The topic of measurement uncertainty of comes up when talking about A+B mode when using two separate flow standards. The following discusses Fluke Calibrations position on this as well as a couple alternative calculations that might be considered:
Option 1 (recommended) - In A+B mode the same pressure transducers and PRT measurement circuits are being used regardless which molbloc is being read. The molbox is also using the same gas property table and the flowing gas will also be the same. Both molbloc's are also independently calibrated within the same spec typically. As a result, Fluke Calibration considers A+B mode uncertainty while flowing through two combined molbloc's to be the same as flowing through a single molbloc. The general uncertainty spec of 0.2% of reading (if using molbloc's with standard gas calibration) or .125% reading (with premium gas calibration) would thus apply to the combined flow reading the molbox would normally display in A+B mode:
Channel A molbloc reading 10 slm but high by 0.2% = 0.02 slm
Channel B molbloc reading 10 slm but high by 0.2% = 0.02 slm.
Sum of errors= 0.04 slm
Sum of flows = 20 slm
= 100 x (0.04 ÷ 20)
= 0.2%
Channel B molbloc reading 10 slm but high by 0.2% = 0.02 slm.
Sum of errors= 0.04 slm
Sum of flows = 20 slm
= 100 x (0.04 ÷ 20)
= 0.2%
Option 2 - Since two separate standards are being combined in A+B mode, there could an argument made that the uncertainties of each molbloc should be Root Sum Squared using the method below or similar. This will produce higher uncertainties than Fluke Calibration recommends and thus could be considered if a more conservative uncertainty spec is preferred for A+B mode:
Quadratic sum (RSS) @ k=1 method: each molbloc is 0.2% reading @ k=2, when you use RSS you bring that back to 0.1% @ k=1, so combining two molbloc's @ 0.1% @ k=1:
RSS = Sqrt (0.1² + 0.1²)
RSS = 0.141 @ k=1
= 0.282% reading uncertainty @ k=2
*** change numbers above if using premium gas calibrations for their spec of 0.125% @k=2 (0.0625% @k=1)
Option 3: Worst case uncertainty would be to directly add the uncertainty of the two molbloc's together. This is another method to consider for the most conservative uncertainty spec for A+B mode:
0.2% + 0.2% = 0.4% reading uncertainty
*** change numbers above if using premium gas calibrations to their spec of 0.125% @k=2 (0.0625% @k=1) It should be noted that when using A+B mode the molbox will take a reading from one channel/molbloc, with some delay as it switches channels, and then it will take a reading from the other channel/molbloc and combine the two readings together. This results in more time or less samples being taken compared to running a single molbloc. There could also be some additional instability when the valves are changed to allow the switching between channels. It is thus recommended to add additional dwell and averaging times to help ensure taking data when stable and get proper number of samples for data averaging. It is also typically recommended to adjust the stability settings to a lower setting so molbox will wait for better stability until it gives the ready state for data collection to further help ensure flow has truly stabilized in A+B mode.
How uncertainty is calculated should ultimately be determined by your company Quality Policy or similar as the above is just our method and some potential options to consider when determining your measurement uncertainties.