# How do multimeters measure very high resistances, and how does (human) body capacitance affect it? I would like a technical answer

Technically speaking, how does a multimeter (Fluke 289 in my case), measure impedance (resistive only), and why does touching a metal case you are measuring with your hand change the reading "drastically"? See below for details on the part I'm measuring.

My educated guess it that the meter is outputting a constant current, measures the volts across the leads that it has to generate to produce the current. Using V=IR (actually R=V/I) it calculates the resistance of the item being measured. Touching the metal object adds your body capacitance, and the current starts charging your body capacitance at a rate given by the resistance of the pin to case and your skin resistance etc. My question is, why do I witness the ohms going downward in some instances? Not switching leads. Repeated touches with finger.

In some instances, the parts we measure are above the 500 MΩ threshold of this meter. The meter shows OL. But when I touch the case, the meter shows impedance (450 MΩ for instance). I suppose that either a) no current is flowing in the OL case (therefore the voltage is outputting it's highest value), or b) the voltage/current combination is outside the parameters that the designer set for accurate measurement. And touching the part causes enough current to flow to bring the parameters into the range that the meter can measure.

Some of you will say, just don't touch the case. I get it. But I want to confirm my technical understanding of this phenomenon. And sometimes I have to hold the case, and I want to know how much I'm skewing the measurement by just holding it, and if this is repeatable (I don't believe it is).

Details on the part I'm measuring: I'm trying to measure leakage for a product line with a Fluke 289. The case is metal, around 1 cm in diameter, and 3 mm thick. There is are a few 2 mm diameter holes drilled, spaced a several mm apart. In each hole, there is a pin (for electrical signal) going through the hole, and a glass insulator material holding the pin centered through the length of the hole. In some of the products, we have electrical leakage occurring between the case and pin(s). When you measure this leakage with a Fluke 289 with the part on the table, it reads, say, 300 MΩ, drifting to say 350 MΩ, over the space of 1 minute. Not touching with your hands. However, when you touch the part with a finger, the ohm reading jumps down in the 100 MΩ range, and fluctuates. Generally it rises, but sometimes it goes down.

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• Is it maybe about the non-negligible DC resistance of the human and floor etc. in parallel with the high impedance? That or a small rectification effect from the large mains frequency currents through the human capacitor. Commented Aug 1 at 17:31
• For high resistance, you may benefit from a controlled electric field environment - something akin to a Faraday cage...and/or you may need a test fixture whose leads to the meter can't move about. A rant: autoranging ohmmeters ought to be banned...I've too often had these go squirrelly, unable to settle on a desired range. Have you tried measuring conductance? Commented Aug 2 at 1:14