Hand-held meters - don't always believe what you read!

[Dan Lenox]

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

[Rob Beckers]

That's weird. It should be more reliable. Any idea why the meter is mis-reading frequency? Is the waveform coming off the turbine so distorted that the second harmonic is strong enough to interfere maybe?

-RoB-

[Dan Lenox]

Rob,

could very well be. I do know that if I put the meter directly across the AC lines it just gets all wacked out, using the step down transformer seemed to stabilize it. I have heard that some people need to use a low-pass filter to get Hz readings on some meters.

At some point I have seen the meter going up from 14hz to 35hz (roughly 250rpm), on the D/A it was reading 17hz, and going out to visually verify saw that the blades apparently were *not* rotating @ 250rpm.

This has happened a bunch of times and each time the D/A was correct.

Dan

[Stewart Corman]

Dan,
Funny that you posted on using the Dataq ...I just got the serial port version DI-194RS and was planning on posting my efforts as soon as I get it passed debug stage and fully hooked up taking real measurements..... details to follow

I am just using the canned program they provided so far, and exporting data to Excel ...planing to correlate with data from LaCrosse weather station.

So far, I set the readings for one a second, and saw voltages from the turbine >300v DC (no load), and used a simple 40:1 voltage divider to scale it down to the 0->20v measuring range ...need to calibrate it later

I did some research on their support forum and see that you recently posted there on frequency measurements.

I am waiting for a response to mine on hardware:
http://support.dataq.com/viewtopic.php?f=3&t=2781

I may be asking you for assistance if I delve into C++ and ActiveX to run outside their canned program ...been a long while since I dibbled in code.

I will open a new thread when I organize my thoughts, but so far, that Dataq looks like it fits the bill!

Stew Corman from sunny Endicott

[Dan Lenox]

Stew,

I would be happy to help.

My input is into port #3, but the hardward scales the signal down cleans it up and shows up on channel #0. every new data event I loop through the dataset returned looking for transition and count the pulses.

Let me know when you are ready and I can give you my C# code for setting up the control and counting pulses from it. it seems that you *have* to put their control on a form and can not instanciate it dynamically.

Only thing about their ActiveX control is that it can hang and reboot your computer.... seems that they don't want to write a .Net device (too bad).

Dan

[Rob Stephens]

Rob,

could very well be. I do know that if I put the meter directly across the AC lines it just gets all wacked out, using the step down transformer seemed to stabilize it.
Dan

I picked up a nice inexpensive DMM at Princess Auto (DT9208) that has a lot of functions including frequency counter, AC amps, capacitance (2nF to 20 uF ranges), and temperature probe besides all the usual. Auto shutoff saves the battery. Has large digit display, memory hold button and a nifty tiltable display feature where the display lifts up while the meter body is still flat on it's back on the bench. Ohms goes much higher than most others too with a 2000 Meg ohm scale and the input impedance is really high on the voltage ranges. I was so impressed for the $$$ I bought three in all for the lab and around the home when they went on sale for $29.99.

I was troubled by the inability for the meter to read the 60 Hz AC mains until I read the instruction manual. Apparently the voltage input allowed on Hz scale is only something like 60 volts AC max. That would explain how a step down xfmer would allow at least this particular meter to read frequency properly. You know what they say, "If all else fails read the instructions!" :weird:

[Dan Lenox]

Rob,

Unfortunately the step down transformer will step down the voltage, but the frequency will remain the same.

As far as ohms goes accurately reading lower rather than higher resistance is typically preferred.

Dan Lenox

[Rob Stephens]

Rob,

Unfortunately the step down transformer will step down the voltage, but the frequency will remain the same.

As far as ohms goes accurately reading lower rather than higher resistance is typically preferred.

Dan Lenox

Dan,

Your comment makes no sense to me. If frequency is the parameter which one wishes to measure, WHY would you want the transformer used to scale the voltage to within the range of the meter so it can read the frequency properly, also CHANGE the frequency you wish to read? My mention of the step down xfmer was in regards to a potential solution to Rob Becker's problem in reading frequency. My point was that he may be overvolting the acceptable input range of the frequency function of his meter.
In the lab work I do being able to measure really high ohms is of value.

As far as reading very low ohms, I made no mention of this. I spoke of the meter having a very high input impedance. This means that it loads the circuit less than most other meters while sampling it to determine the parameters, in this case voltage. The term "ohms per volt" is used to describe this meter input impedance. For electronic work the higher the ohms-per-volt rating of the meter, the better. For electrical power work where circuit impedances are typically very, very low and current available is relatively high a high ohms-per-volt rating of the meter is much less important. When you place voltmeter leads onto a circuit to measure the voltage present the meter represents a resistor. This can load the circuit causing the voltage to droop and thus negatively effect the accuracy of the reading. Higher ohms-per-volt meters reduce or practically eliminate this loading effect so the reading is more accurate.

If one wishes to measure very low ohms accurately then a meter which is deliberately designed for this task must be obtained. That is a rather specialized type of meter. This particular DMM I spoke of is as good as any of the general purpose DMMs I have used when it comes to measuring in the order of a few tenth's of an ohm or so.

Edit: I thought it was Rob's meter that wouldn't read frequency correctly. I see it was Dan's meter as mentioned in the first post. My mistake.

[Dan Lenox]

Rob,

Ohms goes much higher than most others too with a 2000 Meg ohm scale

My comment was that being able to read very low ranges is more desireable - for me, I have no use for reading giga-ohm resistance.

In the lab work I do being able to measure really high ohms is of value.
<snip>
If one wishes to measure very low ohms accurately then a meter which is deliberately designed for this task must be obtained. That is a rather specialized type of meter.

I'm referring to use of meters in average day-to-day stuff, not specialized lab work.

When I was talking about using a step-down transformer, the context was in using a analog to digital (A/D) device. Typically the upper limit of these devices are about 10v, and to use reading a 48v nominal wind turbine, you have to step down the voltage to use with the A/D controller.

With my PowerDashboard software for monitoring/logging renewable energy systems I've adapted it to work with both DataQ and LabJack A/D devices. I use the A/D controller to measure things like turbine rpm, volts, amps, wind speed, wind direction, etc using a connection to a computer (not a meter). If you don't step down the working voltage then you'd likely blow the device.

Dan Lenox