Author Topic: Multimeters With Low Ohms Function  (Read 8380 times)

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Online David Hess

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Re: Multimeters With Low Ohms Function
« Reply #25 on: January 17, 2022, 07:05:21 am »
Some old school meters have a "Low ohms" function that has a compliance voltage below 0.6V so it doesn't switch on semiconductor junctions in-circuit. Seems to have fallen out of favour since the 80's and 90's. My first digital meter, a Soar had it.

I bought my Tektronix DMM916 in the late 90s and it has 4 "resistance" modes:

Ohms   860 millivolts
Low Ohms   280 millivolts
Continuity Test   3.2 volts   Reads out ohms
Diode Test   3.2 volts   Reads out volts

My Beckman RMS225 from about the same age has:

Ohms   650 millivolts
Continuity and Diode Test   3.2 volts   Reads out volts
 
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Re: Multimeters With Low Ohms Function
« Reply #26 on: January 17, 2022, 07:12:47 am »
Ok the tests took less time than I thought, so here are the results.



Thanks for those.
The V in High voltgae will be reduced by divider of the 10M input impedance of the meter unless you used a High-Z meter input.
 

Offline ermionesrl

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Re: Multimeters With Low Ohms Function
« Reply #27 on: January 17, 2022, 12:07:35 pm »
Hello.
May I add some measurements of my own multimeters?

(EDITED: sorry, I mistakenly wrote some units, this is corrected table)

Meter; V in Cont; A in Cont; V in High R; A in High R; V in Low R; A in Low R
Fluke 177; 2.6941; 980.4u; 1.2439; 120n; 1.2440; 311.5u
Fluke 17B+; 0.5316; 159.2u; 0.5301; 45n; 0.5304; 158.8u
Fluke 15B; 0.4410; 140.7u; 0.4408; 45n; 0.4410; 140.7u

I used the Agilent 34401A in >10Gohm input mode, both for voltage and for current. For current, a 100 ohm 0.01% metal foil resistor shunt has been added. In some time I could eventually repeat the measurements with an electrometer, for better current values. I have a second 34401A and eventually I can measure it with its twin.

AVGresponding: you have used the Fluke 289, but its 10Mohm input voltmeter impedance made your measurement of high ohm V almost surely incorrect, the current source got loaded. You see that even the >10Gohm of 34401A (very) slightly load it.
I think that in most (if not all) cases, the burden voltage is the same between low and high ohm. As you own a 34401A, maybe you can repeat the measurements with it at >10G.
« Last Edit: January 17, 2022, 02:06:13 pm by ermionesrl »
 

Offline AVGresponding

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Re: Multimeters With Low Ohms Function
« Reply #28 on: January 17, 2022, 06:02:56 pm »
The V in High voltgae will be reduced by divider of the 10M input impedance of the meter unless you used a High-Z meter input.

AVGresponding: you have used the Fluke 289, but its 10Mohm input voltmeter impedance made your measurement of high ohm V almost surely incorrect, the current source got loaded. You see that even the >10Gohm of 34401A (very) slightly load it.
I think that in most (if not all) cases, the burden voltage is the same between low and high ohm. As you own a 34401A, maybe you can repeat the measurements with it at >10G.


Fair point, I should have thought of that. Just got home from work, and I'm warming up my Fluke 8840A (it's the most recently calibrated high-impedance meter I have).
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Offline Neomys Sapiens

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Re: Multimeters With Low Ohms Function
« Reply #29 on: January 17, 2022, 06:31:28 pm »
One can still test for shorts in circuit, if the open circuit voltage goes higher. A diode junction would still not measure very low ohms, but what you get with some 0.6 V at a resistor and this is usually quite high.

There are 3 points with a limited voltage/power: one is protecting sensitive parts that can not withstand a higher voltage and could be damaged (e.g. BE junction in reverse from more than some 6 V, some LEDs - at least according to the abs. max specs and some were actually sensitive to > 5 V).

The other point is measuring sensors like PTCs / NTCs, where the test current may already cause significant self heating. A small PT1000 tested with 1 mA test current is not ideal. A 10 K NTC with 1 mA test current can be way off. 

