While I have been using this meter I was never happy with the results. The last few days I have been thinking about how it could be improved. The dominant problem has been temperature drift. To combat this, the box has a fair amount of insulation around the sensitive area with a closed loop heater control. For all that effort, the results were not very good. A few ppm hear and there adds up fast.
I am going to attempt to flatten it out using a BAV199 and resistor, similar to what I showed with the UT61E. In the attached graph, the initial ramp is with the boards temperature increasing 2.5C with no compensation. The longer section was with my first attempt at taming it with the BAV199 and a 10K ohm. It will take some trial and error but the initial results look promising.
Dave should do a video showing some basic compensation techniques. I've been a fan of using 4148s for many years as a cheap method.
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The office can get fairly warm and to keep the internal temperature regulated, it will need to run above that. The systems drift is not linear. Next step is to sweep it over a wide range and plot drift and see is there is an area that looks like it may be easier to tame and be at least 10 degrees over my office on a hot day. It will take some time to run all these sweeps. It took several attempts to pick the combo I used on the UT61E.
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To compensate this circuit, I would normally use my HP34401A but the Brymen is more than stable enough in the 500K mode to detect the drift trends.
That's about the only thing that is going smooth so far...