The accuracy level classification of AC voltmeters is based on national measurement standards, which are quantitatively defined through the "allowable error range". The level symbols are usually marked with numbers (such as 0.1, 0.2, 0.5, 1.0, etc.), and the smaller the number, the higher the accuracy. The division logic, core indicators, and level classification are as follows:
1、 The core definition of accuracy level: allowable error formula
The accuracy level of an AC voltmeter essentially specifies the maximum allowable deviation between the measured value and the true value under rated conditions (such as rated temperature, humidity, voltage range). The general calculation formula is divided into two categories, depending on the type of instrument (analog/digital):
Absolute error dominant (commonly seen in digital voltmeters) allowable error=± (a% x measured value+b x minimum division value)
A%: Relative error coefficient (directly related to accuracy level, such as 0.5 level corresponding to a=0.5);
b: Fixed error coefficient (related to the hardware accuracy of the instrument, usually 1-2, representing the "fixed deviation of the minimum division value level");
Example: A 0.5 level digital AC voltmeter with a minimum division value of 0.1V. When measuring 220V, the allowable error is ± (0.5% × 220+1 × 0.1)=± (1.1+0.1)=± 1.2V.
Relative error dominant (commonly seen in analog voltmeters) allowable error=± (a% x full-scale value)
The error of the analog meter is based on the "full range" (rather than the measured value), so the actual relative error will increase in the "non full range range range";
Example: A 1.0 level analog AC voltmeter with a full range of 500V. When measuring 100V, the allowable error is ± (1.0% × 500)=± 5V, and the actual relative error is ± (5V/100V) × 100%=± 5% (much higher than the literal level of 1.0 level).
2、 Key prerequisite for division: "Rated working conditions"
The annotation of accuracy level is only valid under "rated working conditions". If the environment exceeds the range, the actual accuracy will deviate from the level requirements. The rated conditions usually include:
Environmental temperature: Most instruments have a temperature of (23 ± 5) ℃ (laboratory grade) or (0-40) ℃ (industrial grade);
Relative humidity: ≤ 85% (no condensation);
Power supply conditions: AC 220V ± 10%, 50Hz ± 2Hz (or stable DC power supply);
External interference: No strong electromagnetic radiation (such as staying away from frequency converters and high-voltage lines), no severe vibration.
If the ambient temperature exceeds the rated range (such as 45 ℃ in industrial sites), the actual error of some 0.5 level instruments may expand to 1.0 level or even higher.
3、 Core principles for level selection
'Do not choose too high': High precision level (such as 0.05 level) instruments have high costs and strict environmental requirements. If only used for routine monitoring (such as workshop 220V voltage), choose 0.5-1.0 level to avoid resource waste;
'Do not choose too low': Key scenarios (such as power system metering, precision equipment power supply monitoring) should be selected at level 0.2 or above to prevent errors from causing safety risks or process deviations;
Prioritize "range matching" for analog meters: As analog meter errors are calculated based on "full range", if the measured value is only 1/10 of the full range (such as measuring 50V with a 500V range), the actual relative error will be amplified by 10 times. It is necessary to choose analog meters with "range close to the measured value" as a priority.
In summary, the accuracy level of an AC voltmeter is the "standard for quantifying the error range", and the division should be based on national metrological standards. When selecting, a balance should be struck between the accuracy requirements of the application scenario, environmental conditions, and cost, rather than simply pursuing a high level.