The 3 rd point is dry contact testing, so testing the low votlage performance of contacts, where a higher voltage can destroy an oxide layer at the contact and this way hide a contact problem. This dry ohms testing wants a rather low maximum voltage, often realized with a prallel resistor.

A high voltage is nice for diode testing to also measure a blue / white LEDs and maybe a 5 V zener.
On the other side high resistor ranges may need a relatively high voltage to gets low senstivity to hum.
Another very good reason for a low test voltage and limited energy would be the presence of squibs/detonators.

That said, both the Tektronix DM501A and DM502A have a HI/LO section switch for their kOhms ranges. For LO, maximum voltage is stated to be 0.2V.
The Siemens B1043 does it differently: when selecting a Resistance range, a diode symbol is shown if the voltage is above 0.5V. If none is shown, it is a 'diode safe' range.
The Metrawatt M2036 just gives a diagram in the manual from which it can be seen that the open-circuit voltage on all ranges except 30MOhms should be <400mV.

 

Online David Hess

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Re: Multimeters With Low Ohms Function
« Reply #30 on: January 17, 2022, 08:27:39 pm »
Tektronix DMM916:

Ohms   860 millivolts   0.41 milliamps
Low Ohms   280 millivolts   0.03 milliamps
Continuity Test   3.2 volts   0.41 milliamps   Reads out ohms
Diode Test   3.2 volts   0.41 milliamps   Reads out volts

Beckman RMS225:

Ohms   650 millivolts   0.78 milliamps
Continuity and Diode Test   3.2 volts   3.2 milliamps   Reads out volts

Tektronix DM501:

Ohms   10.6 volts   1.0 milliamps   2k ohms range

Tektronix DM502:

Ohms   11.2 volts   1.0 milliamps   200 and 2k ohms ranges

Another very good reason for a low test voltage and limited energy would be the presence of squibs/detonators.

Electrical squibs and detonators are deliberately designed to be as difficult to trigger as possible.  They require high voltage at high current.
« Last Edit: January 17, 2022, 08:54:22 pm by David Hess »
 

Offline Neomys Sapiens

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Re: Multimeters With Low Ohms Function
« Reply #31 on: January 17, 2022, 09:42:14 pm »
The DM501 and DM502 don't have that switch. The 'A' models are completely different.
-> TEKWiki
 

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Re: Multimeters With Low Ohms Function
« Reply #32 on: January 17, 2022, 09:55:12 pm »
Well, that was as much work as doing the whole test before! The settling time is very long, due of course to the low currents involved. Two meters refused to settle, rising to a peak over a number of seconds, then falling continuously (but very slowly) beyond the point of my patience and in the case of the Metrix, beyond the limit of the auto power off.
There is a broad pattern, but with exceptions, as you'd expect.
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Offline ermionesrl

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Re: Multimeters With Low Ohms Function
« Reply #33 on: January 18, 2022, 03:00:43 pm »
AVGresponding:
Interesting: it seems that many multimeters use the same way as 34401A to measure >10Mohm, by inserting a parallel known 10Mohm resistor and do calculations. It's evident when the open circuit voltage is the result of the test current over 10Mohm, and when your previous measurement is the result of the test current over 5Mohm.
About the fact that some didn't settle, I think that the voltage source is not very well stabilized and thus reflect the battery voltage, that fall continuously during the test. If they use a ratiometric method against a known resistor (like original Fluke 87), both the current and the voltage need not to be stable over a timespan longer than the 2 adc samples.
 

Offline Neutrion

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Re: Multimeters With Low Ohms Function
« Reply #34 on: January 18, 2022, 03:01:47 pm »
If the beeping function is as fast as on the Brymen, you can just start to probe around any circuit for shorts, to see wether you get any short false beeps where there are no shorts. But again, this only happens if the beeper function is fast enough. But if you did not design your circuit to avoid it, it would be a wonder if it would work.
the bip on the shorty has an audio tone proportionnal to the value of the short. so if there is a charging capacitor, the tone is changing, and you can hear it.

