Because any power cycle or temperature cycle expected to cause output
shift due to hysteresis and these standards designed to be always
powered on without any interruptions, otherwise uniform constant drift
characteristics cannot be guaranteed. IN CAL LED circuit monitors power
and thermistor value, so when your battery depleted below threshold, or
temperature outside of the normal window LED will be turned off. Fluke
recommends to perform recalibration after standard lost IN CAL LED
status. Typical battery life of 732B is ~3 days and that can be extended
once you have external chargers/+12V DC supply for backup power.
This concept applies not just for 732 series standards, but for any
voltage standard which try to claim ppm annual stability. That also
includes 8.5-digit DMMs like 3458A and multi-function calibrators like
5720A/5730A. Less stress changes in temperature/power - longer life for
instrument. Drift might be bit more, but it would be predicable once
characterized properly.
On 8/6/2019 11:03 PM, Victor Silva via volt-nuts wrote:
Can anyone explain why the Fluke Voltage standards, like 732B, which I use
as my primary standard, turn off the IN-CAL LED when they lose power
(including battery)? I believe the same is true for the 732A, but I do not
have one so I'm not sure on that model.
My 732B lost power during the hurricane in NC last year and the CAL LED is
off. I realize I can press the reset button to turn it on, but that doesn't
really make it in cal if it was out.
Is the CAL LED really just an indicator to warn that the unit has lost power
or does the unit actually go out of spec after a power loss?
I can see a scenario where there was a power loss then power came back,
without that LED you could use the standard before it stabilized.
But if you allow it to stabilize for some time, my question is will it be in
CAL still [but with the LED off]?
Thanks,
Victor Silva
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