hand-held meter for Hz readings

[dlenox]

Been progressing with my Xantrex XW monitoring/logging software, it is coming along quite nicely!

Software has been working pretty well so I recently added the ability to interface to a Dataq DI148 D/A device. I incorporated the device into my software so that I can accurately monitor/log the turbine rpm's, I am using a door bell step-down transformer to bring the voltage down to about 5 volts.

This Dataq device is relatively cheap and comes with software drivers as well as support. Once I got some bugs worked out in my software I got the device working and tracking the rpm's very well, which caused some confusion.

It appears that my Mastech hand-held meter does not measure the frequency coming from the turbine properly, in the past I took the meter reading as gospel - but now highly doubt the readings.

I would watch the meter in higher winds and see that it displayed the fact that my turbine rpms increased from 125 (average) to up to 250rpm, measuring the same thing on the D/A device showed me that the rpm's only went from 125 to 150!!! So in the past when I reported rpm's it was based on the incorrect hand-held meter readings!

Just shows that you can't always trust what a meter displays!

Today we had a good wind blow through and in a 36mph wind I recorded a peak 250rpm with the turbine outputing 3.4kw output with 60 amps!

Dan Lenox

[Flux]

There are many traps in electrical measurement, it was always so even in the days of analogue meters but in those days it was usually caused by using the wrong meter for the job and not understanding the implications. In this digital era the problems are immensely worse and I suspect that even in industry with very expensive equipment the answers may be way off the mark.

Everyone here, except me, never has any trouble measuring frequency from these chopped and crappy waveforms with commutating spikes all over the place. It is reassuring to find that I am not the only one.

I have had lots of trouble with commercial multimeters and frequency ranges on normal waveforms and for this sort of duty I never use them unless backed up by another method. At best they need fairly elaborate filters and even then they will occasionally jump to a harmonic.

Nice to hear from someone occasionally who admits they have problems it restores my faith in human nature.

Flux

[TomW]

flux;

You have a way with words, sir.

I only truly trust measurements made on the CRT of even a low quality oscilloscope. Especially when it gets up near and into the RF range [where I spent most of my career].

You also learn fast not to trust meters that indicate problems your other senses[ors] do not hint at. Check the meter before you start troubleshooting in depth type thing.

Thanks for the input you provide here.

Tom.

[phil b]

It does not matter whether it be a $10 pocket Sears model or the $300 Fluke. It takes someone with common sense to determine if the meters and instruments are accurate. You caught it! The harmonics and dirty sign waves seem to give me the most trouble on generators. Changing the frequency(rpm) can change both and create new problems.

I get better results using optical rpm meters for this application. It means  additional phone cord size wires up the tower, but worth it IMHO. You could tie this into your D/A or A/D, depending on what you find.  They can be purchased rather cheap and there is a big assortment out there. After you determine your true rpm with optical, recalibrate your original signal. Optics tend to get dirty over time when out in the weather.

[bob golding]

tend to agree with tom on this one. use a scope, preferably a digital one. you can get a nice tek digital scope with a printer for only 1500 dollars second hand:-b or if you cant run to that a 20 dollar analog one and a digital camera will do the same job. i tend to play about with caps and resistors to make a crude filter to get rid of the worse of the hash. this is a lot more reliable than a meter. if you really want to build a rpm meter there are some plans in the archives using one chip and a few passives. have fun.

cheers

bob golding