So there are no sudden beeps with your gear at all where there is no continuity, like with fast meters?
Is it latched?

Well, that was as much work as doing the whole test before! The settling time is very long, due of course to the low currents involved. Two meters refused to settle, rising to a peak over a number of seconds, then falling continuously (but very slowly) beyond the point of my patience and in the case of the Metrix, beyond the limit of the auto power off.
There is a broad pattern, but with exceptions, as you'd expect.


Nice spreadsheet, thanks for it!
I also measured the BM789, but I don't have a high Z meter so I measured with the scope instead.

Vcont (with 10MOhm meter) 2,38V  p-p (with scope) 2,56V    A in cont (with 10Ohm meter) 0,35mA   V High Ohm (with 10MOhm meter) 0,221V  p-p(with scope)  3,52V   V in Low Ohm (with 10MOhm meter) 2,38V   p-p(with scope) 2,56V   A in low Ohm 0,35mA.

Edited to make it more clear.

It seems to differ from its specs as the open circuit volts in Ohms supposed to be less than 1,3V and less than 1,5 V for the 600 Ohms range.
And there is no nifference in manual range.

So only NTCs and PTCs are problematic?
« Last Edit: January 18, 2022, 07:07:33 pm by Neutrion »
 

Offline AVGresponding

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Re: Multimeters With Low Ohms Function
« Reply #35 on: January 18, 2022, 06:03:22 pm »
AVGresponding:
Interesting: it seems that many multimeters use the same way as 34401A to measure >10Mohm, by inserting a parallel known 10Mohm resistor and do calculations. It's evident when the open circuit voltage is the result of the test current over 10Mohm, and when your previous measurement is the result of the test current over 5Mohm.
About the fact that some didn't settle, I think that the voltage source is not very well stabilized and thus reflect the battery voltage, that fall continuously during the test. If they use a ratiometric method against a known resistor (like original Fluke 87), both the current and the voltage need not to be stable over a timespan longer than the 2 adc samples.

Given the two meters that were the most difficult in this regard were the Metrix MX 57EX and the Agilent U1401B, I think this unlikely. The Agilent in particular is a process calibrator; its outputs are in my experience very stable. There must be another explanation, perhaps the meters are cutting the current back once the ohms over-range is detected for an extended period, in order to extend battery life a fraction?
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Offline Neutrion

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Re: Multimeters With Low Ohms Function
« Reply #36 on: January 18, 2022, 07:28:07 pm »
When you measure the volts with the BM869 in high Ohms range, is there any difference, if you measure the peak value with a scope?


Should not the name of the thread (and any future video) include the word power or compliance voltage?
As I read it now it sounds like a meter that can show low resistances, like a 4-wire measurement or the dedicated low ohms mode of the Fluke 289?
Good point. And the whole topic started because of the false beeps on continuity mode, which we forgot.
Hovewer it is also an interesting topic, and that is what can be annoying with many meters.
« Last Edit: January 18, 2022, 07:32:36 pm by Neutrion »
 

Online Kleinstein

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Re: Multimeters With Low Ohms Function
« Reply #37 on: January 18, 2022, 08:19:06 pm »
Ideally you want the choice:
High current is wanted to measure small resistors.
High voltage is needed to test higher voltage diodes, maybe even zeners and also high resistors work better this way.
A high voltage (e.g. >5 V) may however damage sensitive parts (e.g. LEDs, Transistors with BE in reveerse).
Not too high a current may be preferred for very small, temperature sensitve resistors.
A very limited voltage is needed for testing contacts in dry ohms, not braking through oxide films.
So no one setting is good for all. Only a few meters have different setting for the different jobs. For the usually limited resolution of hand held DMM the power from the test current is rarely a problem. It is more with some bench meters where the power could go over the top, like 10 mA with 12 V open circuit (and maybe 8 V with current ?) for the 3458 in the 10 Ohms range.

The beeps with a capacitor are a 2 sided thing: they can be confusing if you don't know about it. The can even help to detect capacitors. With a fast reacting beeper, there is no good way around it - waiting for a stable voltage would be a bit over the top and make the reaction slow.
 
