a strange problem

It's stunning to me the number of posters asking advise, then bad-mouthing the response.

Send them back to "basics" ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
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Ross, I have no wish to argue with you - it's not personal. But I must correct your statement about what I am suggesting. It is not *I* who is suggesting that the reading taken with the DMM cannot be relied on, it is *YOU*.

I have said you have to know what you're doing. If you don't, and you think the meter reading means something that it doesn't mean, then you are misleading yourself.

If you want to go ahead and measure things with your DMM and/or analog meters without knowing what you are measuring, no one can stop you. Have a wonderful time. It is glaringly obvious that ignoring _what_ is being measured can deceive you. That is *not* the meter's fault - it is yours. All you said below about what a technician or a user has a "right" to expect is just bullshit. An expectation does not change what the digital meter shows by the most significant digit, least significant digit, or any digit in between, and it does not move the needle on the analog meter movement, at all. It is up to the technician or user to understand what the meter is shoing him/her.

If you want to have a technical discussion, fine. Tell us what the ohmmeter reading should be when you connect it to a resistor in a circuit that has a voltage applied to it by the rest of the circuit. Lets stipulate: said resistor is a bleeder resistor across a cap that is charged to 450 volts, and the resistor is open. Aren't you glad, after your digital meter gives a screwy reading (and hopefully protects itself), that you connected that nice analog meter to measure the resistance? I don't think it's

*fun* to watch the needle wrap itself around the pin. That is one scenario of what can happen if you stubbornly refuse to learn what you are connecting your meter to before doing so.

Ed

[...]

and

Send it back!

That'll show those greedy scamming digital-multimeter-making fat cat bastards...

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John Devereux

Or Nijmegen

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I do not know where you got this steaming pile from. I can see cases that will make almost all digital meters fail with inconsistent results, where analog meters of any topology would do fine. Even in that case it is the technician's issue to understand that the measurement is not valid. Moreover, if they get odd measurements that they cannot figure out they must report it to more skilled personnel. Lab technique for all domains 001.

You seem to know less than nothing about measurement. First, few analog ohmmeters are based on CC excitation. Nor are all digital ohmmeters. It is still the technicians job to report anomalous readings; this requires sufficient skill to recognize the anomalous reading. Hell, i have built computer measurement systems that do this.

On Tue, 06 Jan 2009 19:34:03 +0100, Jan wrote:

:Ross Herbert wrote: : :> On Tue, 06 Jan 2009 05:39:45 GMT, ehsjr wrote: :> :> : :> :Sorry Ross, but that's all bullshit. If you connect the :> :DMM set to measure resistance of a circuit with a capacitor :> :in parallel with a resistor and are mislead by what you see :> :on the meter, that is *not* the meter's fault. It's yours. :> :You seem to think that the meter should show you the value :> :of the resistor, and that, if it doesn't, it's wrong. It's :> :not wrong, you are, for not understanding what you are :> :measuring and how it can affect the reading. :> : :> :Suppose there is a resistor in the circuit and you connect :> :your DMM, set to measure resistance, across it, not knowing :> :that the circuit is applying a voltage across the resistor. :> :Do you expect to get the correct value for the resistor :> :displayed on the DMM? When it is not, do you blame the :> :DMM for being wrong? :> : :> :Ed :> :> What you are suggesting is that a resistance reading taken with a digital :> meter can not necessarily be relied upon. And where an unexpected reading :> does occur the user should completely analyse the system undermeasurement :> in order to determine why the expected resistance measurement is not being :> returned. That is just ludicrous. :> :> A technician using a digital meter to measure a specific resistance :> combination should have no reason to suspect that there should be a :> significant difference in the result comapred to using an analog meter. :> The fact that there may be some parallel capacitance will have no effect :> on the analog meter reading (once the capacitance is charged) and the :> analog meter reading will be accurate. :> :> A technician has every right to expect that a digital meter will also :> present an accurate reading without having to analyse whether or not any :> particular value of capacitance might be present to upset the reading. The :> fact is that digital multimeters, being sampling devices, can be upset by :> a certain combination of resistance and capacitance, but is it wrong to :> say that where an unexpected result occurs, the fault lies with the user :> because he has failed to analyse what might be upsetting the meter? Of :> course not. The digital meter is just a measuring tool the same as the :> analog meter and the technician should not have to be conversant with the :> specifics of the design of the two items in order to determine whether a :> reading is correct or not. :> :> The user has every right to expect the same result (within reason) for the :> same measurement whether using an analog or digital meter, "particularly :> for resistance measurements". If the digital meter produces an unexpected :> result and the analog meter doesn't then where does the fault lie? The :> meter producing the unexpected result is obviously "wrong", despite any :> excuses, legitimate or not, which might be made to explain its erroneous :> measurement. : : :If you don't know how to use a DVM then you're not a technician. :You can't just poke your probes in a circuit and expect to get a meaningfull :reading. : :Jan

I know how to use a DVM thank you very much...

We're talking about measuring the "RESISTANCE" across the output of a fairly simple network here (as indicated in the OP's original post). If you had seen the same erroneous result as the OP when using a digital VOM would you have analysed the network being measured and then been able to determine what was causing the erroneous reading?

"Oh that will be because of the capacitance playing havoc with the settling time", or whatever... Sure, I'll bet you would - ha...

I'll bet my last dollar you wouldn't. And why should anyone be expected to anyway? A meter is a meter is a meter. When measuring resistance as in this case it would be quite reasonable to expect that a reading taken with a digital VOM would be the same as that taken with an analog meter. The fact that the digital VOM doesn't present the right result when there is no fault in the circuit being measured means that the digital meter IS WRONG. And in the end, if all you have is a DVM, and the erroneous reading keeps occurring, just how would you expect any technician to be able to perform the measurement he is trying to accomplish no matter how intelligent he may be? If the meter won't do the right thing it doesn't help him even if he is able to "analyse why it isn't giving the correct reading". He's still stuck with an erroneous reading.

That's why I suggested the intelligent solution - use an analog meter....