine or along a cable tray, and it removes the lead-resistance error that otherwise sits inside the reading. The second is organisational: a 4-20 mA loop can be read by a recorder, a PLC input card, a panel indicator or a SCADA system without any of them needing to know what sensor is behind it, and the loop can be tested with a meter.
The range of transmitters is wide because those reasons are met at different levels of cost. A puck that mounts in the sensor terminal head, a DIN-rail module for a panel, a field housing for a hostile environment and a rail-mounted isolator with several channels all do the same job with different mounting, isolation and channel density. The input is usually configurable rather than fixed, so a range of sensor types is covered by the same module with the input set at order or on site.
Buyers arrive at a transmitter from either end. From the sensor side, it is the answer to a long run or a noisy environment. From the system side, it is the adapter that lets an existing PLC input card read a temperature, and it is then the output form, the isolation and the load that have to be settled rather than the sensor.
The measurement chain starts at the element. A resistance element is read by passing a small excitation current through it and measuring the resulting voltage; the current is kept low so that the power it dissipates inside the element does not raise the element temperature enough to matter. A thermocouple is read as a voltage, and because that voltage is generated between the measuring junction and the terminals where the wires land, the temperature at those terminals has to be measured and compensated for.
The signal is then linearised and scaled. An RTD follows a published curve and a thermocouple follows a polynomial for its letter type, and the transmitter carries the linearisation that the receiving device would otherwise have to do. The resolution of the front end and the accuracy of the transmitter are different quantities: a converter with far more resolution than the accuracy figure is normal, because the resolution has to be non-limiting rather than matched to the specified accuracy.
The output stage drives a current loop. In a two-wire arrangement the same pair of conductors carries the supply and the signal, and the current the transmitter draws is the measurement, so the transmitter has to work on whatever voltage remains after the loop resistance has taken its share. The loop budget is therefore a real constraint: supply voltage, the transmitter’s own minimum operating voltage, the receiving resistor and the cable resistance all add up, and the point at which the transmitter can no longer reach 20 mA is the point at which the arrangement stops working. That is why load and supply are stated together on a transmitter and why a heavier load shortens the permissible cable run.
Two further points decide whether the installation measures what it should. The first is where the cold junction is: a transmitter mounted in the sensor head does its own compensation next to the terminals, while one mounted in a panel relies on compensating cable reaching it. The second is isolation. Where the sensor touches earthed metal, or where several channels are read by one card, a transmitter or an isolator that separates input from output prevents the loop from carrying a path between points that should not be connected.
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Decision | Options to fix | What it actually decides |
|---|---|---|
| Input type | Resistance element types and thermocouple letter types, usually selected in configuration rather than by hardware | Which sensors the module can read. The input has to match the sensor in use: a module configured for one family reports a plausible and wrong value from the other. |
| Input circuit | Two, three or four wire for a resistance element; thermocouple wire direct for a thermocouple | How much lead resistance remains inside the reading. On a resistance element a two-wire connection adds the leads to the element and appears as a fixed offset; a three-wire connection cancels equal leads. |
| Output form | Two-wire 4-20 mA, three-wire voltage, or a four-wire loop-powered or separately powered arrangement | What the receiving device can accept, and how many conductors the installation needs. Two-wire is the usual process answer and the one that most input cards expect. |
| Mounting | Sensor head puck, DIN rail module, field housing with cable entries, or an isolator strip with several channels | Where the conversion happens and therefore where the cold junction and the interference are. A head-mounted transmitter shortens the small-signal path; a rail module concentrates the channels in the panel. |
| Isolation | None, input to output, or input, output and supply all separate | Whether a path exists between the sensor circuit and the control circuit. Galvanic isolation is what makes a floating sensor readable and what stops several channels interfering with each other. |
| Assigned span | The temperature range mapped onto the output, set at order and usually adjustable | The resolution available over the working range and what happens outside it. A narrow span resolves finely and clips sooner; a wide span never clips and resolves coarsely. |
| Factor | How to settle it | Why it decides the answer |
|---|---|---|
| The sensor in use | Name the element type and the connection: a Pt100 in three wires, a Pt1000 in two, a type K thermocouple. | The input configuration follows from it, and so does the wiring count. Getting this wrong is the most common reason a transmitter reads a value that is close but not right. |
| The loop budget | Add the receiving resistor, the cable resistance and the transmitter’s minimum operating voltage, then check the supply can still drive 20 mA. | This is the calculation that decides whether a two-wire transmitter will work at all. A heavier receiving resistor or a longer run can put the arrangement outside the supply, and the symptom is a reading that stops rising at the top of the range. |