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Offline AVGresponding

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Re: Multimeters With Low Ohms Function
« Reply #38 on: January 18, 2022, 08:36:57 pm »
When you measure the volts with the BM869 in high Ohms range, is there any difference, if you measure the peak value with a scope?

I'll give it a poke. I'd expect the relatively low impedance of a scope/probe to have a marked effect.
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Offline Neutrion

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Re: Multimeters With Low Ohms Function
« Reply #39 on: January 18, 2022, 08:50:46 pm »
Ideally you want the choice:
High current is wanted to measure small resistors.
High voltage is needed to test higher voltage diodes, maybe even zeners and also high resistors work better this way.
A high voltage (e.g. >5 V) may however damage sensitive parts (e.g. LEDs, Transistors with BE in reveerse).
Not too high a current may be preferred for very small, temperature sensitve resistors.
A very limited voltage is needed for testing contacts in dry ohms, not braking through oxide films.
So no one setting is good for all. Only a few meters have different setting for the different jobs. For the usually limited resolution of hand held DMM the power from the test current is rarely a problem. It is more with some bench meters where the power could go over the top, like 10 mA with 12 V open circuit (and maybe 8 V with current ?) for the 3458 in the 10 Ohms range.


I hope if Dave does the video, he will cover also some practical and theoretical cases for damage.

But from the cited document above, with the 100uA "for less damage" so far we only got  PTC and NTCs if I understand right. Or are there any other  "temperature sensitive resistors" which we can encounter?
And I don't mean the 12V with 10mA, but the average handheld meters current and voltage.



The beeps with a capacitor are a 2 sided thing: they can be confusing if you don't know about it. The can even help to detect capacitors. With a fast reacting beeper, there is no good way around it - waiting for a stable voltage would be a bit over the top and make the reaction slow.

I was thinking about doing two really fast measurement. If the second shows less current, than there would be  no beep because of capacitance. If they are equal, than comes the beep. Obviously it would need a dedicated circuit.
 

Offline floobydust

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Re: Multimeters With Low Ohms Function
« Reply #40 on: January 18, 2022, 09:42:37 pm »
With low power ohms, I find it's mainly because you need the ability to differentiate between presence of a semiconductor junction or ohmic resistance - in-circuit. For the compliance-voltage limiter.
Damaged semiconductors frequently test as if they're a leaky diode-junction. With multimeters, there is a resistance (say 500R) on diode-test that looks like a good junction. There is a Vf that looks like a resistor. You have to know these values.

Transistor has three leads, six possible readings (both polarities) and double that if you are using both ohms and diode-test. If repairing gear with many BJT's like audio power amplifiers, missing one damaged transistor can be very costly. They will almost test OK, especially some non-symmetrical VBE and VCB junctions which are usually quite close in Vf but not with damaged parts. Or resistive leakage (carbon) is present. Or one junction is low or high Vf. Have to be really thorough sometimes.
 

Offline ermionesrl

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Re: Multimeters With Low Ohms Function
« Reply #41 on: January 18, 2022, 09:45:04 pm »
AVGresponding:
This is very puzzling then. I have not been able to find the schematic of either meter to check, yet it would be very strange if this was intentional. In this case, you would almost surely see a step change.

Neutrion:
If you used a regular x10 probe with your scope, its input Z should also be 10Mohm so your results shouldn't differ. In general scope readings are less accurate than any meter, in particular if your scope has a 8bit adc.

About variable tone beeping, in Art of Electronics II edition, page 954, there is a circuit useful to discover shorts without desoldering parts, measuring millivolts (caused by an external current source). This circuit uses a meter, but I remember somewhere I saw a more interesting version that emitted a proportional tone instead and had an internal current source.
 
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Offline Wallace Gasiewicz

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Re: Multimeters With Low Ohms Function
« Reply #42 on: January 19, 2022, 12:28:29 pm »
Just for attempting to be complete:
My old Fordee PSM 37 is analog and has the low voltage ohms. Manual states it is .094 Volts for Low Ohms.
It has a 20 ohm mid scale reading so it is not that great for anything below 5 ohms.
This unit is essentially a JFET input device, so it has characteristics of both the usual modern multimeters with an analog meter.

