Re: Temperature monitoring

captainvideo462002@xxxxxxxxx wrote:
This is the application:
A nearby apple orchid apparently is in danger of having the trees
damaged due to an early Spring frost. Every year around May they have
gotten this box out. Inside is a temperature monitoring control unit
and what appears to be a low power radio transmitter. There is also a
probe, a thermistor I suspect mounted inside a small piece of PVC pipe
which is connected to about 50 feet of shielded cable. This is placed
out in the field. The transmitter pages out via an antenna which is
mounted in the orchard. The whole business is powered by a 12.0 volt
wet cell. This antenna was described to me as "being about ten feet
tall". So I'm assuming its a CB unit of sorts but until I run an SWR
test I won't know for sure. The temperature control unit has a switch
with positions from 26 to 35 degrees F. You select the temperature you
want to alarm at and a contact closure occurs. This powers the
transmitter and sends a signal to the pagers. The critical temperature
is 31 degrees F.

Lenny -

If I understand the application, you don't need accuracy, but you do need repeatability. In other words, if you want the unit trip when the true temperature is 31 degrees, but the unit tells you that the temperature is 27 degrees - who cares? Just set the trip point indicator at 27 degrees. Or even (dare I say it?) put a new label on the trip point switch. Essentially what you are doing is calibrating the unit.

Repeatability is a different animal. To follow the above example, if the true temp is 31 degrees, and the unit says 27 degrees, the next question is - what does it say tomorrow? And the day after? Does it still say 27 degrees? If it consistently says 27 degrees, then you have what you need. But if the reading drifts from day to day, you are in trouble.

Did that make sense? Accuracy, repeatability, and resolution are not the same thing.

Bill Jeffrey
.

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