| The assigned span | Set the span to the working range of the process rather than to the full range of the sensor. | The output resolution is spread across the span, so a tighter span resolves better within it. The cost is that anything outside the span is no longer represented, and the saturation value has to be distinguishable from a real reading. |
| Where the conversion happens | Decide between a head-mounted transmitter and a panel or rail module. | A head-mounted unit does its compensation at the terminals and shortens the path the small signal travels. A rail module suits many channels in one place and suits an existing compensating-cable run. |
| Isolation requirement | Establish whether the sensor touches earthed metal and whether several channels share an input card. | Isolation is what separates circuits that would otherwise be connected through the loop. Where the probe is welded to a vessel or the machine, it is usually not optional. |
| Environment | State the ambient temperature at the mounting position, the humidity and whether the location is classified or washdown. | A head-mounted transmitter sits at the hot end and sees the process temperature through the head; a panel unit sees the panel. Both have limits that are lower than the limits of the sensor they serve. |
These are the types an enquiry for a temperature transmitter usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a temperature transmitter |
|---|---|
| RTD temperature sensors | Performs the measurement. A transmitter carries no sensing element of its own, so the two are usually specified together: the sensor determines the range and the accuracy, and the transmitter determines what the signal looks like when it arrives. |
| Thermocouples | Generates the voltage the transmitter reads, and is the sensor that most needs a transmitter because its signal is small and its cold junction has to be handled. On a short run and a nearby instrument, the thermocouple is wired directly and no transmitter is fitted. |
| Resistance temperature detectors | The bare resistance element, supplied as a leaded part. It has the same relationship to a transmitter as a complete probe has: the transmitter reads it, and where the element is supplied without a connection head the transmitter is usually mounted away from it. |
| Temperature indicators | Displays the value rather than converting it. An indicator takes a loop signal or a sensor signal and shows it; a transmitter produces the loop signal an indicator would read, and many panel instruments combine the two roles. |
| Temperature switches | Gives a contact at a set point rather than a continuous value, and is chosen for interlocks rather than for measurement. Where both a reading and a trip are needed, the reading comes from a transmitter and the trip is a separate function. |
| Temperature controllers | Reads the sensor and drives a heater or cooler towards a set point. A controller usually takes the sensor directly, so a transmitter is only added in front of it where the run is long or the signal has to be shared. |
The transmitter is chosen last and it is what makes the first reply quotable. State the sensor, the loop load, the supply and the mounting position, and the rest follows.
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
Available products
Temperature Transmitters
Two-wire temperature transmitter module for RTD and thermocouple inputs, converting the sensor signal to 4-20 mA or 0-10 V from 24 VDC, with 0.2%FS accuracy and zero and span potentiometers.
Also in Temperature Measurement
Every type we quote for under Temperature Measurement. A type with no link yet is still quoted for - it is described on the Temperature Measurement overview, and gets a page of its own as soon as a model is published against it.
Frequently asked
Usually not. A Pt100 wired in three or four wires into a panel instrument a few metres away works perfectly well, and a thermocouple wired in matching compensating cable to an instrument with cold junction compensation does too. A transmitter earns its place when the run is long, when the environment between the sensor and the instrument is electrically noisy, when the signal has to be shared by several devices, or when the sensor has to be a sealed assembly with no cable entry for a compensating cable.
That is the loop running out of voltage. A two-wire transmitter needs a minimum voltage across it to drive 20 mA, and the supply has to cover that plus the voltage dropped by the receiving resistor and the cable. When the total exceeds the supply the transmitter saturates and the reading stops following the process, which is easy to mistake for a sensor fault. The fix is a higher supply, a lighter receiving resistor or, failing both, a four-wire transmitter.
Both separate input from output. A transmitter also performs the measurement conversion, taking a sensor signal and producing a scaled loop output. An isolator generally passes a signal through unchanged while breaking the galvanic path, and is used where the signal is already in a usable form but the circuits must not be connected.
On most modern transmitters the input is a configuration rather than a hardware option, so it can be set by the software or by switches on the module. It still has to be set correctly to match the sensor, and where the sensor is a resistance element the number of wires the configuration expects has to match the wiring actually installed, or the lead compensation will not do what it was intended to do.
Where the aim is to shorten the path the small signal travels, which is the usual reason for fitting one at all. It sits in the sensor connection head, does its own cold junction compensation next to the terminations, and sends a loop signal onwards. Its limit is its own ambient rating, which is set by the process temperature conducted through the head and is generally well below the sensor’s range.
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