Fault Finding Guide; The Analogue Current Loop; The 4-20Ma Measurement Range; The Pre-Warning Level - Dräger Polytron Pulsar 2 Technical Manual

Fault Finding Guide

Fault Finding Guide

The Analogue Current Loop

In most installations the first indication of detector condition is the analogue current loop reading. To interpret it fully you need to know
what digital configuration has been loaded into the Receiver. Be sure to distinguish clearly between these four conditions:

The 4-20mA measurement range

Readings in this range indicate gas on a linearised scale between zero and a full scale quantity of a particular gas. That span and the
choice of gas are part of the Receiver configuration. Typically the 20mA reading corresponds to 5 or 8 LEL.m of methane or of propane.

The Pre-warning level

This current is output to warn of conditions that could, eventually, cause an inability to detect gas: misalignments of the Transmitter or
Receiver, dirty lenses, or mistriggering of a flash tube. Note that the detector retains its full sensitivity and that any gas reading above a
low, configurable threshold overrides the warning. Normally the level is 3.5mA and the threshold is 0.5 LELm. However, some control
equipment cannot resolve currents below 4.0mA so 4.5mA, for instance, may be chosen. There is no ambiguity provided the current cho-
sen corresponds to a gas reading below the threshold.
The Beam-block level
An output of 2mA shows that the detector is not able to detect gas for reasons other than a hardware fault at the Receiver. They include
fog or a solid obstruction in the beam path, or that the Receiver has become misaligned by two to three times the amount that initiates the
Pre warning. For compatibility with other Draeger PLMS equipment the 2mA current is fixed, but two time intervals associated with it are
configurable. The first is the time an obstruction must stay in the beam path to cause a Beam-block, normally 60 seconds. The second is
the time a Beam-block must persist to generate a Fault, normally 60 minutes. In installations where beam interruptions are frequent and
tolerable it may be this delayed event which prompts action rather than the Beam-block itself.
A sudden release of a large amount of pressurised and/or refrigerated gas can result in a loss of visibility caused by condensation of
atmospheric water or the released gas itself. As true for all optical open path systems, this may induce a beam block on the Pulsar which
will impair Pulsarís ability to detect the gas. The beam block warning would be activated and reported to the user. Although the scenario
is rather unlikely, choosing shorter rather than longer path lengths when installing Pulsars in this application can further reduce occur-
rence. In environments where fog generated by gas leaks is a frequent problem, beam blocks should be taken as indication for potential
hazards and the use of additional point detectors should be considered.
The Fault level
An output below 1mA indicates that the detector requires attention, either because of a persisting Beam-block (see above) or a hardware
fault. There may be a fault either in the Receiver itself or in the cables and terminations supplying it. Note that a fault which prevents the
Transmitter working at all is not distinguishable from an obstruction in the path, so it will generate Beam-block rather than Fault. Note too
that the fault-tolerant design of the Transmitter ensures that a partial malfunction will not stop the detector working correctly. However it
does generate the Pre warning (see above) and inhibit alignment and re-zeroing.
Be aware that spurious 'faults' may be caused if the Draeger PLMS Dräger Polytron Pulsar 2 is used with control equipment from other
manufacturers without sufficient attention to detail. Analogue loops are inherently prone to small drifts. Thus a system programmed to rec-
ognise any current outside 4 to 20mA as Fault will do so if a zero gas reading drifts to 3.99mA. Likewise, tolerance bands of say ±0.25mA
should be allowed for the Pre warning and Beam-block signals.
Fault finding
Once an abnormal condition has been identified from the analogue loop signal, it is most easily investigated by looking at the digital sig-
nals. The Handheld terminal (HHT) can be connected at the Receiver both to view the data stream and to interrogate the Receiver config-
uration. At the Transmitter the data stream and the Transmitter's (less extensive) configuration are available. If the AI500 Digital Interface
is installed then its Communicator port in the Safe Area is directly equivalent to the HHT connection at the Receiver. For more compre-
hensive diagnostic purposes the AI500 also allows the long-term records from the Dräger Polytron Pulsar 2s internal data logger to be
downloaded into a portable computer via the infrared Data Wand.
Besides measured values and flags, notice that the HHT shows when new data is received with a block in the top right corner of the Flags
display. This useful indicator pulses every few flashes normally, but only a few times a minute if the Receiver is not registering light from
the Transmitter.
Voltage and current are measured most conveniently at the terminations of the field cables in the Safe Area. Be aware, however, that volt-
age measurements here will not take account of the volt drops in the field cables. Direct electrical measurements at the Transmitter and
Receiver terminals will not normally be possible without a safety ('hot-work') permit. Such measurements can be misleading, however,
since the current consumption varies continuously with the internal heater and Transmitter charging cycles.
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