Mass Flow Meters (mfm) Check 02/06/2003

The LST has to work with a certain gas mixture. We choose to use the ternary gas mixture Ar/C4H10/CO2 ( 2.5/9.5/88 % or 3/8/89 % ). In order to make this gas mixture we used the flow mass meter with the following maximum values (FS) of the flux:

Gas	FS (cc/min)
Ar	50
C4H10	50
CO2	200

We don't know if the values of FS indicated are right, then we need to check the behavior of the mass flow meters.

For this porpouse we setup a sort of "instrument" (made at home) to measure the flux regulated by mass flow meters. In the following we report a scheme and two pictures of this instruments:

	The instruments consists of a graduated bottle with two holes: inlet gas and outlet water as indicated in the scheme. To measure the flux the bottle has to be full of water, then we flow the gas and we measure the time and the volume of water which goes out from the bottle. The ratio between volume and time is the flux.

In order to have more information about the behavior of the mfm we will measure the flux for different fluxes fixed in the mfm. Moreover we have to be sure that the variation of the pressure inside the bottle due to flowing out of the water will be negligible, then we will measure different volume for the same fixed flux. The following plots show volume versus time and flux versus volume (or time).

In the following plots the percent x% means that we fixed the mfm at x% of the maximum value of the flux.

The mfm used for the Ar is wrong, the maximum value of the flux is about twice of the indicated value (50 cc/min). Moreover it shows a non-linear behavior for different fluxes, in fact at fixed 25% of FS we measure a flux which is about 1/3 of the maximum measured flux.

The mfm used for the isobutane shows a maximum measured flux which is just a little bit little than the indicated value (45 cc/min instead of 50 cc/min). The behavior seems to be linear for different fluxes. We choose to measure also at 15% instead of 25%, because that is about the working point. Only one point at 15% has been measured, because it needed a lot of time.

The mfm used for the CO2 works well.

From these measurements we conclude that for the Ar we have to change the mfm and check the new ones. For the isobutane we could repeat the measure near the working point, then we have to take into account the quite difference from the indicated value. For CO2 the mfm works well.

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