how to know whether my Digital MultiMeter is correct?

[Marius]

Hi!

 

I have a few DMM's here and I want to know which one is the most accurate when it comes to Voltage measuring.

 

I need a steady source for that.

 

Is an USB output always 5.00V (On a laptop or computer)? Even when nothing is connected to it?

 

If that is true I can check which DMM gives the best result.

 

If you have other (better) idea's... please let me know.

 

Thanks

M.

 

[Rybags]

Under no load it might be higher.

 

Generally the power rails on a PC's PSU won't sit exactly on +5, +12 etc. as reported by sensor software like Speedfan, they usually fluctuate and overall load can be a factor.

I suppose you could use a molex connector from a PC, compare the DMM readings with what the BIOS or Windows based monitoring tool reports.

 

Also, if the battery in a DMM is getting low, it can start to malfunction and give wild results.

Edited November 28, 2014 by Rybags

[Marius]

Ok... thanks for explanation!

[Bryan]

1. Meter calibration. Not a usually cheap option, but meters used in industry are frequently calibrated each year so there are many places which provide the service. Calibrate one, and you'll know how the rest are performing.

 

2. Buy some close tolerance (0.1%-1%)parts and use them to get a rough idea. There are precision resistors, voltage references, etc.. which can be used to verify that your meter is in the ballpark. Buy several and take an average.

 

3. Many new meters come with a calibration certificate and if you're going to pay for calibration, it might just be worth putting that money toward a new, factory calibrated instrument.

 

Don't rely on consumer power supplies to be accurate. For an accurate voltage reference, look at precision shunt regulators like the LM4040-N.

[+Larry]

Why would you not just use 1 or more brand new alkaline batteries? I would think that mfgr's data sheets would provide tolerances. But you may be looking for even closer tolerances...

Larry

[Rybags]

Batteries aren't exactly a reliable test method either, can vary depending on shelf time, temperature etc.

 

IIRC my oscilloscope has a test point, although I think it provides something like a square wave at 5 Volts, 1000 Hz.

[+bob1200xl]

I vote for the voltage reference IC, also. It only takes a couple of parts and you can just use a battery to power it.

 

How accurate do you want it? Why? That would point you at a solution.

 

Bob

[576XE]

Accurate measurements costs very expensive isn't it?

Many thousands dollars.

[Marius]

Thanks all!

 

That LM4040-N seems to be an interesting component. I do not understand (yet) how to make a circuit what gives a perfect 5.000V but I'm not in a hurry.

 

Let me explain:

I have had a lot of issues with my CAR Battery (as long as I have this car). Now I have a NEW battery again and I have decided to keep track of the health of the battery. I have no leaks of current, so when the car is off there is no power consumption. 0.000A ... so that is fine. Probably I use my car not enough, and when I drive it is probably not long enough.

 

Now comes the Digital Multimeter Thing..

 

DMM 1 shows me: Battery Voltage = 12.23 V

 

DMM 2 shows me: Battery Voltage = 12.87 V

 

DMM 3 shows me: Battery Voltage = 13.62 V

 

I think -since this Battery is brandnew- that DMM 2 is right, but I need to be sure.

 

If I notice that the voltage of my battery is getting below let's say 12.40V I'll drive a longer trip to recharge the battery. I understood that this type of battery it is very bad to get too low on voltage for a longer period.

 

So thats the explanation. I need to know which DMM is right.

 

So I measured on my USB

 

DMM 1 showed 4.99V

 

DMM 2 showed 5.04V

 

DMM 3 showed 5.23V

 

Strange eh??

 

Greetz

M.

[+Stephen]

Good lord - that is some crazy variance. Does it do that if you hook up all 3 at the same time? What brands are these? I would say it they are not Fluke, they are not accurate.

[Marius]

All three are brandless crap... hence the terrible results. But I guess they should all three be capable of this simple task right... I agree, I should buy myself some decent equipment

[AtariGeezer]

Have you tried replacing the battery in your DMM's? When the battery voltage gets too low, you'll have different readings...

