Factors to consider when purchasing a digital multimeter
The main indicators of digital multimeter display digits and display characteristics
A digital multimeter's display digits typically range from 3 1/2 to 8 1/2 digits. There are two rules for evaluating the display digits of digital instruments: the first is that the digits that can display all numbers from 0 to 9 are known as integer digits; is the numerator, and the count value is 2000 when the full scale is used, showing that the instrument has 3 integer digits; the fractional digit's numerator is 1 and the denominator is 2, so it is known as "three and a half digits," and the highest bit can only.The highest digit of a digital multimeter, called 3 2/3 digits (pronounce "three and two-thirds digit"), can only display integers ranging from 0 to 2, hence the highest display value is 2999. It is 50% higher than a 3 1/2 digit digital multimeter's upper limit under the identical circumstances, which is particularly useful when measuring 380V AC voltage.
For instance, the highest number of a standard 3-1/2-digit digital multimeter can only be 0 or 1 while measuring grid voltage. You can only use three digits to represent grid voltage when measuring 220V or 380V. only 1V. The top digit of a 3 3/4-digit digital multimeter, in contrast to a 4 1/2-digit digital multimeter, can display 0 to 3, allowing it to display the grid voltage in four digits with a resolution of 0.1V. equal force.
Popular digital multimeters often fall under the category of hand-held multimeters with a 3 1/2-digit display. Digital multimeters with 4 1/2 and 5 1/2 digits (below 6 digits) can be classified as either handheld or desktop. Benchtop digital multimeters with more than 6 1/2 digits constitute the majority.
The digital multimeter uses cutting-edge digital display technology, providing a clear, user-friendly display and precise readings. It not only caters to readers' reading preferences and assures the objectivity of the reading, but it can also cut down on reading or recording time. These benefits aren't offered by conventional analog (i.e. pointer) multimeters.
Accuracy (Precision)
The accuracy of a digital multimeter is a combination of systematic and random errors in the measurement results. It indicates the degree of agreement between the measured value and the true value, and also reflects the size of the measurement error. Generally speaking, the higher the accuracy, the smaller the measurement error, and vice versa.
There are three ways to express the accuracy, which are as follows:
Accuracy = ± (a% RDG + b% FS) (2.2.1)
Accuracy = ± (a% RDG + n words) (2.2.2)
Accuracy = ± (a% RDG + b% FS + n words) (2.2.3)
In the formula (2.2.1), RDG is the reading value (that is, the display value), FS represents the full-scale value, and the previous item in the brackets represents the A/D converter and functional converter (such as voltage divider, shunt, true effective value converter), the latter is the error due to digitization. In the formula (2.2.2), n is the amount of change reflected in the last digit of the quantization error. If the error of n words is converted into a percentage of full scale, it becomes formula (2.2.1). Formula (2.2.3) is rather special. Some manufacturers use this expression, and one of the last two items represents the error introduced by other environments or functions.
Digital multimeters are far more accurate than analog analog multimeters. Taking the accuracy index of the basic range for measuring DC voltage as an example, 3.5 digits can reach ± 0.5%, 4.5 digits can reach 0.03%, etc. Example: OI857 and OI859CF multimeters. The accuracy of the multimeter is a very important indicator. It reflects the quality and process capability of the multimeter. It is difficult for a multimeter with poor accuracy to express the real value, which may easily cause misjudgment in measurement.
Resolution (resolution)
The voltage value corresponding to the last digit of the digital multimeter on the lowest voltage range is called resolution, which reflects the sensitivity of the meter. The resolution of digital digital instruments increases with the increase of display digits. The highest resolution indicators that digital multimeters with different digits can achieve are different, for example: 100μV for 3 1/2 digit multimeters.
The resolution index of the digital multimeter can also be displayed by resolution. Resolution is the percentage of the smallest number (other than zero) that the meter can display to the largest number. For example, the minimum number that can be displayed by a general 3 1/2-digit digital multimeter is 1, and the maximum number can be 1999, so the resolution is equal to 1/1999≈0.05%.
It should be pointed out that resolution and accuracy belong to two different concepts. The former characterizes the "sensitivity" of the instrument, that is, the ability to "recognize" tiny voltages; the latter reflects the "accuracy" of measurement, that is, the degree of consistency between the measurement result and the true value. There is no necessary connection between the two, so they cannot be confused, and the resolution (or resolution) should not be mistaken for similarity. Accuracy depends on the comprehensive error and quantization error of the internal A/D converter and functional converter of the instrument. From the perspective of measurement, resolution is a "virtual" indicator (which has nothing to do with measurement error), and accuracy is a "real" indicator (it determines the size of measurement error). Therefore, it is not possible to arbitrarily increase the number of display digits to improve the resolution of the instrument.
Measuring range
In a multi-function digital multimeter, different functions have their corresponding maximum and minimum values that can be measured. For example: 4 1/2-digit multimeter, the test range of DC voltage range is 0.01mV ~ 1000V.
measurement rate
The number of times a digital multimeter measures the measured electricity per second is called the measurement rate, and its unit is "times/s". It mainly depends on the conversion rate of the A/D converter. Some handheld digital multimeters use the measurement period to indicate the speed of measurement. The time required to complete a measurement process is called the measurement cycle.
There is a contradiction between the measurement rate and the accuracy index. Usually, the higher the accuracy is, the lower the measurement rate is, and it is difficult to balance the two. To solve this contradiction, you can set different display digits or set the measurement speed conversion switch in the same multimeter: add a fast measurement file, which is used for the A/D converter with a faster measurement rate; Improving the measurement rate, this method is relatively common at present, and can meet the needs of different users for the measurement rate.
input resistance
When measuring voltage, the instrument should have a high input impedance, so that the current drawn from the circuit under test is very small during the measurement process, which will not affect the working status of the circuit under test or the signal source, and can reduce measurement errors. For example: The input resistance of the DC voltage range of a 3 1/2-digit handheld digital multimeter is generally 10μΩ. The AC voltage file is affected by the input capacitance, and its input impedance is generally lower than that of the DC voltage file.
When measuring current, the instrument should have a very low input impedance, so that the influence of the instrument on the circuit under test can be reduced as much as possible after being connected to the circuit under test. Burn out the meter, please pay attention when using it.
