Glyn and I have been going through the data here from the system we have access to to try and work out how prevalent this issue is. It does seem that it’s affecting a larger number of kamstrup units than we thought.
50% of kamstrup systems are not showing the issue, 22% of systems did show the issue early on but the issue has now resolved and 28% of systems still appear to show the issue to varying degrees.
The Sontex systems do not appear to have this particular issue, at least not visibly, though the DECC report below suggests that air issues can manifest as an error in the readings rather than no readings. There is another thread on here about potential issues for Sontex meters if there is dirt in the system, see Sontex heat meter accuracy over time.
We think that most of the issue is likely caused by remaining trapped air in the system and this might explain why 22% of systems improved as air is removed over time. There’s probably linked issue with turbulence caused by flow disruptions here as well, perhaps if there is air, other disruptions may have greater effect?
The systems that improved suggest that there is hope for resolving this issue. We will update our guidance to highlight that special care is needed here.
This DECC report is interesting and discusses the issue of air (gas entrainment) and highlights some of the other challenges with ensuring accurate heat measurement
An_Investigation_into_Heat_Meter_Measurement_Errors_Final_Report_AECOM.pdf (5.1 MB)
Laboratory Tests
To provide an indication of the effect of free gases on the test meters, air was deliberately introduced into the test rig and the reaction of the meter noted.
Little effect was observed with the turbine meter, but more significant effects were seen with the ultrasonic and vortex meters.
The ultrasonic meter stopped reading and showed an error code. The error code is a standard feature of
the meter and is intended to alert the user to a problem rather than attempt to give spurious readings.The vortex meter showed a relatively small error at low air volumes, increasing as the volume of air
increased, eventually causing the reading error to exceed the MPE for a Class 2 meter.