Flow Meters - Tech Talk 13
Most flow meters are calibrated with no back pressure on the outlet side (unless specified). Rarely, however, are flow meters used without some back pressure. Because pressure compresses the gas being measured, the measured volume will change depending on the pressure at which it is being used. At high pressure, the meter may read five cfh, but the flow may actually be ten cfh. When used in vacuum applications, flow meters are off even further. Therefore, you should never expect the meter to provide an accurate reading, unless it has been calibrated for the same gas at the pressure and back pressure at which it is to be used. As inaccurate as these meters may be, they are good tools for "relative" flow adjustments and for resetting the same flow used in the past.