Here's a riddle for the masses (which I don't know the answer to). I have 3 meters, a HP 3478a bench meter, a U1253A handheld meter, and a Fluke 87-V. I'm curious to measure the input impedance of all of them (of course, I know that the agilent is 10Meg, the Fluke is 10Meg unless you select Hi-Z mode and the mV range, and the HP is 10Meg in the 30V and above ranges, but in the gigaohms in the lower ranges).
To measure A's impedance, I need to use meter B. But B sources some particular test current, which will present to A a voltage, thus potentially changing A's range automatically.
Let's start with the Agilent in ohms measuring the HP. The Agilent sources 1.85V in its highest ohms range, which leaves the HP in its 3V range, which is very high impedance (>500Mohm, which is all the agilent can tell me). However if I manually range up on the HP to the 30V range, I see the voltage sourced by the Agilent drops to .93 or so, and it shows 10Mohm on the nose. So far so good.
Let's swap. With the agilent on volts, the HP sources 1.14ish volts and reads 11odd Mohm. So far so good.
Now, the Agilent doesn't have a high-Z mode, but the Fluke does. If I plug them together, Agilent on ohms and flip Fluke to mV, I see 10Meg on the nose (11.1Meg in Volts mode). If I turn the fluke to mV while holding the Hz/% key (to put it in High-Z), both meters show overload (the Fluke because the Agilent is sourcing >600mV, the Agilent because the Fluke's impedance is >500Mohm). So again, so far so good.
Now the fun is when I try the same measurements Fluke vs HP.
With the Fluke in mV High-Z and the HP autoranging in Ohms, the Fluke says OL but the HP says 27Mohm. According to the Agilent (and anecdotes about what the real value is for the Fluke's high-z), the measured value should easily be overload, >500Mohm. What if I flip it around? Fluke in Ohms, HP in volts, HP shows 2.6 while the fluke shows 10Mohm [sidenote: this 2.6 is JUST below the hysteresis mark for the HP to range down from 30V to 3V, except as soon as it does, the impedance shoots up and thus the measured voltage with it, kicking it back to 30V range. This happens very quickly, and makes a chirping noise in the HP when it does, so if autoranging is on these two machines will just sit there flopping back and forth, chirping once or twice a second. Neat!]. 10Mohm on the 30V range is correct, so what if I range down to 3V? Suddenly the Fluke reads 27.3Mohm! That's not >Gohm at all!
In short, when measuring Agilent vs HP or vs Fluke, everything is completely normal. But when measuring Fluke v HP or vice versa, suddenly what should be many Gohms (well, overload ohms anyway) is appearing as 27Mohms, on both meters! What gives?! When the HP is in autoranging Ohms measuring the Fluke in mV or mV-HighZ, it shows 26Mohms as well! Oddly, with the Fluke mV (not high-z) range in overload, the HP seems to think it drops to around 3Mohms rather than 10M. Not sure what that is, either.
So the riddle is, why does impedance on the fluke seem to drop when the rang is in OL, and where is this magical 27Mohm coming from *on both meters* when I think I should be getting >Gohms?