Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2023-04-06.
There are several ways to get a result out of a monitor. The simplest, once reading a local display is set aside, is a hard-wired signal: a voltage or a current on a dedicated pair of wires, proportional to the reading, with status carried by mechanical relays that open or close. Hard wiring was the industry standard and is still found on simple sensors such as temperature and pressure devices. A complex analyser, however, may output several continuous values and several states, and giving each its own pair of wires is impractical and error-prone, so digital communication has become the norm, even on simple sensors. For continuously variable values the 4-20 mA current loop is the common standard, scaled linearly to the range in the datasheet. Starting at 4 mA instead of 0 makes a broken wire detectable — a reading of 0 mA means a fault, not a low temperature — and a current below 4 mA can indicate an error state. Voltage signals of 0-10 V or 0-24 VDC exist but suffer from cable resistance, which a current loop avoids. Digital links are mostly serial: RS-232 and RS-485, increasingly Ethernet with TCP/IP, each with a protocol that defines how the two ends talk.
Interface choice in Chinese projects is usually decided together with the data acquisition and reporting requirements. A CEMS has to meet HJ 75 and HJ 76 for acquisition, processing and transmission, and uploads to the regulator's platform generally follow HJ 212. At acceptance and comparison, the range, unit, dry or wet basis and oxygen reference of every signal have to match the datalogger and the platform configuration; a signal that arrives correctly but means something different is far more time-consuming than a wiring error. In practice the most common combination is 4-20 mA plus RS-485/Modbus: analogue for point-to-point values with defined timing, digital for many devices on one twisted pair. Use shielded twisted pair with single-point earthing and route it away from power cables, and remember that a change of range must be made in the analyser, the datalogger and the platform at the same time. A 0 mA reading must be treated as an instrument or wiring fault, never as a valid low concentration. Where an analyser has many signals, digital output is usually cheaper and removes terminal faults, while relays are worth keeping as an independent path for calibration status and alarm contacts. With Modbus RTU, confirm baud rate, parity, slave address and register map before delivery — small details that stop commissioning. Note also whether a transmitter is two-wire, powered over the signal pair, or four-wire with separate supply.
Instrument Interfaces — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2023-04-06. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
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