L60iF Issue 9 November 2018 5
Low and high flow alarms
If for any reason, the flow drops to a very low level, or rises to an abnormally high level it is desirable
that the test should be aborted.
Low flow
Apart from mischievous blocking of the air inlet, the most likely cause of low flow will be excessive
loading of the filter following an event that releases large clouds of dust or particles.
The supervisor can select an alarm level a few litre/minute below the working flow for the filter in
use. For example, if the flow is normally 64 l/min when the pump is hot, a low flow alarm limit of 60
l/min could be set. The limit can be set down to 20 l/min, if required.
The results of the test up to the time at which the alarm condition occurs will be stored and the code
LF appended.
High flow
The high alarm level is not set as an absolute value, because of wide range of flows likely to be
encountered in normal operation. For example, the cold airflow may be 5 l/min more than the hot
flow, and the flow will also be lower with a high restriction filter.
The high alarm level is set as the change of flow (increase) that must occur within a 2 second period
during sampling to trigger the alarm. Thus the change always ‘rides’ on the present flow setting. If
for example, the alarm level is set to 5 l/min and the cold flow is 69 l/min, the alarm level will be 74
l/min. If the airflow drops to 65 l/min for any reason, the alarm level will likewise drop to 70 l/min.
Apart from damage to the filter, the most likely cause of a high flow alarm is an extension hose
being accidentally detached from the front of the sampler. The results of the test up to the time at
which the alarm condition occurs will be stored and the code HF appended.
If a limit less than 1 l/min is attempted, the high alarm will be disabled.
Alarm Sounder
The audible alarm that sounds when a flow failure is recorded can be muted.
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