Current measurements with fluke 87V

[cdntech]

I noticed while taking a DC current measurement that I am getting different readings depending on which way my probes are hooked up. When the meter displays a (-) current reading the values are lower than when it is displaying a (+) reading.

 I tested it with my benchtop supply and a 10 ohm 5 watt resistor and came up with this.

At 2 VDC through the 10 Ohm resistor with the negative probe connected to negative on the PSU and the positive connected to the load side of the resistor I get the following readings;

A range: 0.1910

Ma range: 160.80

Probes reversed;

A 0.1863

Ma 160.76


This may be a stupid question but why are the readings lower when the probes are reversed and the meter displays a (-) symbol? My other meters show the same reading either way.

[alm]

Does the same happen with voltages? In a perfect world the meter would have the same response to positive and negative values, but in practice some variation is acceptable. The differences mA values (note capitalization) are totally insignificant. The differences between the 6 A range and 400 mA range are probably due to the higher burden voltage (shunt resistance) of the lower range.

The ~5 mA difference is close to the 4.4 mA tolerance for measuring 0.190 A on the 6 A range. So I do not really see a reason to worry (the true value could have been .189 A and both readings would be in spec). If the 6 A range displays the same relative difference at larger currents, then I might start suspecting the 6 A range is out of calibration. There could be a parallel protection diode across the shunt that is leaky.

[cdntech]

Thanks for the reply, I did notice something in the manual on page 26 note 5. "Reversing the leads will produce a negative reading but will not damage the meter".
 Does this mean with the positive lead connected to the more negative side of the circuit that the reading may be lower?

Thanks!

[alm]

Well yes, -1 A is lower than +1 A . But the absolute value should be pretty similar. That note is only to indicate that you will read a positive current as negative if you reverse the leads. So a 1 A current would be read as -1 A. To not confuse yourself, it is helpful to use a consistent polarity. The comment about not damaging your meters is probably because old analog meters could not read negative voltages: you had to reverse the leads to read a negative voltage.

[saturation]

To be sure nothing is wrong with the DMM, such as a calibration problem, short the lead and use the relative mode to zero the input, then try again and report back. Readings should be essentially identical with the polarity reversed for any DC in the 87V, current or voltage.

If the relative mode works, it means the DMM is in cal but it is picking up interference.   

[cdntech]

Interesting...that was what was confusing me. The values are a bit different when the probes are reversed. I just purchased this 87V and am not familiar with it yet...I thought that maybe it had something to do with it's internal circuitry hence the note. Even at 1 Amp it reads a bit higher on the positive side.

[cdntech]

With relative mode the values are much closer than they were. When out of relative mode there is no change.

[JFJ]

... why are the readings lower when the probes are reversed and the meter displays a (-) symbol?

The amplifier that buffers the meter's A-D converter must have a small input offset voltage. Fluke does not provide any means to trim that out during calibration. Nevertheless, the small offsets in your readings (40uA on the 60mA range, and 4.7mA on the 6A range) are well within Fluke's specification for the DC current ranges of the meter.

A far greater error is being introduced into your reading by the resistance your meter's shunt resistors and that of the test leads. On the 60mA range, the shunt resistance is 1 ohm - which is significant, here, because it's adding 10% to your circuit's load resistance.

[cdntech]

Thanks for the info!

[saturation]

There isn't as big a difference if you measure a true DC source like a battery, the output is fairly the same in reverse and forward polarity without the REL mode, with bench supplies it depends on how pure the DC is. 

Without REL as another stated, the accuracy of the LSD float is still within spec, but if you want to squeeze the limits of its accuracy while watching current source or sink, using REL is easy to do.

Older model 85 I and 85 III do not behave this way, as well as the competing Keysight meters like the 1272a, 1252a, or newest 1282a: ammeter all read 0.000/0 with leads off,  all meters are accurate compared to each other without using REL, read about same in either polarity.

In archive


https://www.eevblog.com/forum/chat/fluke-87v-non-zero-current-reading/

With relative mode the values are much closer than they were. When out of relative mode there is no change.

[eKretz]

Just checked my several year old 87V, it is pretty close to bang on with both polarities when checking that level of current. With a Sorensen DC power supply supplying current, the DC A range in + and - polarity measured .187 A  and -.188 A; mA range measured 186.7mA and -186.6mA.

[macboy]

To be sure nothing is wrong with the DMM, such as a calibration problem, short the lead and use the relative mode to zero the input, then try again and report back.  Readings should be essentially identical with the polarity reversed for any DC in the 87V, current or voltage.

If the relative mode works, it means the DMM is in cal but it is picking up interference.

Right idea, but not quite.
When using REL mode to zero voltage mode, you short the leads to ensure you get "zero" voltage between them. For zeroing in current mode, you leave the leads open to ensure zero current between them. If you short them together, you provide a low impedance path for current to flow, just as if you leave them open, you provide a high impedance path to allow a voltage between them. So, open the circuit, zero (REL) the meter, then measure current.

[saturation]

You're right, MB I stand corrected.

To be sure nothing is wrong with the DMM, such as a calibration problem, short the lead and use the relative mode to zero the input, then try again and report back.  Readings should be essentially identical with the polarity reversed for any DC in the 87V, current or voltage.

If the relative mode works, it means the DMM is in cal but it is picking up interference.

Right idea, but not quite.

[cdntech]

Thanks for all the info! I was playing around with it again and noticed while giving a negative reading it is spot on to the current display on my bench supply. In the positive it is a few counts higher.

Edit: Tried again with a battery and now the measurements are cooperating between + and -