Extending the flowmeters calibrated range – an expert view

It’s not an uncommon request.  When the enquiry comes in, the client thinks the required flowmeter 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 flowmeters 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.

VFF Analysis:

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.

Linearisation:

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.

Here’s another, more extreme, example:

The client bought a series of LF15 VFFs in 2005 with  2500 pound flanges and a flow rate range of 1 to 3 litres per hour on 10cSt fluid. In 2017, on one of the lines, there was a change in viscosity and flow range.  The fluid became methanol and the flow rate range expanded to 0.5 to 10 l/hr. The change in viscosity affects positive displacement meter performance at the lower flow rates. In this case, an analysis of the performance curve from the original calibration indicated that it was unlikely to reach 0.5 l/hr on 10 cSt so extending the calibrated range was not possible.

Litre Meter constantly develop and improve the VFF. In 2012, the LF05 was introduced. It was followed in 2015 by the LF03. These are smaller rotors and chambers with lower flow capabilities but manufactured in the same size module as the LF15.

We were able to offer a replacement module with the lower flow ability, calibrated at the factory, for the customer to swap on site, leaving the 2500 pound flange body in situ. This dramatically sped up the upgrade process and met the new range requirements.

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