DMM accuracy changing over time?

[trjones1]

I've been having some strange readings from my DMM in the voltage setting recently.  Firstly, I made  multi-face and in biasing Q2 I have only read voltages on the base of the transistor in the range of 1.6-1.85V.  Despite these readings it sounds fine and I can accept this as the vagaries of electronics that I don't understand.

The next incident I have no clue.  I've been running a Rebote delay off of a 9v battery for probably a total of an hour.  I've heard the Rebote eats up batteries, so I measured the voltage of the battery and got a reading of 10.3V.  Now, I didn't measure the battery when it was new, but I'm willing to bet it wasn't over 10.3V.

Do DMM's lose their accuracy over time or as a result of low batteries, or some other factors?  I've never noticed readings that are obviously wrong before.

[Mark F]

I would assume DMM's,like any other electrical device, would be subject to drift over time. It would depend on the quality of the meter among other things how drastic this drift might be. Also, as you mentioned, low battery may give you erroneous readings. I would suggest you put a brand new battery in the meter and check at least one,preferably more,  known stable voltages and see how accurate your readings are. I would trust these readings as a reflection of accuracy more than the readings in a circuit that are more liable to drift than the meter.

[niftydog]

voltage readings might change due to a dying battery, but in the DMMs I've used they are setup and require no further calibration post-factory.

Put the two probes together before making a reading to check that the display is showing zero, or very close to it. This should work in both voltage and resistance ranges. If there is an offset, take note of it and subtract it from the final reading to get an accurate answer.

[petemoore]

Resistance seems to measure fine with a decent battery IN the Dmm.
 Voltage however has been stepped up to a 9/16ths division...IOW, where I used to read 8's and 10's for voltage measurements, now I'm reading 14 or 16.
 I still seem to get 'close enough' ratios [like FF at 12v would have a 6V bias at Q2] but I don't know how much 'curve' there is to the drift...maybe lower voltages are 'graded at the bottom of the curve'.
 Get a new DMM.
 You need a new DMM dude.
 I decided to work with it...I just work with fractions like 1/2 instead of "4.5'" when measuring voltages.

[bioroids]

I rememeber reading on my DMM's instruction manual that you will damage the unit if you try to read resistances with power on the circuit... I should have read it before using it though... anyway the multimeter is the one thing that keeps falling off my desk... but still seems to work fine when using fresh batteries.

Luck

Miguel

[R.G.]

Like so many things in life, the best advice for this situation is to
(a) buy cheap, rent, or borrow until you learn that you really need the unit, be it a DMM, soldering iron, guitar, or welder.
(b) when you discover that you're going to need one of these long term, buy a good one and take good care of it.

DMMs do not drift much, not like the old analog meters we used to use. but drift does happen. Usually mechanical damage is what changes things, rather than drift for all but the cheapest DMMs.

I personally could not live without a reasonably accurate DMM. I've had five over the years. Each was promptly canned when I suspected it was becoming unreliable in any significant way. When I bought, it was the best I could reasonably afford. Other people may have different levels of need for a good DMM.

[Paul Perry (Frostwave)]

I'm sorry to say, that sounds like a bad meter.
I have been caught out using a meter while ignoring 'battery low' :oops: but, if a meter is consistently severely out with new batteries, I would say it is &*%$d.
Is the error the same on adjacent ranges?

[trjones1]

I changed the battery and now the readings are accurate.  I measured the old battery and it was at 5V.  The moral of the story is change the battery in your DMM every 6 or 7 years!

I don't know exactly the quality of my DMM.  My father bought it for me when I was 12 and for the first 10 years I had it I was like "what the hell am I going to do with this thing?"  Then I got into building stompboxes...  I assume it is of reasonable quality and it serves me well as I am still a rank amateur.

Now to go and get accurate bias readings on my multiface.