L60iF Issue 9 November 2018 11
6. Re-calibration
The L60iF uses a four point linearisation algorithm to compensate automatically for changes in restriction of
different filter media, and for increasing restriction as a filter collects particulate during sampling. The
calibration can be checked and corrected by the user providing that a flow meter (rotameter) of suitable
accuracy and a means of varying the restriction in the air path are available. Temperature calibration can also
be carried out if required. As this will only be affected by long term changes in pump performance, it is
recommended that it be only carried out infrequently, say twice a year, or after servicing of the pump.
Notes on accuracy
The accuracy of the flow indication of the L60iF will be almost as good, (but not quite) as the flowmeter used
to calibrate it. Consider a flow gauge with a full-scale reading of 100 litre/minute, the most likely range for
calibrating the L60iF. A typical commercial rotameter flow gauge will have accuracy to VDI/VDE 3513 Class
1.6. In practice this means that the accuracy will be 1.6% at full-scale (100), 1.8% at 65 and 2% at 45 l/min.
Thus over the working range of the L60iF, the calibration accuracy will be r2%.
This accuracy cannot be assumed from a single calibration. For full confidence the calibration would be
performed at least three times and readings of the calibrated flow gauge made over the working range of
the L60iF. If the errors are small and repeatable, the calibration can be accepted with confidence. Of course,
small and repeatable would need to be defined. For example, if the flow at any chosen calibration point was
always within r0.2 l/min and the difference was less than
0.1 l/min between runs, the error could be considered small and the repeatability acceptable. These figures
are illustrative only; the user might finally decide the figures to be used in relation to the work being carried
out. If there is not time for more than a single calibration, lower accuracy must be assumed.
At the other end of the scale, accuracy can be improved by having the rotameter calibrated in a test house,
which would provide a correction chart for any part of the scale specified by the user (more calibration
points, higher cost!) This higher accuracy would in turn be transferred to the L60iF. Comparison checks could
then be made at intervals to ensure that the instrument is maintaining its accuracy. The flow gauge would
need to be recalibrated at intervals not greater than 18 months.
Rotameter flow is measured from the top level of the float unless otherwise stated by the manufacturer. To
ensure maximum accuracy, read the float at eye level. This will avoid parallax errors. The accuracy of the
L60iF is only as good as the flow settings.
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