| Description: | During 30 June and 02 July, 1994 a meteorological condition occurred that resulted in one
minute and 30 minute aspirator values to be below acceptable levels. As has happened
before, 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.
However, under such rapid change conditions it is virtually impossible to tell if T or RH
have been affected. I know from my past experience with the flow sensor in the aspirator
unit that 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, the flow sensor
indicates an improper flow condition. The latter is the culprit during the specified times.
There is a tremendous variation in raw output of the flow sensor as a result simply of
very small fluctuations in aspirator flow; the flow sensor is a very sensitive device.
Therefore, there are periods such as those below when aspirator flow will be indicated as
incorrect, when the the aspirator is probably functioning properly and all other data values
are correct.
The affected aspirator values are (times are GMT):
1 minute 30 minute
30 June 1994 0221-0224
02 July 1994 0547-0551 0600-0630
0553-0555
0557-0558
0610-0611
0613-0616
0619
0621-0623
0628 |