EM-M21A is a configurable temperature transmitter for Pt100 resistance sensors. It converts the measured temperature into a standard 4–20 mA current-loop signal.
The Pt100 sensor is connected using a three-wire circuit, which compensates for cable resistance. Four temperature spans can be combined with either a 0 °C or −50 °C starting point, providing eight selectable measuring ranges.
The EM-M21A provides four selectable temperature spans. A selectable −50 °C offset allows each span to be used with either a 0 °C or −50 °C starting point.
The available temperature ranges are:
The Pt100 sensor is connected to the transmitter using a three-wire circuit. This connection method compensates for the resistance of the sensor cables and reduces the measurement error caused by long connecting wires.
The converter supplies the Pt100 sensor with an excitation current of approximately 1 mA.
The resistance-to-temperature characteristic of a Pt100 sensor is not perfectly linear. The EM-M21A includes compensation for this non-linearity so that the 4–20 mA output follows the measured temperature more accurately across the selected range.
The selected temperature range is converted into a proportional 4–20 mA signal. The converter obtains its operating power from the 12–32 VDC current loop and requires no separate auxiliary supply.
With a 24 VDC loop supply, the specified maximum loop resistance is 700 Ω.
The 4–20 mA output can also be converted into a 1–5 V voltage signal by connecting a suitable resistor according to the connection diagram.
A 250 Ω conversion resistor produces a nominal voltage of 1 V at 4 mA and 5 V at 20 mA.
Separate zero and gain adjustments are provided for calibration. The adjustments are designed to operate independently, making it easier to set the lower and upper values of the selected measuring range.
The converter includes protection against incorrect connections and voltage transients.
| Current-loop supply voltage | 12–32 VDC |
|---|---|
| Sensor type | Pt100 |
| Sensor connection | Three-wire |
| Temperature range 1 | 0 … +100 °C |
| Temperature range 2 | 0 … +200 °C |
| Temperature range 3 | 0 … +300 °C |
| Temperature range 4 | 0 … +400 °C |
| Temperature range 5 | −50 … +50 °C |
| Temperature range 6 | −50 … +150 °C |
| Temperature range 7 | −50 … +250 °C |
| Temperature range 8 | −50 … +350 °C |
| Current output | 4–20 mA |
| Optional voltage output | 1–5 V using a conversion resistor |
| Linearity error | < 0.1% |
| Thermal drift | < 100 ppm/°C |
| Cable compensation error | 0.1% per 10 Ω of cable resistance |
| Pt100 excitation current | Approx. 1 mA |
| Maximum loop resistance | 700 Ω with a 24 VDC loop supply |
| Operating ambient temperature | 0 … +60 °C |
| External auxiliary supply | Not required |
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