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Manual 111-082010_Nall Mass Flowmeters Page 17 of 23
8.2.2 Decrease in Line Pressure
The relationship stated in Section 8.2.1 also shows that DP increases proportionally as absolute line pressure
decreases. This situation is true until the DP becomes a significant part of the total absolute line pressure.
When this occurs, the relationship becomes more complex but the effect is still the same. The change in DP
does not affect the accuracy of the mass flow indications, but causes a change in the minimum upstream
pressure necessary to force a given amount of flow through the transducer.
8.3 DP Increase Due to Fouling of the Transducer:
A DP measurement across the transducer can often provide a good indication of fouling inside the
transducer. The values given in the Table on the preceding page are typical and may vary as much as 50%
for any given model. However, an increase in DP of two or three times the typical value is definitely an
indication of fouling. When checking for changes in DP, the flowmeter should be in operation at or near
standard conditions of 0oC and 760 Torr in order to relate the measured DP to those listed in the
Table. If there is an indication of fouling, refer to Section 10.1.