[volt-nuts] Two 4338B high resistance meters fail on the same range - Keysight can't adjust EEPROM of either.

[Dr. David Kirkby]

On 2 May 2018 at 14:25, <geoelectronics at rallstech.net> wrote:

> "This reflects my experience when using
> it to measure close tolerance resistors - measuring them at low voltages
> gives poor results, but at higher voltage, the resistances are measured
> more accurately."
>
> I noticed similar results in general, over years ad several instruments.
> Always wondered why, but we have a rule-of-thumb: if the part is to be
> used at X KiloVolts, test it at X Kilovlolt.
>

I am sure your advice is good, but I based measurements on the best
resistors I could find at sensible prices from Farnell.

The issue on the 4339B is the accuracy of the internal 0-1 kV voltage
source, if set to low voltages. I don't have the time at the moment to hunt
for the exact specifications, but the calibration certificate  give the
output voltages the meter is set to, and the voltage limits. I calculated
them as a percentage.

0 V -> +/- 0.1 V (infinite percentage)
10 V -> +/- 0.12 V (+/- 1.2%)
25 V  -> +/-  0.14 V (+/- 0.56%)
50 V -> +/- 0.18 V (0.36%)
100 V -> +/- 0.26 V (+/- 0.26%)
...(I will miss out 200 V, 201 V, 250 V for safe of brevity)
500 V -> +/- 1.3 V (+/- 0.26%)
1000 V -> +/- 2.1 V  ( +/- 0.21%)

The basic uncertainty of the instrument is 0.6%, but clearly if the output
voltage is set to a low value, the percentage error in the voltage is high,
so the percentage error in the resistance will be high. The above would
suggest using a voltage under 50 V is going to compromise accuracy and
using 100 V or more is better.

The uncertainty of the ammeter also depends on the range it is on, and not
surprisingly that has a higher percentage error on its lowest range (100
pA) than on its highest range (100 uA).

The Agilent 4339B is said to work from 1e3 to 1.6e16 ohm. Clearly to
measure 1000 ohm, any voltage above 0.1 V would exceed the full scale of
the ammeter on its least sensitive range (100 uA). Setting the source
voltage at 0.1 V is likely to result in significant errors reading a 1000
ohm resistor. But clearly a "high resistance meter" is not designed to
measure 1000 ohm resistors. I expect a £5 handheld multimeter from China
would do a better job at measuring 1000 ohm than what this instrument
does.

Sorry, I don't know about the HP 4328A. Personally I would have no concerns
about breaking the seals on an instrument of that age. I would consider it
prudent to check for any leaking electrolytic capacitors or other nastiness
that may reside inside an old instrument.

Dave