Wally
 

Offline AVGresponding

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Re: Multimeters With Low Ohms Function
« Reply #43 on: January 19, 2022, 02:34:40 pm »
AVGresponding:
This is very puzzling then. I have not been able to find the schematic of either meter to check, yet it would be very strange if this was intentional. In this case, you would almost surely see a step change.

Neutrion:
If you used a regular x10 probe with your scope, its input Z should also be 10Mohm so your results shouldn't differ. In general scope readings are less accurate than any meter, in particular if your scope has a 8bit adc.

About variable tone beeping, in Art of Electronics II edition, page 954, there is a circuit useful to discover shorts without desoldering parts, measuring millivolts (caused by an external current source). This circuit uses a meter, but I remember somewhere I saw a more interesting version that emitted a proportional tone instead and had an internal current source.

You'd be unlikely to see a step change because of the very low currents involved, 41 nanoamps and 55 nanoamps for the Agilent and Metrix respectively, and the very high impedance (≥10GΩ) means that even with very low input capacitance it takes a while for the indicated voltage to drop.
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Online Kleinstein

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Re: Multimeters With Low Ohms Function
« Reply #44 on: January 19, 2022, 03:02:16 pm »
The high Z meters have a high input resitance, but often also some input capacitance that may start in some not so well defined state. So one can still see an initial change in the voltage from the capacitance and just a possible slow setting to a final voltage. For the open circuit voltage the ouput impedance of the meter under test may no longer be that high in resistance even with a low current setting.
 

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Re: Multimeters With Low Ohms Function
« Reply #45 on: January 19, 2022, 07:22:37 pm »
Here's a couple of pics of the BM869S high ohms output as seen by an oscilloscope.



The high Z meters have a high input resitance, but often also some input capacitance that may start in some not so well defined state. So one can still see an initial change in the voltage from the capacitance and just a possible slow setting to a final voltage. For the open circuit voltage the ouput impedance of the meter under test may no longer be that high in resistance even with a low current setting.

Yes, I might have a go at measuring the input capacitance, since it's not specified in the manual (for DC anyway, AC specifies <100pF).
« Last Edit: January 19, 2022, 07:24:25 pm by AVGresponding »
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Offline ermionesrl

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Re: Multimeters With Low Ohms Function
« Reply #46 on: January 20, 2022, 10:34:18 am »
Kleinstein, AVGresponding:

What I expect to be inside the multimeters is a voltage limited, step controlled current source. The current is controlled (influence measurement) and different on many ranges, while the voltage is just limited (by giving a limited voltage input to the current source) and has not any requirement for accuracy or stability (in the 34401A the voltage is derived from +18V power with a simple 5.1V zener, I don't think in the U1401B took care of deriving it from a reference).

Unless there is a reason (like not to push semiconductors in conduction on some ranges, or the opposite) I expect it to be the same for all ranges even if might seem to have changed, in case multimeter insert a parallel resistor in highest range.

About falling time, if total capacitance is 10nF and total "leakage" resistance is 10Gohm, we would have a tau of 100s. Long, in effect, but I suspect both values being lower, in particular capacitance, so you should see voltage to settle in a reasonable time, if it is really stabilized.

It would be interesting to test again the U1401B with different condition batteries, to see if and how much the testing voltage change, as well as in the last but one range.
 

Online Kleinstein

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Re: Multimeters With Low Ohms Function
« Reply #47 on: January 20, 2022, 12:39:51 pm »
For the input capacitance I would expect maybe 100 pF to 1 nF. When at the limit the current source would also be no longer very high impedance. This is what limits the voltage.
depending on how the current shouce is made, the voltage may change. A common configuration is to start the current source from the positive supply and than get a defined drop on a switchable resistor. The voltage drop can change with ranges, with possibly less voltage lost in the lower current ranges in a way of switching rangs.  The 34401 uses such a circuit. So there can be a slightly different maximum voltage with the ranges.

