Temperature Transmitter Module

A temperature transmitter module takes the raw signal from an RTD or a thermocouple and turns it into a standard loop signal that the rest of a control system can read. It exists because the two sensor families produce signals that a PLC input card, a recorder or a panel indicator cannot use directly: an RTD returns a small resistance that depends on lead length, and a thermocouple returns a few tens of millivolts that depend on the temperature of the terminals as much as on the temperature being measured. Conditioning that signal at the sensor, rather than at the far end of a long cable, is what the module is for.

The module on this page is a two-wire design on a 24 VDC supply, with an output of 4-20 mA as standard and a 0-10 V version on the same housing. Inputs cover the common platinum and copper resistance elements and eight thermocouple types, with tungsten-rhenium types and Pt1000 available to order, and the accuracy figure stated in the material is 0.2%FS. The base is 44×40 mm with an 8 mm pitch terminal block, two adjustment potentiometers and M3×5 mounting screws, and the housing is PA66 flame-retardant nylon.

Because the input type is a configuration rather than a separate instrument, one module type serves a wide span of process temperatures: the same housing covers a Pt100 at -200°C and a type B thermocouple at 1800°C, differing in the input set at order and in the output span assigned to it.

How a temperature transmitter works

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 material gives that current as 0.2 mA, which is low enough that self-heating in the element stays small. A thermocouple is read as a voltage, and because that voltage is generated between the hot junction and the terminals where the wires land, the terminal temperature has to be known and compensated for. That compensation is the reason a thermocouple is connected to a transmitter with the correct thermocouple wire rather than with copper: copper joints introduce their own voltages at the point where the thermocouple wire ends.

The signal is then linearised and scaled. An RTD follows a published curve and a thermocouple follows a polynomial for its type, and the front end resolves the incoming signal finely enough that the linearisation is not the limiting error: the supplied material states a 24-bit sigma-delta converter, which is far more resolution than the 0.2%FS accuracy figure needs. Resolution and accuracy are different quantities, and it is the accuracy figure, together with the temperature coefficient, that describes what the module actually delivers.

The output stage drives a current loop. In a two-wire arrangement the same pair of wires carries both the supply and the signal, and the current drawn by the module is the measurement, so the module has to work on the voltage that remains after the loop resistance has taken its share. That is why the load and the supply are linked: a 250 Ω or 500 Ω load plus the cable resistance is what the 24 VDC supply has to drive, and the material states a limit on the lead resistance for that reason rather than as a cable recommendation.

Two potentiometers sit on the module for zero and span. They are there because the transmitter is the point in the loop where the installed assembly can be trimmed: the element’s tolerance, the lead resistance and the loop resistor all end up as a single offset and a single gain error, and both can be corrected at the module against a reference thermometer.

How to select the right transmitter

Start with the sensor, because the input type is fixed for the module. A Pt100 is the usual choice where the temperature has to be defensible and stays within its range; a thermocouple takes over where the temperature is high enough or the point is small enough that a resistance element is impractical. The material lists Pt100, Cu50, Cu100 and Pt1000 on the resistance side, and types B, E, J, K, N, R, S and T plus WRe3-25 and WRe5-26 on the thermocouple side.

Then the output span, which is the decision that most affects the reading in practice. The 4-20 mA loop carries a fixed current range, and the span assigned to it decides how much temperature each milliamp represents. Assigning a narrow span around the working range gives a finer reading over that range, but anything outside it clips. The material lists the assignable spans from -20-60°C up to 0-600°C in the steps given in the selection table, and a 0-100°C marking appears on the 0-10 V version.

Then the loop itself. The supply is 24 VDC and the load is stated as 250 Ω or 500 Ω, with a lead resistance limit to respect. Where the run is long, or the receiving instrument presents a heavier load, the available voltage at the module is what constrains the arrangement, and the material offers a build to order where the lead resistance on a Pt100 run exceeds 35 Ω.

Then the environment and the mounting. The material gives an operating range of -30 to 80°C in the parameter table but describes -20 to +60°C as the usual working range, and it also lists a transport and storage range of -40 to 120°C. The housing is a 44×40 mm base with M3×5 screws, which suits a panel or a field enclosure rather than a wall.

Finally the mating probe. The material documents probes to go with the module, including a rolled Pt100 of φ4×30 mm on a 0.5 m waterproof lead, a flat Pt100 of 7.5×20×3.6 mm, a threaded waterproof version on a 1.5 m lead with an M6×1 thread, and a high-temperature version on an M12×1 thread rated to 420°C. The lead insulation is not the same across those options, and only the stainless braided leads carry the high-temperature rating.

Limitations and points to confirm

The accuracy claim needs a reference to be meaningful. 0.2%FS is a percentage of the assigned span rather than of the measured temperature, so a 0-600°C span and a 0-50°C span produce quite different absolute errors from the same figure. The material states no reference standard behind it, so the figure should be read as the supplier’s specification and confirmed against a certificate where the application depends on it.

The two temperature coefficient figures are not reconciled. A general ≤25 ppm/°C FS and a Pt100-specific ≤250 ppm/K FS on a 100°C span differ by an order of magnitude, and the material does not say which inputs the general figure covers. Where the module operates away from the calibration temperature, that gap matters more than the accuracy figure does, and until it is resolved the wider figure is the safe one to assume.

Some limits in the material need reading carefully. The lead resistance limit is given as under 80 Ω for both a 500 Ω load and a 250 Ω load, which is unusual: the allowable lead resistance normally falls as the load rises, because both consume the same loop voltage. The figure has been flagged rather than published as two separate verified limits. The 20.3 mA output current limit is likewise a saturation figure, not an operating point.

Several parameters that a specification usually states are absent: ingress protection, humidity limits, the isolation voltage between input and output, and long-term stability. They appear on this page as information required rather than as estimates. The material also states that the components are imported without naming a grade or an origin, which is not published here because it cannot be substantiated from the supplied document.

Two practical points about the assembly. The material states that the mating probes are hand-assembled and may carry measurement variation, so the paired probe and transmitter should be treated as an assembly to be trimmed rather than as two independently interchangeable parts. And the product carries no model designation in the supplied document: the identification on the housing is a configuration label, so an order should state the input type, the output form and the assigned span rather than a code.