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can anyone recommend a good DMM for low voltage, current measurements please? Under £150

george2525

Jan 30, 2015
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Hi

Ive been searching around for a new DMM but there are so many options

I wonder if anyone knows / has experience of a good DMM for measuring low voltages of around 0 - 50mA with good accuracy. EG to at least 2 decimal places.

and currents of around 30uA also with accuracy

the voltage measurements are the most important

ive looked at a few but its always good to ask someone whos actually used a model

Thanks
 

Harald Kapp

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measuring low voltages of around 0 - 50mA
low voltages as measured in Volts or low currents as measured in mA?

At your budget almost any DMM will be suitable, even for less.
A nice feature that may come in handy in some applications is an interface for transferring measured data to a PC for evaluation, or even better a remotre control interface so you can set up automatic measurements should the need arise.
Make sure any such interface is electrically insulated to prevent any possibly hazardous voltages on the DMM to enter the PC.
 

Harald Kapp

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I have to set a 0.032mA reference current
The lowest range of this multimeter is 40 mA with a resolution of 1 µA or 0.001 mA. At a first glance this seem accurate enough, but the accuracy is +- (0.5 % of reading + 10 digits) in the DC range -worse for AC.
At 32 µA this amounts to +-(0.16 µA + 10 µA) as 1 digit equals 1 µA. The error is +-10.16 µA which is approx. 30 % of your measured value. This meter is definitely not suitable for your purpose.

A Fluke 115 for example is much better suited for yout application.
 

george2525

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A Fluke 115 for example is much better suited for yout application.

Thanks. I was under the impression most of the flukes are designed for larger applications and not good for small values. I will look into it further

Im looking at this one which is quite cheap at £42 but has decent reviews and the spec states:

dmm1.PNG

and

dmm2.PNG





This seems ok to me but you are better at interpreting this kind of thing so if you have a minute id appreciate your opinion.

Thanks

https://www.amazon.co.uk/dp/B00L7R1G4E/ref=pe_385721_37986871_TE_item
 
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Harald Kapp

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The data above look impressive. Almost too good to be true at that price o_O

In the 220 µA range the error at 32 µA measured current is +-(32 µA*0.5 % +0.1 µA) = +-0.26 µA which is in total ~1 % of your measured value. This is assuming the percentage in the accuracy is percent of measured value.
(If it were percent of range, another method of detailing measurement instrument specs, the result is worse.)

check some step voltages in 0.0015V increments
The increment is one parameter. But what is the absolute voltage to be measured? This determines the range and hence the resolution. (Sorry, I should have asked that in the first place.)
 

george2525

Jan 30, 2015
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Thanks. im confused about the "x%+10" stuff. what does the +10 mean in math terms?

Anyway this seems good as its says its for "reading"

dmm3.PNG

Im not sure what you mean by "absolute voltage"

the steps are discrete and range from 0.0015V to 0.0465V
 

Harald Kapp

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what does the +10 mean in math terms?
+10 = + 10 digits. What a digit means depends on the measurement range. In the tables you supplied, it is stated for example: range = 220 µA, resolution = 0.01µA. This means 1 digit = 0.01 µA or 10 digits = 0.1 µA. This is an absolute error that has to be taken into account regardless of the reading.
Therefore with a small reading the relative error is higher than with a high reading as, to stay within this example, 10 digits = 0.1 µA is a high relative error of 10 % when measuring 1 µA (within the 220 µA range) but is very small when measuring 100 µA (still within the 220 µA range) where it contributes only 0.1 %.

Im not sure what you mean by "absolute voltage"
the steps are discrete and range from 0.0015V to 0.0465V
It is easy to measure a voltage step of 1.5 mV when the instrument is set to the 220 mV range.
If your absolute voltage is e.g. 10 V and you want to measure the same step (from 10 V to 10.0015 V), you need to set the meter to the 22 V range. In the 22 V range the resolution is 1 mV (=0.001 V). Taking into account an absolute error of 2 digits = 2 mV and a relative error of 0.1 % => 10 mV, the total error when measuring 10 V in the 22 V range is 12 mV which renders the measurement of a voltage step of 1.5 mV useless.
If, however, your max. voltage is 0.0465 V, as I assume from post #7, you can set the meter to the 220 mV range with a max. error of 0.1% of 46.5 mV = 0.0465 mV plus 5 digits = 0.05 mV. In total 0.1 mV (generously rounded up). Your smallest value to be measured is 1.5 mV, therefore the max. error is on the order of 6.5 %.
 
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