Most handheld meters have a fixed voltage (like 200 mV with 2000 count) for the full scale reading on the ranges.
The fixed range corresponds to the configuration with a fixed votlage range that can be measured without a divider in front and thus with very high impedance.
Better bench DMMs have a higher voltage with the higher resistance ranges (e.g. 10 V for 20 M). This avoids very small current sources for the high ohms part.
 

Online David Hess

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Re: Multimeters With Low Ohms Function
« Reply #48 on: January 20, 2022, 01:57:59 pm »
A common configuration is to start the current source from the positive supply and than get a defined drop on a switchable resistor. The voltage drop can change with ranges, with possibly less voltage lost in the lower current ranges in a way of switching rangs.  The 34401 uses such a circuit. So there can be a slightly different maximum voltage with the ranges.

In older multimeters, the configuration I regularly see bootstraps the switchable input divider (1k, 10k, 100k, etc.) to apply a fixed voltage across it.  So the operational amplifier buffers the voltage at the high input, and applies that voltage plus a fixed voltage to the switchable input divider and applies it back to the high input.  The example below is comparable.

Autoranging multimeters which use a grounded input divider must be doing it in a different way.
« Last Edit: January 20, 2022, 02:20:37 pm by David Hess »
 

Offline AVGresponding

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Re: Multimeters With Low Ohms Function
« Reply #49 on: January 20, 2022, 02:14:53 pm »
Well, I haven't charged the battery in the U1401B for some time, so I'll top it off, but I doubt it'll make any measurable difference.

I'm warming up my main LCR meter for a couple of hours, then I'll get a decent reading on the Fluke 8840A input capacitance.

Tried to get a nS reading on the Fluke 8840A to work out the input impedance, but both the Fluke 289 and Fluke 87V gave a settled reading of 0.00nS, and the Brymen 869S wouldn't settle, fluctuating between 0.02 and 0.07nS. Either way, It looks like the ≥10GΩ impedance is more > than =.

Also had to edit the spreadsheet, as I realised the Tek 912 had a LV ohms mode...   :palm:



EDIT: The Agilent U1401B with freshly charged battery peaked at 4.2788V, with an identical 41nA current. The voltage drops off at two or three hundred microvolts per minute, much more slowly than if the Fluke 8840A had no input.

EDIT 2: It looks like I should have had more patience the first time around with the Agilent!   :-DD  It stabilised at 4.2767V after around 10 minutes, then climbed back to the original figure over the next 20 minutes.
I'll leave it to run for a while and see where it ends up. Looks like it's just drifting around a bit under warm-up.

EDIT 3: It peaked at 4.3001V after about 3 hours and 40 minutes, then started a very slow (10-20uV/minute) drift down again. Not really a practical warm up time for a battery DMM! The current was still 41nA.
Naturally I had to unplug from the Fluke 8840A measuring voltage to plug into the Keithley 197 measuring current to check that, and when I plugged back into the 8840, the voltage went to 4.2995 pretty quickly. I'm confident to call it warm up drift and nothing else at this point.

EDIT 4: Hopefully the last edit for this post, but well...  Tried measuring the input capacitance of the Fluke 8840A; much more problematical than I expected. My main LCR meter (Iso-Tech LCR819) says slightly under 51nF, however checking with other meters to look for a mean value threw up various non-responses. Fluke 87V and Fluke 289 both went over-range,  Metrix 57 said about 430nF, Brymen 869S and Tek 912 both cycled through capacitance ranges without giving a reading, and the 328-based component tester said 150uF...
I have recently acquired a GenRad 1657 DigiBridge but I haven't yet had a chance to fettle it and check its reliability, so haven't tried it with that.
« Last Edit: January 20, 2022, 07:54:39 pm by AVGresponding »
nuqDaq yuch Dapol?
Addiction count: Agilent-AVO-BlackStar-Brymen-Chauvin Arnoux-Fluke-GenRad-Hameg-HP-Keithley-IsoTech-Mastech-Megger-Metrix-Micronta-Racal-RFL-Siglent-Solartron-Tektronix-Thurlby-Time Electronics-TTi-UniT
 


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