[Bryan]

To use a shunt regulator, you just need a battery a bit higher than the regulator voltage, a resistor, and the regulator itself. If you got an LM4040 of the 5V/0.1% variety, and put a 470 ohm - 1 kohm resistor in series with it and connected it to a 9V battery, you'd have a very accurate 5V across the LM4040 (look at the datasheet for an example circuit).

 

A meter like this would be affordable and accurate:

http://www.amazon.com/Precision-2703C-Ranging-Digital-Multimeter/dp/B004TMT592

[+bob1200xl]

An LM4040 circuit would tell you which meter is the most accurate, at the time of the test. I'm not sure absolute accuracy is needed as much as relative, long-term consistency. In other words, when the car battery shows xx.yy volts, it is well charged, and when it shows aa.bb volts, it needs charging. The absolute values of xx.yy and aa.bb do not matter so much as them being repeatable over time. If the meter drifts all over the place with time, temperature, and internal battery voltage and such, it isn't going to be too useful for your needs.

 

Oh - get a trickle charger for you car... ??

 

Bob

[russg]

Thanks all!

 

That LM4040-N seems to be an interesting component. I do not understand (yet) how to make a circuit what gives a perfect 5.000V but I'm not in a hurry.

 

Let me explain:

I have had a lot of issues with my CAR Battery (as long as I have this car). Now I have a NEW battery again and I have decided to keep track of the health of the battery. I have no leaks of current, so when the car is off there is no power consumption. 0.000A ... so that is fine. Probably I use my car not enough, and when I drive it is probably not long enough.

 

Now comes the Digital Multimeter Thing..

 

DMM 1 shows me: Battery Voltage = 12.23 V

 

DMM 2 shows me: Battery Voltage = 12.87 V

 

DMM 3 shows me: Battery Voltage = 13.62 V

 

I think -since this Battery is brandnew- that DMM 2 is right, but I need to be sure.

 

If I notice that the voltage of my battery is getting below let's say 12.40V I'll drive a longer trip to recharge the battery. I understood that this type of battery it is very bad to get too low on voltage for a longer period.

 

So thats the explanation. I need to know which DMM is right.

 

So I measured on my USB

 

DMM 1 showed 4.99V

 

DMM 2 showed 5.04V

 

DMM 3 showed 5.23V

 

Strange eh??

 

Greetz

M.

I guess you don't drive your car very much. In that case, like Bob 1200XL says, get a trickle charger (less than 3 amp), or a small car battery charger

(like 3 amp, which actually charges at about 1.5 amp).

My 3 amp charger, it has a ammeter on it, if it gets to .5 amp charging, it is done. Such charger plugs into your wall socket, off 120v. Looks like DMM 1 and 2 are close.

DMM will vary on the condition of its battery. Fresh batteries are good to have. There are car battery testers that put a load on the battery when testing. That type of

tester is superior to just a volt reading. I go by 12.3 volts being the bottom of what I don't have to charge. Lower than that and on goes the 3 amp charger. Car battery

chargers come in more amps, and more expensive, but 3 amp is sufficient. With battery at 12.3 volts to start, you can charge 10 hours (overnight) with a 3 amp

charger. More expensive car battery chargers would sense the voltage and quit, but my 3 amp one I have to control it. A fully charged car battery is about 12.8 volts.

It will read 13 or 14 volts right after you take the charger off it, let it sit an hour before reading.

It should measure about 14v with alternator (engine running). With my stupid 3 amp charger, you don't want to charge too long.

Car batteries are six 2 volt cells. A 10. something volts indicates a dead cell. Then there are battery hydrometers to test the level of charge (a bulb tube

with markings on it for testing the solution of open cell batteries.)

Edited November 29, 2014 by russg

[chevymad]

Test leads can vary widely in their resistance as well. Better meters have a way to zero the meter when you touch the leads together. How old is this car? Almost no car made in the last 20 years has a 0 draw. Any radio, clock, or ecm in the car will draw some for memory. The more computers in the car the more it will draw. A 2000 era caddy might drain a battery til it won't start in just a couple months. While a 68 chevy could sit all winter.

