Measuring Fan RPM help

[tigerstyle]

I've got a fancy EBM DC fan here that spins at around 1000RPM
I'd like to measure this RPM accurately as I've been told is possible from the EBM info.
I just want to make sure my thinking is on the right track to do it.
I need a DMM that can read Frequency (Hertz) but very low Frequency, starting at 10Hz and going up.
Most of the cheap meters  I've seen only start measuring at 1MHz, that is far too high for my readings right?



The only meters starting low I can find are Flukes that start at 10Hz but thy cost a lot! I need a cheaper solution!
I love the EEV videos, but find them hard to search+navigate, too many to go through!

Hope someone can fix my understanding of the EBM spec  if  I'm wrong!

[amspire]

All DMMs with Frequency measurement can measure down to below mains frequencies (50/60Hz), so they can probably do down to near 10Hz - check the meter specs. Obviously though, you are not going to be happy with the accuracy if you can only read to the nearest 1 or 10 hz. None of them start at 1MHz - I am not sure where you got that number from.

I wouldn't spend a lot of money just to get something that has better resolution  as there are alternative ways to measure fan speed.

One is to let a bit of paper or thin plastic touch the fan blades and record the clicking sound on a smartphone or PC mic.

Then look at the waveform in an audio editing package like the free Audacity or any other audio editor.

You will see the clicks caused as the blade hits the paper.  Just measure the period. Multiply it by the number of blades, and invert and you have the fan speed. It is just as accurate as any meter.

Richard

[SgtRock]

Dear Tigerstyle:
--The comments by Richard are correct. If, however, it turns out that you need a DMM, please consider getting a used Fluke at a good price. Watch eBay carefully, and "Strike while the iron is hot". There are very good reasons why Dave and the other "Old Hands" recommend used Flukes over HK "One Hung Low" products.

--With regard to DJ's videos; I recommend

[tigerstyle]

Sorry,  other cheap meters would have something like this "min range   2kHz" Mhz is obviously wrong...

I like the sound idea though, would be fun to try out!
The other method  I've seen is with a light beam/LED style device.

Just as  I need a good DMM, I'd like to try one out that has every function I'd need. On the spot instant RPM readings would be a bonus!

[Uncle Vernon]

http://themakersworkbench.com/?q=node/350 How someone has done something  similar with an arduino.

[nixxon]

I bought this RPM reader: http://www.dealextreme.com/p/digital-laser-tachometer-with-pouch-2-5-99999-9rpm-range-0-1rpm-resolution-18239

When tested on a 5200 RPM hard drive platter with a reflective sticker, it reads "5200". Extremely accurate...

I

[Alex]

This is the sort of measurement that you can start a competition on. First the 555, then 7400 logic, now 'Find the most innovative way to measure RPM'.

For this application an o-scope would be a simple and accurate solution, for even more accuracy a good quality pulse counter would be my choice. Really, the o-scope way will give you such good results so quickly, it is worth borrowing one if one is not available. Nixxon's tachometer is very cheap too and accurate within +/-0.5 RPM @ 1000RPM.

[kaptain_zero]

I picked up a hobby type tachometer meant to check the rpm on propellers, and seeing props have different numbers of blades, the meter can be set for 2 to 9 blades on the prop.  For the measly sum of $8.99 plus shipping... it's a bargain. Of course, I also have a Fluke 87V so I could use either for your purpose. As I dabble in R/C hobbies, a dedicated optical tachometer was a no brainer for me.  Just make sure you use the sun or other steady illumination as it picks up on the frequency of a fluorecent light source. My meter pegs my ceiling lights at 3600 rpm using the 2 blade setting..... that means the frequency is 1800hz.


http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=10783


Regards

Christian

[Zero999]

I think you've got that backwards, most cheap meters don't work up to 1MHz but are good for lower frequencies.

I used my cheap Chinese Mastech 320 to measure the flash rate of my motorbike indicators which is around 1.6Hz. I did this because I'd recently bought LED indicators which need a resistor, otherwise the flash rate is too high as the relay is designed for old fashioned incandescent lamps.

http://www.alibaba.com/product-gs/207961230/M320_Pocket_Size_Digital_Multimeter.html
http://www.rapidonline.com/Tools-Equipment/Mini-multimeter-74585

[nixxon]

This is the sort of measurement that you can start a competition on. First the 555, then 7400 logic, now 'Find the most innovative way to measure RPM'.

For this application an o-scope would be a simple and accurate solution, for even more accuracy a good quality pulse counter would be my choice. Really, the o-scope way will give you such good results so quickly, it is worth borrowing one if one is not available. Nixxon's tachometer is very cheap too, but only accurate within +/-50RPM @ 1000RPM. Maybe this is insufficient for your application.

My disk spinning at 5200 rpm is reported by this tachometer as 5200 rpm 90% of the time (No decimals). The remaining 10% of the time, it reports either 5199 or 5201. Please look at the enclosed picture of a typical reading with the disk spinning @ full speed. You can see the one reflective sticker at the top of the exposed platter.

To increase accuracy, I guess I could place more reflective stickers on the surface of the hard drive platter, and divide the readout of the meter with the number of reflective stickers. E.g. 10 stickers should give the readout 52000 rpm/10 stickers=5200.0 rpm. If I ever bother to try this, I will let you know the results...

The rated accuracy is, by the way, +/- 0.05%. 5200rpm * 0.05% = 5200rpm * 0.0005 = +/- 2.6 rpm @ 5200 rpm.

An error of +/- 50 rpm @ 1000 rpm would reflect an accuracy of 5%, not 0.05%. At 1000 rpm, 0.05% = +/- 0.5 rpm.

In other words, this gadget is pretty accurate

[Alex]

My disk spinning at 5200 rpm is reported by this tachometer as 5200 rpm 90% of the time (No decimals). The remaining 10% of the time, it reports either 5199 or 5201. Please look at the enclosed picture of a typical reading with the disk spinning @ full speed. You can see the one reflective sticker at the top of the exposed platter.

To increase accuracy, I guess I could place more reflective stickers on the surface of the hard drive platter, and divide the readout of the meter with the number of reflective stickers. E.g. 10 stickers should give the readout 52000 rpm/10 stickers=5200.0 rpm. If I ever bother to try this, I will let you know the results...

The rated accuracy is, by the way, +/- 0.05%. 5200rpm * 0.05% = 5200rpm * 0.0005 = +/- 2.6 rpm @ 5200 rpm.

An error of +/- 50 rpm @ 1000 rpm would reflect an accuracy of 5%, not 0.05%. At 1000 rpm, 0.05% = +/- 0.5 rpm.

In other words, this gadget is pretty accurate

Yes you are right nixxon, post corrected.