measuring Megohms using what DVM?

[PrescottDan]

When measuring in the megohms, most DVM meters just display O.L meaning overloading

The manuals on the DVM meters say they can measure more then 10meg ohms, but when measuring 150megs or 300 megs , the meters will just display O.L

When an Input Or output of a circuit is Open, how many megohms would it measure? 150Megs or 300 megs?

Analog Meters have a different input impedance, are these better to use when measuring megohms?

The plug in the wall bench type DVM have a different input impedance then the hand held Fluke DVM meter, when measuring in the megohms it just Displays O.L

 

[dick_freebird]

At some point you may be better off with a precision power
supply (or just an accurately measured one) and measuring
current (you can get picoammeters, but tough to find giga-
ohmmeters).

 

[betwixt]

OL = Open Leads = Nothing detected between the probes. Normally they would display zero if the probes were shorted or the resistance was 'under' range.

Up to about 200M a normal DVM can cope, beyond that you really need a dedicated instrument with low internal leakage or the effects of measuring mask the value you are trying to measure. An analog meter is usually significantly worse at very high resistances because the current used to displace the meter needle is so tiny. There are ways to take such measurments though, most using fixed voltage references and current monitoring as dick_freebird explained.

When an Input Or output of a circuit is Open, how many megohms would it measure? 150Megs or 300 megs?

Could be Giga Ohms, or even much, much, much higher. What is the resistance between the Earth and Moon for example!

Brian.