PDA

View Full Version : Why you should not buy a cheap multimeter







star882
01-06-2007, 05:58 PM
Here's a reason why you should not buy a cheap DMM. The input filter in cheap meters easily gets confused and attenuates the signal, and worse than that, the amount of attenuation is unpredictable so you have no way to know the real measurement!
To demonstrate, I'll use a cheap meter (about $35) and an expensive meter (about $80) to measure the normal AC line.
http://i12.tinypic.com/688cdfs.jpg
So far so good, right? That's because they're measuring an almost pure sine wave.
But measure the UPS output and look what happens!
http://i8.tinypic.com/4p16byc.jpg
That's because they're measuring a square wave, which confuses the input filter in the cheap meter. The expensive meter has better input filters that are not confused by high frequencies in the signal being measured.
This inaccuracy is confusing at best and dangerous at worst. A cheap meter can easily cost you a lot in the long run!

chillin out
01-06-2007, 08:13 PM
I wonder what the 15 one I got today would do...lol

Good explanation of "You only get what you paid for".

Chillin:):)

frank
01-06-2007, 08:25 PM
Quite right Chillin'

But I don't think that $80 is what you would call an "expensive model".

I paid 126 for my Fluke ($245) so

US Iceman
01-06-2007, 08:55 PM
Expense is one of those relative terms. My old Fluke DMM was about $600 US almost 25 years ago. I understand they are a little cheaper now!

I would not use any meter that was in the price range described by Star882. And, I would not buy one unless it had the Fluke name on it. They build very good meters!

The MG Pony
01-06-2007, 11:00 PM
I got a circuit test that cost me 300 cad 10 or so years ago. So far it works great and messures pretty much every thing reliably and has a built in frequancy counter :)

star882
02-06-2007, 01:13 AM
Quite right Chillin'

But I don't think that $80 is what you would call an "expensive model".

I paid 126 for my Fluke ($245) so
It all depends on the application. The $80 one described works fine for most personal uses. But if you need NIST calibration, standalone data logging, LCR measurement functions, etc., by all means get it! Just overkill for the average user. Most amateur PC technicians get away with $10 specials. (I can't wait for one to get confused when measuring a UPS,then he calls me over and I show him that it's working correctly.)

NH3LVR
02-06-2007, 01:46 AM
Here's a reason why you should not buy a cheap !
Star882;
Once again you bring controversy to the forum. (I like that in a person)
Will bring my Fluke 66 home next week and check it on my UPS's. Never have done that, I always assumed that the output was a true Sine Wave until the power failed. Perhaps I should bring my old Oscilloscope up from the basement and take a look.
I noticed years ago that I could not measure the output of a Variable speed drive with my voltmeter. I assumed that this was because the output was not a true sine wave. Are the high peak voltages I observed real or the fault of my meter? I assumed that they were real as I soon found out that non VFD rated motors failed very quickly when connected to a VFD.

analog
02-06-2007, 02:49 AM
Star882;
Once again you bring controversy to the forum. (I like that in a person)
Will bring my Fluke 66 home next week and check it on my UPS's. Never have done that, I always assumed that the output was a true Sine Wave until the power failed. Perhaps I should bring my old Oscilloscope up from the basement and take a look.
I noticed years ago that I could not measure the output of a Variable speed drive with my voltmeter. I assumed that this was because the output was not a true sine wave. Are the high peak voltages I observed real or the fault of my meter? I assumed that they were real as I soon found out that non VFD rated motors failed very quickly when connected to a VFD.



I, for one, have always used the cheapest tools available.

Except for my lucky pen. I've had it for over 10 years. It writes pure gold. It can turn a blown transformer caused by my own stupidity into "performed low voltage circuit trace", that sentence alone has got to be worth 150 quid.

As far as meters go, I've always relied on my second hand amprobe to tell me if a wire is hot, when in doubt, my helper James performs a "wet finger" test.

His new pacemaker is working out nicely.

star882
02-06-2007, 03:09 AM
It depends on design. The UPS in the pictures is old so it outputs a square wave. A few new ones, however, output sine waves. For computer equipment, however, a lightly filtered 170v peak square wave is actually better than a 120v RMS sine wave. It does increase the chances of an EMI problem but I have never seen it actually cause system problems.
BTW, most UPSes first rectify the incoming AC to charge up a capacitor. Then it uses a H bridge to recreate AC. A DC/DC converter provides switchover by converting 12v from the battery pack to 170v. Another DC/DC converter charges the battery from 170v. (Some UPSes use a bidirectional DC/DC converter.)

analog
02-06-2007, 03:25 AM
It depends on design. The UPS in the pictures is old so it outputs a square wave. A few new ones, however, output sine waves. For computer equipment, however, a lightly filtered 170v peak square wave is actually better than a 120v RMS sine wave. It does increase the chances of an EMI problem but I have never seen it actually cause system problems.
BTW, most UPSes first rectify the incoming AC to charge up a capacitor. Then it uses a H bridge to recreate AC. A DC/DC converter provides switchover by converting 12v from the battery pack to 170v. Another DC/DC converter charges the battery from 170v. (Some UPSes use a bidirectional DC/DC converter.)

I've always found the sine wave protocal (based on the works of Dr. Kurt Jurger) to contain to many variables when contrasted with the UPSes used today.

Rectification of capacitance is a Tessla theory that holds no water.

The H bridge has pretty much been replaced by the M


jmho

ultralo1
02-06-2007, 04:18 AM
Fluke True RMS meters are the way to go.
Another thing that i hate about cheap meters is the sampling rates. I hate having to wait for the display to settle in. It is like watching a cap charge up.

Toosh
02-06-2007, 07:03 AM
GeeZ I must be gettin old I still swear by my old Simpson meter And my year1972 amprobe, well I been out the business for nye on 20 years suppose being senile helps I guess

Toosh

FrankWSmith
13-07-2007, 01:18 PM
Fluke stays the best, not cheap but the best, own three models at present.

Richard Hillsid
22-09-2007, 07:50 PM
Fluke at 250 Euros, is good but I wont trust my life on it.

Tycho
22-09-2007, 10:42 PM
http://www.bis.fm/assets/images/productphotos/Fluke%20787%20Process%20meter_l.jpg

$624

had it for six years now... co-worker borrowed it and blew something inside, causing it to make a beep beep beep sound every now and then, it's been back to fluke, but got it back with a "No fault found, changed fuses" made the beep beep beep sound for a few months, but stopped now :)

wingman
28-04-2009, 08:31 PM
I never leave home without these. For amps I use an ordinary clamp meter.
http://www.kew-ltd.co.jp/en/products/multifunction/images/6010B.jpg

http://www.vellemanprojects.com/images/products/0/hps50.jpg