Copyright © Romilly Bowden 1997.
One of the features of smart field devices is their ability to detect faults, either in the device electronics or in an associated sensor. Using HART, such faults are reported in the device status byte in every message (assuming that communication is still possible!). But, for the benefit of systems not using HART communication, it is still useful to follow the convention of indicating fault conditions by setting the analogue output current to a value which is recognisably beyond the normal operating range (including the small amount of linear over-range commonly allowed). If it is still alive, the microprocessor program should set the current output value to an appropriate value. Otherwise, a watchdog timer circuit may be able to do this, even if the microprocessor itself has failed. The intention is that a host system should be able to set alarm thresholds just outside the normal 4 mA to 20 mA range, to indicate measurement out-of-range, and to set further alarm thresholds to indicate a fault condition.
NAMUR Recommendation NE43 (18.01.1994) suggests the following:
However, not all instrument manufacturers follow the NAMUR recommendation. In particular, the 3.6 mA downscale fault indication value is regarded as difficult to implement in a smart transmitter, where every microamp of current consumption is carefully accounted for to support the demands of the microprocessor and sensor circuitry. One well-known supplier typically uses the values:
Fortunately, alarm threshold levels can be chosen which accommodate both these sets of values. A host system can set out-of-range alarm thresholds at 3.95 mA and 20.3 mA, and can differentiate between an out-of-range measurement and a fault, by further alarm thresholds at 3.78 mA and 20.9 mA.
In percent of range terms, these thresholds are -0.3% and +101.9% for out-of-range, and -1.4% and +105.6% for a fault. But remember to check that the values you use match the values actually used by your instrument supplier, if they don't follow NAMUR.