Fluke 87v AC Current Measurement

[albell67]

Hi all. Name here is Al. Been following the blog for quite a while but this is my first post.

Tonight I was measuring a 10-15ma AC current and found a strange problem with my Fluke 87v. With the lead plugged into the amp jack it took about 10 or so seconds to settle to a steady reading, in the ma/ua jack the measurement was pretty much instant. I thought maybe it was a case of use the right jack for the proper range. So just to see what would happen I pulled my 83v out and it showed an almost instant response in both the amp and ma/ua jack with the same current. Once again the 87v took quite a while to settle on the proper reading in the amp jack and when it did settle it agreed with the 83v. So now completely forgetting what I had set out to do in the first place I then checked with a higher current and the 87v behaved just like the 83v with a pretty much instant reading from both jacks. This seems to only happens with very small AC current higher current and DC is pretty much instant.

I seldom measure AC current and even less often at such a low level so its not a real big deal to me however my question is: Has anyone else noticed this with their 87v? I don't know if it matters but its a rev 10.

Thanks all for any input on this.

[rastro]

My 87V does the same.  On the Amps plug it starts around 0.060A and drops to 0.023 in about 10 seconds.  Reading from the mAuA is almost instant.

[albell67]

Thanks for the reply rasto. Good to know that mine is not the only one that does that. I do find it odd that my 83v does not exhibit the same behavior. I always figured (wrongly?) it was just a dumbed down 87v.

[Wytnucls]

Probably something to do with the true RMS to DC converter in the Fluke 87 V.

[Wytnucls]

I suspect this is what is happening:
On the A range, the shunt has a low impedance and with AC currents in the mA range, measured voltages are very low too. The true RMS to DC converter has a longer settling time with low voltages, hence the behavior you have observed.
On the mA/uA ranges, the shunt impedance is higher and thus measured voltages higher too, shortening settling time.
I'm not sure what converter the 87 V is equipped with, but it probably behaves in a similar manner.

Here is the graph for typical settling time versus voltage:

[Wytnucls]

The Fluke 87 V has the AD737 installed, so the converter settling time applies.
With an AC current of 10mA and an Amp range shunt resistance of approximately 0.03 Ohm, the measured voltage is 300uV, giving a settling time of about 10 seconds with a CAV of 10 uF, as you experienced.
Here is the datasheet:

[albell67]

Wow!! Thank you very much for the information, it pretty much explains what I was experiencing to a tee.