Quick answer
A 4-20mA gas detector signal is an analogue current output used to represent a configured measurement range. In the normal linear mapping, 4mA corresponds to the lower range endpoint and 20mA to the upper endpoint. The receiving equipment needs the matching gas, unit and scaling information. Fault and special-state currents are model-specific and must not be guessed from the normal span.
The signal carries a value, not a gas name
The current is an electrical representation of the measurement. It is not the concentration unit itself. The receiving input must be configured to interpret it against the detector's documented measurement span; the same current can represent different concentrations in instruments with different ranges.
Analog Devices' current-loop application note explains the use of this analogue signalling method. Its electronic examples are not UTAI wiring diagrams or evidence that a particular gas detector has a current output.
What is meant by the normal span?
For the normal linear convention, the two endpoint currents represent the configured measurement endpoints. Intermediate values are interpreted between those endpoints. This explains why an input range mismatch can produce a believable-looking but incorrectly scaled display.
The nonzero lower endpoint is often called a live zero. It distinguishes the normal measurement baseline from no loop current, but does not prove sensor health or specify every diagnostic state. A lower endpoint need not be interpreted as 'no gas' unless that is how the instrument's range is defined.
Which documents must agree at both ends?
The detector output and receiving input are a pair to review. UTAI's editorial checklist below identifies the information needed for that review; it does not give connection instructions or approve any particular combination.
| Review item | Why it matters |
|---|---|
| Gas and measurement unit | The current alone does not identify either |
| Lower and upper range endpoints | Both ends must interpret the same span |
| Output and input electrical types | Signal compatibility is not established by the range label |
| Power and load requirements | The complete loop must meet the applicable electrical conditions |
| Fault and special-state definitions | An abnormal signal must not become a misleading normal reading |
Why is a current loop not immune to installation problems?
The current-loop method can provide robust signalling, but the complete circuit still has electrical limits. Analog Devices explains that sufficient supply voltage must remain available for the voltage drops around the loop. The actual allowable supply, load and connection arrangement come from the equipment documents.
Do not infer the number or function of power conductors from the phrase 4-20mA. There are different powering arrangements. A qualified person must establish the supplied instrument's requirements without trial connections on an operating gas-alarm system. No resistor value, terminal order or live measurement procedure is supplied here.
What about faults, warm-up and conversion modules?
Normal measurement scaling is not a universal status-code table. The treatment of faults, warm-up or other special conditions must be established from the applicable detector and receiver manuals. This article deliberately gives no diagnostic-current values or alarm settings.
A conversion module adds another interpretation boundary. UTAI's 4-20mA to RS485 module page provides existing product context, but a conversion route still needs verified range, status and protocol mapping. RS485 does not by itself establish Modbus compatibility.
For the wider system context, see the UTAI controller guide. Competent personnel must verify the complete measurement and warning path under approved arrangements. If a gas alarm occurs or a release is suspected, follow the site's emergency procedure rather than interpreting a current value as clearance.