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MM001: Valuable Information for Selecting a DMM (digital multimeter)

Number of digits:
This number indicates how many measuring signal digits can be displayed at the DMM.

Example:
The DMM shown at the left has 4½ places with 12,000 digits. Four full places are displayed from 0 to 9, plus one additional place (in this case a 1). As of a value of 11,999 (e.g. 11,999 V), automatic switching to the next higher measuring range takes place (100 V).

DMMs which display mean value:
In the case of voltage and current measurement, only purely sinusoidal signals are correctly measured. Due to todayís demanding requirements, this is no longer in keeping with the times. Even electronic low-voltage transformers read out extremely distorted sinusoidal signals from their outputs. With DMMs of this sort, either no value at all, or entirely incorrect values are displayed. A DMM with TRMS should be used for this reason.

Where can the multimeter be used?

Specifications indicating under which conditions the digital multimeter (DMM) can be used are included in the operating instructions and printed directly onto the instrument. Maximum permissible voltages in the various measuring ranges (measuring categories) must also be adhered to.

CAT I
Measurements in electrical circuits which are not directly connected to the mains, e.g. batteries etc.

CAT II
Measurements in electrical circuits which are directly connected to the low-voltage mains. Via plug, e.g. in household, office and laboratory applications etc.

CAT III
Measurements in building installations. Stationary consumers, distributor terminals, devices connected permanently to the distributor

CAT IV
Measurements at power sources for low-voltage installations. Meters, mains terminals, primary overvoltage protection devices.

This multimeter can be used in the CAT III range at up to 1000 V, and in the CAT IV range at up to 600 V.
Three Connector Jacks with Automatic Blocking Sockets (ABS) 1)

All current ranges are implemented via a single connector jack which prevents any possibility of operator error. Beyond this, the automatic blocking sockets prevent incorrect connection of the measurement cables, as well as selection of the wrong measured quantity. Danger to the user, the instrument and the device under test resulting from operator error is thus ruled out. 
1) Patented (patent no. DE 10 2005 062 624, US 7,439,725)
 

Intrinsic error (measuring accuracy):

Intrinsic error specifies maximum permissible error which may occur under certain conditions. Its value is specified as a percentage.

Examples:
With an intrinsic error of ±1%, an actual value of 100 V could be displayed within a range of 99.0 to 101.0 V. With an intrinsic error of ±1% +2 digits, an actual value of 100 V could be displayed within a range of 98.8 to 101.2 V.

DMMs which display TRMS / bandwidth:

Sinusoidal and non-sinusoidal signals are correctly measured and displayed by these instruments for voltage and current measurement. And the bandwidth is just as important as TRMS measurement! The signalís frequency band is an important factor when measuring alternating current and alternating voltage. For example in the case of a low-voltage halogen transformer, a multimeter with a bandwidth of greater than 24 kHz is required.

Current measurement > 10 A:

Measurement must frequently be performed for current which exceeds the DMMís measured value, or where the current carrying cable may not or cannot be interrupted. In this case, a current clip must be closed around the current carrying cable at a 90° angle in order to assure highest possible measuring accuracy.

Current clip as current transformer: measurement of alternating current only

Current clip as Hall effect current sensor: measurement of alternating and direct current
Important: These current clips have a voltage output (volts per ampere). Set the function selector switch to VAC+DC for this reason! The transformation ratio, for example 1:1000 V / A, can be selected in the setup menu, and the measured current value is then automatically calculated and displayed.