Adam Savage on multimeters

[Scherms]

Dave found that the 88v RMS control is a chip separate from the main ICU and it has on the side 2 blank solder pins to solder on a 33uf capacitor to change from AVG to RMS mode...

Just remember that you have to recalibrate the whole meter.

Here's what you get in the 88V:
A 4.99 Ohm resistor in series with a small ceramic cap that wouldn't measure in circuit and a spare footprints for your missing tantalum.

Here is an updated picture of the area in question on the Fluke 88v, there are differences than the 87v! What are the cap and resistor (blue box) doing there between the blank pads and the AD737 chip? Would they need to be removed for the mod?

[EEVblog]

Can you measure the value of that cap?
Manual says: "AC conversions are ac coupled and are average responding, rms indicating. "

[Scherms]

Can you measure the value of that cap?
Manual says: "AC conversions are ac coupled and are average responding, rms indicating. "

Sorry I did not go to the trouble of measuring the input coupling, RMS averaging and output filter capacitors on my meter, didn't want to go to the trouble of desoldering them. I expect them all to be in the 1-10nF range.
Have yo read the AD737 datasheet? I have partly read it, as I understand, in the 88V they have compromised for mainly average AC measurements but with a little twist to aid in catching fast pulses. (Common-rail injection voltage or current pickup on ignition maybe?)

I couldn't really make sense of this phrase on page 67 of the 88V user manual though: "AC conversions are ac coupled and are average responding, rms indicating."

I wouldn't go to the trouble of removing the existing RC if you want to change to true RMS, they're not going to have a notable effect parallel to 33µF. I'd be worrying more about being set for the readjustment.

Sorry no, but Shakalnokturn might do... and if only there was a circuit diagram for the 88v!

If it is average responding then WTH is "rms indicating" supposed to mean here (oxymoron)?

Do you have a link to the spec page of the AD737 chip indicated as that might shed some light?

 

[HKJ]

If it is average responding then WTH is "rms indicating" supposed to mean here (oxymoron)?

This means the meter is calibrated to show equivalent to rms with a sinus input. I suppose all average responding meters are calibrated that way.

[Scherms]

EXPLAINED

I couldn't really make sense of this phrase on page 67 of the 88V user manual though: "AC conversions are ac coupled and are average responding, rms indicating."

AC coupled = any DC components in the signal will be ignored for the measurement

Average responding = the results will be proportional to the average of the (absolute value of) the signal: if the average value is twice as high, the reading will be twice as high.

RMS indicating = although the value is proportional to the average value, it's adjusted so it matches the RMS value for sinusoidal signals.

So for a sine wave with a 1V amplitude, the average value is 0.637V, which the meter multiplies by 1.111 to give 0.707V (the RMS voltage) a true RMS meter would read the same. If you measure a square wave with the same amplitude, the average value is also 1V, which the meter will multiply by 1.111 to give 1.111V. But the RMS voltage is also 1V, so the meter would read 11.1% high. A true RMS-responding meter like the 87V would indicate 1V.