Once in a while I have an opportunity to help a customer with a technical question when the applications team is overwhelmed with calls. Today I had such an opportunity, and I found myself explaining to a customer why two flow meters read differently under what appear to be the same conditions. This comes up quite a bit so I thought I’d write about it.
Why two mass flow meters or controllers don’t agree is a great question and not something people really think about until they are faced with the issue. It’s like looking at two identical copies and trying to figure out which one is a fake. You know one is a phony, but you can’t figure out which one. Or worse, what if they are both phonies?!
I’m going to take a stab at briefly explaining why you can get different readings from two different mass flow meters, under seemingly identical flow conditions.
The first thing that you’ll need to be aware of is the standard conditions to which your particular devices are calibrated. There is no single correct standard temperature and pressure (STP) so each manufacturer takes an educated guess as to what their customer would like to have. Some choose 0C and 1ATM, others choose 25C and 1ATM, while yet others choose 21.1C and 1ATM. The STP defines the amount of molecules of gas in a given volumetric amount of gas under a given set of temperature and pressure conditions. If one meter is using 0 C and 1 ATM as its definition of mass flow, and the other is using 25C and 1 ATM, then identical volumetric flow rates through those two meters may appear to have very different mass flow rates based upon their difference in STP.
The next most commonly overlooked component is dead volume. Dead volume is the volume of tubing that connects the two devices being compared together. If you start your flow source and immediately start to compare the two devices it is not unusual to see a rather large difference between the two devices. You will need to allow time for the dead volume to fill completely prior to taking measurements. Depending upon how the process is arranged it could take some time. The best practice when comparing two devices is to reduce the dead volume to an absolute minimum. We recommend that users quite literally connect one unit to another via a common threaded fitting and eliminate any tubing altogether.
Although there are a number of other factors that come into play when comparing two mass flow meter or controllers, the two that I’ve described above tend to cover the vast majority of problems that customers encounter.
If you find yourself in this unenviable position, tried what I’ve suggested, and are still having trouble then give us a call or write us an email. We would be happy to try to help you figure out why your mass meters or controllers can’t seem to agree.