How to use digital multi meter for rpms

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How to use digital multi meter for rpms

Post #1 by 80corvette » Wed Jul 17, 2019 5:23 am

Where do you connect the multi meter to measure rpms

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Re: How to use digital multi meter for rpms

Post #2 by desertrefugee » Wed Jul 17, 2019 10:59 am

Cant get there with a DMM. An oscilloscope is needed. You need to measure pulses.
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Re: How to use digital multi meter for rpms

Post #3 by heraldhamster » Fri Jul 26, 2019 12:22 am

hmm, really?
I never knew this.
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Re: How to use digital multi meter for rpms

Post #4 by flyin900 » Fri Jul 26, 2019 7:15 am

Don’t believe that is correct as my DMM has a section for RPM on a variety of multi cylinder engines. Don’t ask me how to use it though, as I have never hooked it up for that purpose and don’t have the instruction anymore.
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Re: How to use digital multi meter for rpms

Post #5 by pidjones » Fri Jul 26, 2019 7:18 am

So, not just any DMM, but rather one that has the tach function.
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Re: How to use digital multi meter for rpms

Post #6 by Shadowjack » Fri Jul 26, 2019 8:06 am

A very specialized tool, that. I've never seen one, and I used to use electronic test equipment professionally.

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Re: How to use digital multi meter for rpms

Post #7 by Whiskerfish » Fri Jul 26, 2019 9:09 am

Several of the Fluke DMM's have a RMS function that can be used for this.
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Re: How to use digital multi meter for rpms

Post #8 by desertrefugee » Fri Jul 26, 2019 9:45 am

RMS function won’t get you there. As noted, you’ll need the specialized instruments which can be expensive have to be configured to measure ignition pulses going to the coil. RMS simply gives you another way of looking at AC power other than peak to peak.

Since those aren’t typical DMMs, I did not consider them in the discussion.
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Re: How to use digital multi meter for rpms

Post #9 by Whiskerfish » Fri Jul 26, 2019 11:24 am

All you do is take the rms and multiply by 60. Then correct it for the wasted spark.
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Re: How to use digital multi meter for rpms

Post #10 by flyin900 » Fri Jul 26, 2019 12:23 pm

Here's the one I have, yet only use it for electrical/impedance stuff. Should try to find the instructions and see how this other stuff works.

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Current Bikes:
1967 CL175E Low production number with #802 as engine serial- winter 2019/2020 project.
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1981 GL1100 Standard model - Sept 2018 start of cosmetic refresh, nice original condition.
1983 CB1000 Custom-Low mileage full cosmetic restoration back to factory new.
1984 GL1200 Standard model in showroom condition-two owner bike.
1984 CX650E Restored summer 2017 - a rare Eurosport model-excellent one owner bike.

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Re: How to use digital multi meter for rpms

Post #11 by desertrefugee » Fri Jul 26, 2019 2:41 pm

Whiskerfish wrote:All you do is take the rms and multiply by 60. Then correct it for the wasted spark.


Hmm. That works? I'll be danged. I guess its using the RMS function to measure the pulse frequency and then mutiply to get one minute. If youre only looking at one coil, maybe the wasted spark correction isnt needed?
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Re: How to use digital multi meter for rpms

Post #12 by mikenixon » Fri Jul 26, 2019 7:55 pm

As I recall, Fluke sells an rpm attachment for their digital multimeters. Never tried it though. Most shops use standalone rpm test tools. Used a lot on ATVs and other vehicles to set idle speed carefully so as to be compatible with CVT (automatic transmission) systems.

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Re: How to use digital multi meter for rpms

Post #13 by Shadowjack » Sat Jul 27, 2019 8:09 am

In electronics RMS stands for Root Mean Square and is the DC equivalent of an AC wave. Nothing to do with frequency.

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Re: How to use digital multi meter for rpms

Post #14 by desertrefugee » Sat Jul 27, 2019 11:14 am

This got my interest up. Poked around a bit. You can calculate frequency using a true RMS meter. Since most alternating current sources are not a clean, pure sine wave, straight RMS calculations will not work. Or at least would not be very accurate. That’s why true RMS methods are used. Also, the sample window must be short and defined or the frame needs to be short enough so that voltage changes cannot corrupt the calculation - a constant, stable peak to peak amplitude is essential.

I suspect that the generic DMM versions of "tachs" are adequate, but not entirely accurate. I could be wrong, but if you're looking for precision, a dedicated instrument would be the route to take.

I spent some time (decades) in an engineering lab, but mostly slinging code.
Last edited by desertrefugee on Sat Jul 27, 2019 5:43 pm, edited 1 time in total.
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Re: How to use digital multi meter for rpms

Post #15 by DUGG » Sat Jul 27, 2019 12:10 pm

My fluke 78 has functions for rpm and dwell but it was built for the auto repair industry. To use the rpm it has an inductive clamp that goes around a plug wire. Some timing light have rpm functions as well.
To use a Dmm it would have to be able to have graphing features so you could see the waveform. Seems in this day and age there must be a cheap inductive tach out there.


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