How do I review a DMM

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Some reviews focus on verifying as many ranges as possible on the DMM, in my reviews I will not use much space on that. Instead I will focus on what ranges and functions it supports, including a schema that show the most important ranges.

DMMschema1

The first part will show all the common ranges with the maximum reading for each range. When possible I will verify the maximum reading and include the value I see (Many DMM's allows a little bit higher reading than specified).
If I find some range with strange behavior or large errors I will mark them with a red background.

DMMschema2

For DMM's with temperature function this section will contain data. The first line will show the possible formats, i.e. where the comma is placed.
The second line will show the supported sensors. Any DMM that supports a thermocoupler (the letters) will have a built-in sensor, but it may not be possible to read it directly (Shorting the thermocoupler input will always show the reading). Thermocouplers are not very precise.
PT100 & PT1000 are very precise standard sensors.
Thermistors are not standarized, the meter will probably support one from the same manufacturer.
The "Build-in" means that the meter can directly show ambient temperature.

DMMschema3

This is a list of various functions that the meter supports.

DMMschema4

If I test a meter with interface this will show what is supported:
DMMschema5

Tolerances for some of the ranges. Both lines are for 25C (or thereabouts), one year calibration and without special functions activated. Function like Low-Z and LPF will usual reduce precision.

DMMschema6

This is the data sheet values for frequency for voltage, current and frequency counter input. A rms meter will usual be able to measure sinus voltage at considerable higher frequency.

DMMschema7

How much voltage is dropped across the meter when measure current. To make it easy to compare meters I have selected some fixed current, not the maxium for the ranges. Keep an eye on the 1mA and 100mA values, and multiply them with the maximum range, some meters have more then 1 volt in burden voltage at maximum current!

DMMschema8

What type of power source the meter uses and how long it is expected to run on it (I use measured power consumption), this will usual be:
AA: AA alkaline batteries, assuming 2800mAh capacity.
AAA: AAA alkaline batteries, assuming 1000mAh capacity.
9V: 9V alkaline block, assuming 600mAh capacity.
Mains: Bench DMM's are usual mains powered




Testing parameters

Max/min or Peak capture time: This is done with a function generator at maximum ampliture (20Vpp) pulses and then adjusting the puls width until the meter shows 95% or more of the signal voltage.

Input impedance: A DMM is used to measure the voltage range, to measure the mV range I uses a Source meter that can test with a low voltage (Due to input protection the high input impedance will drop drastic when going over range).

Open voltage in ohm, continuity and diode is tested with a high impedance DMM

Continuity speed: I use a mosfet to short the input, it is trigged from a 1Hz pulse with variable length. I look for the shortest pulse where the meter will beep for at least 20 pulses in a row (I only check with 1 digit precision).



Equipment

Even though I do not verify all ranges I do some checking.

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This meter secures that I have enough precision, even without a reference.
It can also do 4 terminal ohm, making it easy to get precise resistance measurements of the probes.

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To generate precise currents and voltage this meter is very useful. It can also be used as a ohm meter.

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For frequency, RMS/average and other test where I need AC or pulses I uses this generator.

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Probes may be fine for voltage measurement, but when testing the frequency counter into the MHz range, it is not a good solution.
The DMM range will depend on input voltage, to keep that fairly predictable I use a coax cable from my generator and first convert to banana plugs at the DMM connectors.

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For DC voltages in the 0-10V range this handy little reference works very well.

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This box can simulate thermocouplers and allows me to check temperature ranges. It can also compensate for ambient temperature.

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For testing resistance and capacity I have a box with fairly precise resistors and capacitors.

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For checking when the DMM change range and continuity this decade box is more useful.

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And for capacity a decade box is also useful, together with some loose electrolytic capacitors for the high values.

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When checking continuity speed I use a MOSFET driven from a pulse generator. To make it easier to use I have put the MOSFET in a box.
I try to find the pulse with where it beeps each time.

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This box has the mains voltage on two terminals (0 & 230VAC) and a voltage doubler output on 620VDC with a divider to 200VDC.
A DMM on DC range must show:
A DMM on AC range must show:
If the meter works as above it will get the true-ac checkmark.
All the above voltages are not precise and I might use 120VAC as input, instead of 230VAC (For safety reasons this box is always feed from my AC generator or isolation transformer).

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I have power supplied that can deliver a lot of current (50A), but when testing a clamp meter it is not enough. This small coil makes it very easy to check a clamp for a few hundred amperes. Because I do not now the exact number of turns (It is about 200) I do not know exactly what it is supposed to show, but I can see how it behaves at maximum current.



CAT rating

DMM's usual has a CAT rating printed on them, this defines where it can be safely used and for what voltage.

The ratings are (Explanation is simplified):
The difference between the above CAT's is transient voltage and impedance.

CatRatings

A CAT III 1000V or CAT IV 600V meter must be able to handle 8000V from 2ohm (i.e. up to 4000A) on any range, including the ohm and current ranges. This requires some good protection and fuses that can break high current.