Dwyer TFM-LP - Manual (Page 16)

F-20-TF-L, pg. 16 of 30
b) Flows Below the Minimum Rated Flow.
Signal outputs may be obtained at flow rates below the minimum specified
for your unit. The amount of flow that may be measured below the minimum
specified varies from unit to unit.
5. Zero Adjustments
It is impossible for there to be any zero drift so zero adjustments are never
required.
6. Recalibration
The calibration of the flow sensor may be easily verified using empirical
methods.
It should be noted that the use of empirical methods outside a certified
calibration laboratory is subject to inaccuracy and error. These methods
should only be used to gain an indication of a sensor’s performance. Please
contact Dwyer Customer Service if accurate, certified recalibration is
required.
A typical empirical calibration check may be carried out as follows:
Carefully adjust the flow rate so that the maximum output signal is obtained.
With the flow rate constant, liquid flowing through the unit should be
gathered in a container over a timed interval. The total volume flowed over
the timed period should then be measured using a measuring cylinder. The
actual flow rate (in the same units as the flow sensor calibration certificate)
may then be calculated. The actual maximum flow rate should then be
compared to the specified maximum flow rate to determine the error in
calibration. For example:
For a flow range of 20-200ml/min with a 0-5VDC output:
With a constant flow and constant 5VDC output the actual volume
measured over 30 seconds was 102ml.
Therefore the actual flow rate is:
(102 / 30) x 60 = 204ml/min
The error is therefore 204 – 200 = 4ml/min or 2%