Re: a strange problem

On Tue, 06 Jan 2009 19:34:03 +0100, Jan <Nospam@xxxxxxxxxxx> wrote:

:Ross Herbert wrote:
:
:> On Tue, 06 Jan 2009 05:39:45 GMT, ehsjr <ehsjr@xxxxxxxxxxxxxxxxx> 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....
.

Relevant Pages

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    ... :>:DMM set to measure resistance of a circuit with a capacitor ... :>:You seem to think that the meter should show you the value ... :>:measuring and how it can affect the reading. ... :>determine why the expected resistance measurement is not being returned. ...
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