| Description: | There are two general conditions under which the Central Facility 60 meter tower aspirator
data indicates as lower than acceptable, even though aspirator flow is acceptable and
T/RH data are correct. I think that the explanations below should accompany all Tower
T/RH/aspirator data that is distributed, perhaps in the metadata. I would appreciate it if
the PRB would discuss this possibility.
During April 14, 1995 the Central Facility experienced high winds (10 to 14 m/s at ground
level), which resulted in one minute and 30 minute aspirator values being below
acceptable levels.
This is not the only condition that can cause the aspirator to falsely indicate
insufficient air flow. As reported in a DQR on July 8, 1994, during times of rapid change in
temperature and relative humidity (the former is mostly responsible) the aspirator value can
fall below acceptable levels, even though there is no evidence in the T/RH data that
aspirator flow has been reduced. During rapid temperature decreases, the heater in the flow
sensor cools off and does not respond rapidly enough to warm up to the acceptable
temperature that would indicate proper flow. The flow sensor determines flow by sensing the
temperature of the heater; if the temperature is within an acceptable range, which is
calibrated to flow, the unit indicates proper flow. If the flow is too small (such as from a
failed aspirator fan) or if the heater cools off from a rapid ambient temperature decrease
or enhanced ambient wind flow (which is the case in this DQR), the flow sensor indicates
an improper flow condition. There is a tremendous variation in raw output of the flow
sensor as a result of very small fluctuations in aspirator flow; the flow sensor is a very
sensitive device (almost too sensitive).
Therefore, there are periods when aspirator flow will be indicated as incorrect, even
though the aspirator is probably functioning properly. Data values are correct in this
situation, as they are for the data period listed above. |