 

I've got several cars that dont get driven much anymore. A few years ago I bought a few battery maintainers from harbor freight. They were about $7 each. I put them on 2 of my cars and the riding mower. They really have helped. I thought the mower battery was trash when I started using it, but its still going now! These will not charge a battery but will keep it charged.

[Marius]

The problem is, my car is always outside, parked in the street. I cant trickle charge due to this setup.

 

It is a Fiat Panda 1.1 from 2002 (new in 2002) and it had the car-radio originally bypassed (so it can play even when the car is off), but I connected it different, so car off = radio off. The electronics in the car shut off around 30 seconds after the engine is stopped. After 30 seconds no current is used.

 

I drive very short trips (too short), AND this car lacks a battery-meter on the dashboard. So I have no idea what the state of the battery is. Almost every 2 years I have to replace the battery. So now I want to change my strategy, and check the voltage of the battery once a week.

 

As soon as the voltage is around 12.4V or so, Ill make a longer trip (let's say 45 minutes or so) so the battery will get a chance to recharge.

 

I understand this is still not the most ideal solution, but it is in my circumstance the only way of increasing my battery-lifespan.

 

Another solution would be a trickle-charger with a battery. I would first charge that device in my home, and then I could connect that device to my cigarette-lighter inside my car (there is an open connection from that to my battery). But I'm not aware of the existence of such a device.

 

Does exist a trickle charger that charges my car-battery, using a built-in-charged Battery (which can charged inside the house) so I can trickle-charge my car-battery without the need of a wire from my house-to-the-car?

 

This type of car btw.

 

 

I have heard of Solar-Cell-charge devices for the Car Battery, but I have also heard those are pretty useless. Anyone with a positive experience with these?

Edited November 29, 2014 by ProWizard

[Bryan]

Test leads can vary widely in their resistance as well. Better meters have a way to zero the meter when you touch the leads together.

Test leads won't affect a voltage reading since the internal resistance of the meter is very high. You should zero the meter when taking low capacitance or low resistance measurements.

[russg]

The problem is, my car is always outside, parked in the street. I cant trickle charge due to this setup.

 

It is a Fiat Panda 1.1 from 2002 (new in 2002) and it had the car-radio originally bypassed (so it can play even when the car is off), but I connected it different, so car off = radio off. The electronics in the car shut off around 30 seconds after the engine is stopped. After 30 seconds no current is used.

 

I drive very short trips (too short), AND this car lacks a battery-meter on the dashboard. So I have no idea what the state of the battery is. Almost every 2 years I have to replace the battery. So now I want to change my strategy, and check the voltage of the battery once a week.

 

As soon as the voltage is around 12.4V or so, Ill make a longer trip (let's say 45 minutes or so) so the battery will get a chance to recharge.

 

I understand this is still not the most ideal solution, but it is in my circumstance the only way of increasing my battery-lifespan.

 

Another solution would be a trickle-charger with a battery. I would first charge that device in my home, and then I could connect that device to my cigarette-lighter inside my car (there is an open connection from that to my battery). But I'm not aware of the existence of such a device.

 

Does exist a trickle charger that charges my car-battery, using a built-in-charged Battery (which can charged inside the house) so I can trickle-charge my car-battery without the need of a wire from my house-to-the-car?

 

This type of car btw.

panda.jpg

 

I have heard of Solar-Cell-charge devices for the Car Battery, but I have also heard those are pretty useless. Anyone with a positive experience with these?

Don't know of a battery-oriented charger. If you get a 3 amp indoor battery charger, then take the battery out once a month and charge it over night, that would work.

Or, you could test your battery once a week and if below 12.3 volts, take it in to charge it. You need some non-driving way to charge the battery when you go for

extended non-driving period. Your DMM doesn't have to be perfect. .... OR, lemme see. Buy a second battery and a really cheap small trickle charger. Keep

a battery inside on the charger and swap batteries once a month.

