Which one is right?

[gar]

090215-1503 EST

chris:

I read 9.33 on a new Energizer.

Change your range to AC and see what you read on the battery.

Do you have a low voltage transformer as a supply to do a comparison? A bell transformer with nothing else connected would work. If this correlates, then how about 120 V isolation transformer.

What is the Fluke T+? Is it a useful comparison?

.

 

[LarryFine]

The Greenlee bangs around the job sites with us, the Ideal stays in my briefcase.

Well, that explains it: the Ideal is out of whack. :smile:

 

[chris kennedy]

090215-1503 EST

chris:

I read 9.33 on a new Energizer.

Change your range to AC and see what you read on the battery.

Do you have a low voltage transformer as a supply to do a comparison? A bell transformer with nothing else connected would work. If this correlates, then how about 120 V isolation transformer.

What is the Fluke T+? Is it a useful comparison?

.

The T+ is like a Wiggy. I have a bunch of transformers here, no isolation though. Hang on.

 

[gar]

090215-1535

chris:

The transformers will provide DC isolation unless there is some direct connection between primary and secondary. I just want you to be sure there is no DC component. However, I think that there are few AC VOM meters that read DC on an AC range. The Simpson is one that does unless you use the output terminal and then there is a series 0.1 mfd capacitor which I believe has a 400 VDC rating.

Do you have a way to chop a DC source. For example a 555 timer driving a 2N7000 FET and a large coupling capacitor?

.

 

[chris kennedy]

090215-1503 EST


Change your range to AC and see what you read on the battery.


.

On the AC scale, both just zero out. I was wrong about the transformers, took them to the shop. I have some old computers and amplifiers here. Could rob a transformer out of one of those.

 

[gar]

090215-1555 EST

Consider your doorbell transformer if diodes are not used anywhere in the pushbutton loop.

Look at these meters:
http://basic.product.agilent.com/handhelddmm/1250_features.html
http://basic.product.agilent.com/handhelddmm/hhdmm_product_comparison.htm
I have not found the price yet. The 1250 (1253A) series is probably the one to consider because of the bright high contrast display. This is the instrument division of the orignal HP company, now a separate company with a poorly selected name.

OK on nothing with a DC input. This means any DC component in combination with an AC signal will have the DC component removed.

Do not rob a transformer.

.

 

[gar]

090215-2315 EST

chris:

Here is a means to check your meter calibration assuming that the AC waveform at your shop or home is sufficiently clean.

Make a half wave bridge rectifier consisting of a 1N5060 diode with a 2 megohm load. I used two 1 meg 1/4 W resistors in series. Connect the 10 meg input DVM in DC position across the 2 meg resistor. Read the DC voltage from this. When I ran my experiment it was 54.7 V. Multiple this by 2 for the full wave rectified equivalent. Then multiply by 1.111. This should be close to the RMS value. For my measurement this calculates to 121.6 . Then without much delay, because line voltage jumps around a little, I switched to AC position and directly measured the line voltage and it was 121.9 .

This kind of test should be sufficient to identify which meter is good in the AC position, if either is.

I expect your meters are reasonably good on DC from the battery measurement you made.

A 1N4004 diode might be equally good, and the 2 meg resistor was a judgement call. The last digit of the 1N400x defines the voltage rating.

.

 

[Speedskater]

One other thing to keep in mind when measuring waveforms with harmonics:

AC voltage and current accuracies are affected by the "Crest Factor" of the waveform, the ratio of peak value to the RMS value.

Some meters deal with high "Crest Factor's" better than other.

 

[gar]

090216-0825 EST

chris:

I found a 1N4004 this morning and the results were about the same as for the 1N5060. Probably a better estimate of the difference between the DC average as transformed to an equivalent RMS and the true RMS at 120 V is about 0.4 V based on several measurements this morning.

This corresponds with a separate measurement I made on the diode for its DC forward drop at 50 microamps of 0.45 V. In the half wave rectification cycle for a 2 meg load the current varies from virtually 0 to 85 microamps each cycle. Thus, 50 microamps is a nice judgement for an average effect.

You should see the true RMS value about 0.4 V above the calculated value from the DC measurement. My guess is that the error from this method on a pure sine wave and using the 0.4 volt difference should be less than 0.2 V. This is 0.16 %.

.

 

[chris kennedy]

gar, I certainly appreciate all the time you spent on this. I don't keep diodes at the house or shop for that matter. The links to the testers you provided seem to be cat III. The closest company for calibration I found today was in Tampa.

To all members, DMM of choice on a $300 budget?

 

[wallyworld]

I have that same Ideal meter, I think its a piece of crap. Its slow to come to steady state and Amprobe part of it always seems low.

 

[byourdesky]

I have that same Ideal meter, I think its a piece of crap. Its slow to come to steady state and Amprobe part of it always seems low.

I too have that meter....everyone that uses it thinks its crap but usually cause they don't know what buttons to push.....it is pretty close most the time to a co-workers Fluke T5, but the T5 seems to be the choice of most people

Also I have seen alot of people wondering where to send their meters to be calibrated but not alot of answers...so do you send it to the manufacturer or to a "big box" place?

 

[zog]

gar, I certainly appreciate all the time you spent on this. I don't keep diodes at the house or shop for that matter. The links to the testers you provided seem to be cat III. The closest company for calibration I found today was in Tampa.

To all members, DMM of choice on a $300 budget?

Fluke 87, but it depends on your needs and what you use it for. get the Fluke DMM that best suits your needs, a 87 runs closer to $400 but you can find tham on ebay, calibrated, for around $300, might be an older model but 87IV or 87III is still a good meter.

 

[zog]

I have used these guys at every place I have worked (Besides one), dont know how much it costs to send something in, maybe $20 or so, they come to our shop and calibrate everything, charge us by the day.

http://www.etiinc.com/index.html

Also I have seen alot of people wondering where to send their meters to be calibrated but not alot of answers...so do you send it to the manufacturer or to a "big box" place?

You miss this? About every testing company I know uses them. (Hey my first multi-quote post, cool )

 

[electricalperson]

buy the fluke 117 and the 337

 

[byourdesky]

You miss this? About every testing company I know uses them. (Hey my first multi-quote post, cool )

No I saw it....just was hoping for a quick fix

 

[dbuckley]

Being honest, for general electrical troubleshooting, the meter you use for everyday tasks doesn't have to be very accurate, just dependable, and be able to bounce well when dropped. Or cheap, so when it doesn't bounce you just get another $20 meter out of the truck...

Edited to say - but do buy a decent set of test leads, which will outlast several meters...

 

[steelersman]

Or cheap, so when it doesn't bounce you just get another $20 meter out of the truck...

I'd just spend $2 on one of those DMM's that Harbor Freight has. No need for a fancy shmancy Fluke.

 

[quogueelectric]

The red and black leads are reversed on the meters so meters are reading inverted signals from each other assuming the same colors are in the same side of the receptacle.

 

[LarryFine]

(Hey my first multi-quote post, cool )

Myagi have hope for you!