# Ammeter time limits - how to understand them [duplicate]

On my multimeter, near the 10 A connector it says 10 A MAX 10sec EACH 15min. I usually measure currents less than 1 A so I never cared about this.

Is it correct to assume that if the measured current is lower than 10 A, I can take the measurement in more than 10 sec and I don't have to wait 15 minutes?

I measured only a few times a current of about 4 A and I'm sure I didn't follow those time limits and the multimeter is still working (so far).

• It's a thermal rating. As a VERY rough guide you could probably expect time to vary with the square of the current. So at 4A ratio of squares = (/10)^2 = 0.16 ~=1/6. So you can probably safely use the meter to measure 4A for 10s every 15/6 =~ 2.5 minutes. Also, if you reduce the measurement time to less than 10 seconds a proportionally faster measurenet rate can be used. – Russell McMahon Jan 3 '15 at 12:44
• Which model of multimeter is this? – neverMind9 Sep 13 '18 at 13:36
• @neverMind9 it's Uni-T DT-830B – Cornelius Sep 13 '18 at 16:41

The short-term specification — 10A for 10 seconds — is an indication of how much energy the meter can absorb without going outside its accuracy specifications (and without damage). You can probably assume that for lower currents, you can extend the time proportionally to the square of the reduction, since power is proportional to current squared. E.g., at 5 A, you can probably make measurements in the short term that add up to 40 seconds.

The long-term specification — each 15 minutes — is an indication of how quickly the meter can dissipate the absorbed energy and return to thermal equilibrium. This is equivalent to an average power rating. For example, if you measure 5A for 10 seconds, which is 1/4 the energy, you should be able to do this every 15/4 = 3.75 minutes.

• Highly, highly useful answer. Thank you. But is there a point (let's say ~0.5A), where the measurement duration gets indefinite due to faster downcooling than heating up? And what about the temperature of surroundings? – neverMind9 Sep 13 '18 at 13:39
• @neverMind9: Yes, there's probably a power level that the meter as a whole can dissipate continuously. But it isn't possible to infer what that value might be from the specifications as given. – Dave Tweed Sep 13 '18 at 15:13

First time I measured a 10A power supply (using a virtually bombproof AVO-8 ) I got a surprise after several minutes when I burnt myself on the test leads! The probes were overheating too, but the meter was made of stronger stuff!

Anyway after that, I'm not surprised to see limitations like that on anything but the most rugged grade of multimeters. At 4 amps I'd keep it down to maybe a minute or so and let it cool off, or if I was making quick measurements I wouldn't worry about it.

• The less rugged ones (\$2.50 meters) probably have a similar limitation but it's not written on the front. – Spehro Pefhany Jan 3 '15 at 16:11

If you measure 10A, you can do so for 10s, then you must leve the multimeter to cool down for 15min, I believe.

If I remember correctly measuring continuously 10A will not destroy your unit (at least my survived), but will affect the accuracy.

If you measure lower currents, you will hit a different delta temperature. This deltaT may or may not be enough to affect the accuracy of your unit, but it is not going to be necessarily linear - it depends on the overload.

I would expect to see something similar to the pulse derating curve for a resistor, but those are only my expectations.