FYI, Multimeters with Hz, can read Fan RPM

[Martinm210]

I was tooling around with my old Mastech Multimeter, since I wasn't completely satisfied with my Kaze Master's 30RPM increment and figured out another way to more accurately measure RPM without a computer.

I've never used the Hz function of this multimeter before, although I did try this once before. I never could figure out what it was telling me.

Well Hz is cyles per second, and apparently there must be one cycle per two RPMs, or something. Regardless...multiplying by 30 gives the correct answer...

I first did a quick linear correlation between my Kaze Master and and what the meter was reading and it was very linear at 30.511. The .511 seemed like it might have been error, so I then checked it against both my Crystalfontz and Speedfan through the MB, and both checked out almost spot on using the 30 factor.

Anyhow, someone with any electrical know how probably already knew this, but I was pretty happy with my greenhorn self.. Figured I'd share, with a Hz capable multimeter, you can read RPM without a computer.




I may include this meter as a more accurate RPM readout in my fan tests. The scythe Kazemaster will be there for quick glance approximation, but this will give me a more precise RPM number.

This old meter may be worthless for sound measurements, but I'm going to put it to work for RPM duties..

[Martinm210]

Actually, I just realized I don't even need ground, just the positive to the yellow tach wire will do..

[jumper2high]

The PC fan tachometer gives out 2 pulses per rotation. It doesn't work with all multimeters, as I've tried it a couple of times on cheaper equipment and got nonsense results (like a fan spinning at 50.000RPM or so ) but I have created custom measuring circuits that would count the number of pulses per second or time the interval between two rising edges of the pulse, and it worked...


EDIT: You should always put the GND wire on the GND port, even if it works without it, for more precise measurement. Since the voltage is a difference of electrical potential, as long as the GND lead is on something *constant*, for the purposes of frequency measurement, it might not make a difference, but the electrical potential of air, or your hand, might not be constant, so holding the wire or leaving it hanging around might easily skew the results.


Also, remember when calculating the speed that the motherboard can actually make considerable mistakes when measuring the RPM. This has to do with the way most of those things work. They count the total number of pulses per second and then calculate the RPM. This means that if the count is 'off' by just 1 (for the purposes of this example, let's say it's 1 less) on a 3000RPM fan, that'll equate to a calculated RPM of 2790RPM. Depending on the update speed of the measurement, this "step" may be anything between 30RPM and 300RPM (faster updating measurement is less precise). Your instrument, on the other hand, measures the frequency, down to the decimal, so it's indeed a lot more precise since the PC has no means of measuring 'half a pulse'.

[Martinm210]

The PC fan tachometer gives out 2 pulses per rotation. It doesn't work with all multimeters, as I've tried it a couple of times on cheaper equipment and got nonsense results (like a fan spinning at 50.000RPM or so ) but I have created custom measuring circuits that would count the number of pulses per second or time the interval between two rising edges of the pulse, and it worked...


EDIT: You should always put the GND wire on the GND port, even if it works without it, for more precise measurement. Since the voltage is a difference of electrical potential, as long as the GND lead is on something *constant*, for the purposes of frequency measurement, it might not make a difference, but the electrical potential of air, or your hand, might not be constant, so holding the wire or leaving it hanging around might easily skew the results.


Also, remember when calculating the speed that the motherboard can actually make considerable mistakes when measuring the RPM. This has to do with the way most of those things work. They count the total number of pulses per second and then calculate the RPM. This means that if the count is 'off' by just 1 (for the purposes of this example, let's say it's 1 less) on a 3000RPM fan, that'll equate to a calculated RPM of 2790RPM. Depending on the update speed of the measurement, this "step" may be anything between 30RPM and 300RPM (faster updating measurement is less precise). Your instrument, on the other hand, measures the frequency, down to the decimal, so it's indeed a lot more precise since the PC has no means of measuring 'half a pulse'.

Will do. I noticed it worked without the ground when testing in my office, then I went back to my fan test rig and it suddenly wouldn't work without the ground. Interesting, I plan to test with both, but I'll probably use the mastechs results for charting..

That's cool..

[wez]

Nice, didnt know this! Will give it a try

[Akula]

Will do. I noticed it worked without the ground when testing in my office, then I went back to my fan test rig and it suddenly wouldn't work without the ground. Interesting, I plan to test with both, but I'll probably use the mastechs results for charting..

That's cool..

My guess is that you had a ground, just not on purpose. The unconnected lead making contact with something evenly mildly conductive would probably work with a high impedance function like frequency on the multimeter.

[Martinm210]

Interesting, I gave it a quick run this afternoon and it seems to be plotting very precise RPMs..much much better than the Scythe Kaze Master could do. I will definitely be including it as the RPM meter in addition to the Scythe.

[MrBean]

Hello Martin,
Yes, I use a Fluke 787 Process Meter, wich has built-in Hz function, and it works much better than Motherboard "calculated" rpm's

Nice to share with eveyone though, sometimes we get so used to being techies, we forget not everyone knows these things

[Akula]

On a somewhat related note, you could use the HZ function to make a cheap (not counting the multimeter) and reasonably accurate flow meter using a koolance FM16 or FM17 http://www.koolance.com/water-coolin...product_id=288 and small battery.