Deep cycling a car battery, going below 12 volts, and then charging it isn't good for the battery. Your battery should last for at least five or six years.

Probably ten years. Keep the water level where it is supposed to be, with distilled water. Always charge it after adding water. (Atari?)

Edited November 29, 2014 by russg

[UNIXcoffee928]

It is a Fiat Panda 1.1 from 2002

...

This type of car btw.

panda.jpg

 

Ahhhgggk!

 

Easy solution: sell the Lunchbox-Deathtrap, buy an inexpensive, safe, powerful, fun to work on used Porsche 928, and bank the rest to fill the refrigerator.

 

 

= )

[_The Doctor__]

You should put a main terminal disconnect in that car.. that will keep the battery draw down to a minimum.... as for the solar charger... use two of them in parrallel...

Your car should be happy to start.... thats what a few friends in the upper penninsula do and they are happy... remember to wipe off the panel and glass from time to time and point 'em in the right direction.... resting voltage of a car battery should be about 12.7 v... this can vary by battery type and cell health.. I could make this a five page battery report about sulfation, crystallization, thresholds and the like but most peoples eyes will glaze over at that point and suffer cerebral shutdown...

Try to use a 3 or 4 step charger on your car batteries...

[russg]

You should put a main terminal disconnect in that car.. that will keep the battery draw down to a minimum.... as for the solar charger... use two of them in parrallel...

Your car should be happy to start.... thats what a few friends in the upper penninsula do and they are happy... remember to wipe off the panel and glass from time to time and point 'em in the right direction.... resting voltage of a car battery should be about 12.7 v... this can vary by battery type and cell health.. I could make this a five page battery report about sulfation, crystallization, thresholds and the like but most peoples eyes will glaze over at that point and suffer cerebral shutdown...

Try to use a 3 or 4 step charger on your car batteries...

OK, I learned a little about 3 stage charger (IUoU). Basically it is a smart charger, won't over charge and start gassing. I thought gassing was indication of full charge and

a good thing, which it isn't, it damages the battery if done too long. I was wrong about each cell being 2 Volts. It is 2.1 volts, for a 12.6 volt fully charged battery. I

guess 12.8 volts is too high.

I still believe the OP needs some type of indoor charger.

[chevymad]

Good to know on the lead resistance. Cause some of mine have been 15 ohms or more. I think a solar charger is the answer. Also just because the electronics "shut off" after 30seconds doesnt mean they won't draw. Capacitors won't hold their charge forever and they'll suck up a bit of power again. Also if your radio remembers which station you were on last time you were in the car... it has a memory function and that too draws power. Any clock in the car, draws power. Also if its fuel injected the ecm will also draw some power. Working in a shop, I find car batteries generally last just about the warranty period. About 5 years unless a cheapie. Yes, there are some that last 10 years or more. But you have no way to know if you have one of those. Doesn't seem to matter by brand or design, just luck of the draw.

 

A high amperage/small physical size battery won't last as long in your application either. The plates are made closer together to get more of them into the battery. This makes it easier for the battery to sulfate if it sits for long periods. Kind of funny but an inferior battery alot of times lasts longer in this situation.

 

One last thing, an addon 2" voltmeter gauge is pretty cheap. You could install that right into your car and have a means of checking both your charging system and battery condition without the multimeter.

[Bryan]

Leads should be well under an ohm, so a 15-ohm lead needs to go in the garbage.

[Marius]

 

Ahhhgggk!

 

Easy solution: sell the Lunchbox-Deathtrap, buy an inexpensive, safe, powerful, fun to work on used Porsche 928, and bank the rest to fill the refrigerator.

 

 

= )

 

Haha… I am so fond of that car. It's my 2nd…. I have never owned another type/brand car. They are AMAZING. Check this ad from 1981… can't beat that right?

 

 

or check this… that is even cooler!

 

Edited November 29, 2014 by ProWizard