How do I review a DMM
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.
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.
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).
The "Build-in" means that the meter can directly show internal temperature.
This is a list of various functions that the meter supports.
- min/max: Standard min/max
- Peak: Fast min/min, usual 1msec or less to measure voltage
- Average: Show average value, often part of the min/max function.
- Relative: Zero the display at any value and show voltage relative to this.
- Hold: Freeze the display, usual not very useful
- Auto-hold: Automatic freeze the display when a stable value is detected, can be very useful.
- Trend-plot: Show a curve of the last captured values.
- Hi-Z: Some range has very high input impedance (Gohm or more), often in mV range and for some meters the lower volt range.
- Low-Z: Meter has a special AC range with lower input impedance, very useful for measuring on main voltage. A standard 10Mohm input impedance may show voltage just because the cable is close to another cable with mains voltage, the Low-Z prevents that.
- Freq-V: It is possible to select frequency measurement in Volt ranges, the frequency range on with this selection will usual be lower than a dedicated frequency input, but can work with higher voltage.
- Freq-A: It is possible to select frequency measurement in Current ranges.
- Freq-DC: Frequency measurement works with single sided pulses, they do not have to cross zero volt.
- Duty-cycle: Meter can measure duty-cycle, a DMM that can measure frequency will usual have this.
- True-RMS: DMM can measure correctly on any AC waveform.
- True-AC: DMM will not register DC when AC is selected
- Backlight: A backlight can be turned on in the display.
- Square-out: The meter can output a square wave
- Led-test: The meter can display led voltages in diode test. Even without this function the voltage may be high enough that a led will glow, check the notes in the measurement section.
- Auto-off: The meter will turn automatic off
- Manual-off: The automatic off can be disabled, i.e. the meter will stay on until manually turned off.
- LPF: Low pass filter, used on variable frequency drives.
- NCV: Non Contact Voltage detector, i.e. the meter will give an alarm if near mains voltage.
If I test a meter with interface this will show what is supported:
- RS232: Old style serial port.
- USB: Usb with HID interface.
- USB/com: Usb with a virtual com port.
- LAN: Direct commands over LAN.
- LAN/LXI: Use the LXI protocol to give commands.
- GPIB: Old style professional interface.
Tolerances for some of the ranges. Both lines are for 25°C (or thereabouts) and without special function activated. Function like Low-Z and VFC will usual reduce precision.
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.
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.
Even though I do not verify all ranges I do some checking.
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.
To generate precise currents and voltage this meter is very useful. It can also be used as a ohm meter.
For frequency, RMS/average and other test where I need AC or pulses I uses this generator.
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.
For DC voltages in the 0-10V range this handy little reference works very well.
For testing resistance and capacity I have a box with fairly precise resistors and capacitors.
For checking when the DMM change range and continuity this decade box is more useful.
And for capacity a decade box is also useful, together with some loose electrolytic capacitors for the high values.
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.
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.
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):
- CAT I: Electronic and other equipment after the mains transformer.
- CAT II: Anything connected to normal household outlets.
- CAT III: Industrial and other high current installations.
- CAT IV: Distribution and outdoor conductors.
The difference between the above CAT's is transient voltage and impedance.
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.