Nearside rear window goes down only

My Hall clamp is made by Fluke, no shabby name, and it too is finicky about zeroing in the lowest amp range, and also never quite gets there. Seems to be the nature of the beast. I'll often flip it around and average the two readings.

 

True enough. Plus there's drift due to temperature change.

I've got a bunch of amp clamps for diagnostics. Pico, Snap On, AEMC, Amprobe. The ones with physical adjustment knobs I can understand being off zero some when you power on (depending on what the range & temperature was previously).

The ones that have a "push button" zero function usually are at zero when turned on. The Uni T always has an offset when switching to DC amps (just checked - right now it's 0.341 amps & is the same on all ranges)

I can zero it by pushing a button, just find it odd that I have to.

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Exact method was:

1. set it to 2A range, DC mode
2. clamp around the rubber hose on the hinge side of the door
(this part didn't work well, the clamp closed, but not convincingly so)
3. zero button (verify that the display is at zero)
4. press switch and measure

Normally I zero before clamping, to measure the current already running in a wire, but in this case I wanted to zero any small one way current into or out of the door that might have been present before the door switch was enabled.

 

Could be a cost saving measure in the design.

The meter must always be zeroed before the last slight displacement to apply the clamp, so that whatever extraneous magnetic fields which are around don't throw off the measurement. So why add circuitry to zero it once at a random location in space, which is probably not the proper place to do that operation? At least for me, I usually hold the thing up in front of my face so that I can read it better when changing modes. The screen is pretty reflective, I'm often wearing polarizing sun glasses, and it has to be held just so so that I can read it clearly to verify that it is in the right mode.

If I had to design that circuit, on entering DC clamp mode it would show XXXXX or DOZERO or some other flag to indicate the unit was not yet zeroed. I wouldn't set it to 0.000 because the naive user might not then zero it properly when placing it for the measurement.