It’s not an uncommon request. When the enquiry comes in the client thinks the required range will be 1 to 10 but, once installed, it’s clear that he should have stated 2 to 20. Then, the question is: what do we do?
First: it’s unlikely that the range can be extended beyond the meter maximum rated flow rate. Some flowmeter principles can cope with excessive flow rates, probably at the expense of pressure drop, but most can’t.
Second: Consider the magnitude of change and the expectation of accuracy. If it’s a change from 1 to 10 to 1 to 11 with a 5% accuracy requirement when the maximum rate is 12 then that’s an easy one – check with the factory on how the range can be changed within the instrumentation but the range extension will be OK. On the other hand, if it’s a ±0.2% meter then it’s unlikely that any range extension will be within that value without a return to the factory for recalibration.
Here’s an example:
The client had purchased an LF03 VFF positive displacement meter for the measurement and control of corrosion inhibitor. The viscosity was 55cP and, although the meter is capable of 18 litres per hour, had specified an operational range of 0.2 to 2.3 l/h which we calibrated over.
A year later we had a request for a range change – could they up the calibration range to 5.6 l/hr? Of course, yes. Could we provide a statement to this? See below:
VFF Flowmeter Extended Range Accuracy – 50cP viscosity
The original calibration for meter VFF5112 was up to 2.3 l/hr on 55cP. Litre Meter have analysed the last 17 LF03s calibrated at or around 50cP and can confirm that re-ranging to 5.6 l/hr will have only a minor effect to system accuracy.
It can be seen from the aggregated performance curves above that extending the flow rate above 2.3 l/hr up to 7 l/hr produces little change in the meter linearity. We would suggest that, in the absence of any higher flow rate information above 2.3 litre per hour, that the meter is unlikely to be outside of ±2% of the 2.3 l/hr pulses per litre value up to 7 litres per hour. Increased confidence and accuracy can be obtained by recalibration.
Litre Meter produce a document LM0688 “Technical Description – Linearisation” that explains the linearization process and the flow rate versus pulses per litre table. Also here.