Here are specs from an older General Radio Bridge for observation.

GR 1644-A specifications:

Resistance Range: 1 kΩ to 1000 TΩ (10^3 to 10^15 Ω) in ten ranges.
Accuracy: 10^3 Ω to 10^10 Ω, ±1 %. After self-calibration: 10^10 to
10^12 Ω, ±1%*; 10^13 Ω, ±2%; 10^14 Ω, ±10%; 10^15 Ω, ± one scale
ΔR% Dial: ±5% range; accurate to ±0.2% or, for small changes,
to ±0.1%.
Test Voltage: Voltage accuracy is ±3% ±0.5 V.
Fixed Voltages** 10 20 50 100 200 500 1000 v

Best regards,


-----Original Message-----
From: volt-nuts [] On Behalf Of ed breya
Sent: Friday, March 02, 2018 12:23 PM
Subject: Re: [volt-nuts] Precision high resistance measurements / calibration 
of HP 4339B high-resistance meter.

Oops - I think I didn't send this message properly yesterday - here goes again. 

Yes, David, unless you go to very extreme measures, you won't see real R values 
that have any practical meaning beyond E12 ohms or so. Most practical 
insulation Rs may be around E12-E14 tops, unless you go to sapphire. Up in that 
region, the R may be all within a material, or include surface components like 
a film of dirt or moisture, or a fingerprint.

E11 resistors can be made to fairly high precision, and maybe E12 nowadays. In 
the old days, higher values were made by stacking E11s - like ten in series to 
get E12 with decent precision. The glass packaging also limits how high it can 
go, due to leakage within and on the surface. I once used a glass reed relay 
capsule as an ultra-high resistance in a circuit. There was no precision or 
stability at all, but it made a nice high resistor (probably E14-ish dry) even 
though there was no element in there, and the circuit didn't care, as long as 
it was very high, but not infinite.

The specs on this HP unit are likely just the most extreme capability taking 
maximum voltage over minimum current resolution, but any measurements would 
tend to be very noisy and unstable anyway. Also, testing at the extreme 1 kV 
makes the numbers seem more impressive, but the voltage coefficient of 
resistance will pretty much be unpredictable.

If this is a digital meter, then the other spec trick that tends to obscure the 
real performance limit is that the ultimate resolution and noise is that last 
digit - or even last two or three - that may may be pretty jumpy, unless very 
long averaging time is used.

There may be newer, fancier electrometers nowadays, but Keithley used to be the 
standard for these in the old days, before several digits of DVM resolution 
complicated the specs. They had a vibrating capacitor electrometer with 
all-sapphire input structure back in the 1970s/80s I think, that was the 
epitome of electrometers. I forget the model number, but vaguely recall that it 
could reach the aA region full scale - not that last digit of resolution thing. 
It's long obsolete, and I don't think they ever made anything actually better - 
only added DVM digits to less capable, conventional semiconductor amplifier 
techniques. If you can find info on it, it's an interesting read. I found a pdf 
of the manual years ago, but have no idea where it is now, or what info may 
still be around.


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