16 Agilent ADM Flow Meter Operating Instructions
About Flow Measurements
When compared with readings from the ADM Flow Meter, measurements taken
with bubble-type flow meters may show a difference. This does not indicate a prob-
lem with your ADM Flow Meter. As a consequence of the ideal gas law, measure-
ments made with both soap bubble meters and the ADM Flow Meter are
temperature sensitive. However, bubble-type flow meters add water vapor to the
gas being measured, which introduces error into the flow rate measurement.
Even when accurately calibrated, soap bubble meters typically read slightly higher
than the ADM Flow Meter. The higher readings arise from relatively high concen-
trations of water vapor present in the soap bubble apparatus. At room temperature,
water vapor can raise readings by nearly 4%. This unfortunate property is amplified
by the effect of temperature. As the measured gas or the flow meter itself is
warmed, the amount of water vapor increases, and the readings can be much
higher than the true flow rate.
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