# How to Select a Digital Multimeter 2?

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## How to Select a Digital Multimeter 2?

Apart from digit and resolution, another factor to judge the accuracy of a multimeter is precision, i.e. the degree of closeness of the measured value to the actual value of a signal that has been measured. To take ZOTEK and ZT109 as an example, their precisions for direct voltage is 0.5%+3; when it is used to measure 100.0V direct voltage, its reading will be in the range of 99.2V-100.8V. Occasionally, one might find the precision of the multimeter he was using fell short of the precision range given by factory. There are three causes for this situation. First, the product is defective; second, the user slipped up in his reckoning; third, the standard value that the user adopted was wrong. In the case of the first cause, the user should get the seller for changing or refunding. In the event of the second cause, the correct precision calculation formula should be the difference value/ratio of the measured value to the correlative value; the user should look to adopting same units in calculations; for example, when the measured value is 9.98mV and the correlative value is 0.010V, the unit of the correlative value should be converted to mV, i.e. 10mV, before calculation; then the result will be (10-9.98）/10=0.002=0.2%. In the event of the third cause, the user should choose correct contrast objects. Many users slipped out in the multimeter selection, that is, the precision of an internationally known brand must be the highest, and thus, they regarded the measured value of famous brand multimeter as a correlative value. But a famous brand multimeter with 1999 counts may not be higher than that of a common brand with 6000 counts. Assuming that the actual value is 10mV, the famous brand multimeter will get it 9.92, and a common brand 9.98; if the correlative value 9.92 is used in calculations, we will get that the precision of the common brand will be (9.98-9.92）/9.92≈0.6%, which differs substantially from the prior result (0.2%). Therefore, the values obtained from the multimeter with more digits and higher precision should be used as the correlative values in the precision calculation.

True RMS is a notion relative to the average value. The measurements to describe alternating current cover peak value, the average value and virtual value. Among others, the peak value refers to the maximum value; the average value is the average of absolute values of alternating current in a certain period, while the virtual value is defined via heating power. When direct and alternating currents separately go through the same resistance device, if they generate equivalent heat in the same period of time, then the voltage and current values of the direct current will be regarded as the virtual values of the alternating current. When people refer to voltage or current of alternating current, they generally mean the virtual values. However, it is not easy to measure virtual values, compared to the peak and average values. In sinusoidal wave conditions, the peak, average and virtual values may have a correlation. When the peak or average value is known, the virtual value of alternating current may be reduced by a specific formula. Thus, parameters of alternating current measured by lots of multimeters on the market are actually values deduced from the peak or average value by the sinusoidal wave relation. But if the non-sinusoidal wave is measured, the measured “virtual value” will vary tremendously from the true virtual value, even as much as 40 percent. The so-called true virtual value multimeter is to measure the true virtual value by a specific means, rather than a deduced one from the peak and average values. For a user who needs not to measure the parameters of alternating current frequency, or just needs to measure the standard sinusoidal wave, a multimeter without the function of presenting true virtual value will make no big difference; but if high precision is very important for a user and complicated waveforms need to be processed, then it is better for the user to select a multimeter that serves the function of presenting the true virtual value.