In brief
A transmitter converts a sensor signal into a standardised, interference-resistant output signal such as 4–20 mA that control systems can process directly.
Contents
What is Transmitter?
Many sensors only deliver weak or non-linear raw signals, such as the millivolts of a thermocouple or the resistance of a Pt100. A transmitter amplifies, linearises and scales this signal and outputs it as a standard signal. It can sit directly in the sensor head, be installed as a field device or be mounted on a DIN rail in the control cabinet. This also allows longer cable runs to the control system to be bridged without significant signal loss.
Common outputs are 4–20 mA, 0–10 V and digital protocols; some devices transmit additional digital information superimposed on the current signal. The current signal is insensitive to lead resistance, and the live zero at 4 mA makes it possible to detect wire breaks. Key ratings are input type, measuring range, output signal, accuracy, galvanic isolation, supply in two- or four-wire technology and Ex approval. Isolating amplifiers additionally separate circuits galvanically.
A faulty transmitter shows up as frozen, jumping or out-of-range values. For a replacement, it is crucial that measuring range and configuration are set again exactly. Many devices are configured via software, operating buttons or a programming adapter; without these tools, a replacement unit can hardly be adapted.
In practice: maintenance and procurement
Keep the parameter settings ready for every transmitter: input type, measuring range, output signal, damping and behaviour on sensor break. In an enquiry, state whether a head-mounted, DIN rail or field device is required and whether it should be supplied preconfigured.
Synonyms and abbreviations: measuring transmitter · signal converter · signal conditioner · measuring amplifier · 4–20 mA transmitter
Frequently asked questions
Why is 4–20 mA used instead of 0–20 mA?
With a 4–20 mA signal, the start of the measuring range corresponds to 4 mA instead of zero. If the current falls below this value, the control system detects a fault such as a wire break or loss of supply. With 0–20 mA, a cable break could not be distinguished from a genuine measured value of zero. In addition, the base current can power a two-wire device.
What is a two-wire transmitter?
A two-wire transmitter draws its power via the same two wires over which it outputs the 4–20 mA signal. This saves wiring and is very common in the field. Four-wire devices have separate supply and signal lines and are used when the electronics require more energy.
What information is needed to replace a transmitter?
The manufacturer part number, input type such as Pt100, thermocouple type or voltage, measuring range, output signal, supply, design, Ex approval and, where applicable, communication protocol are needed. So that the replacement unit is ready for use immediately, the current parameters should be documented and passed on. Otherwise the device has to be reset on site.