1. What is the accuracy level of an electrical instrument?
When the electrical instrument is working under specified conditions, the percentage of the maximum basic error that may occur and the upper limit of the meter's measurement is called the accuracy level of the meter. Its calculation formula is:
The accuracy level of the ±K-meter in the formula;
Δm - the maximum basic error expressed in absolute error;
Am-meter's upper measurement limit.
Therefore, the accuracy level of the electrical instrument is the value of the maximum reference error when the instrument is used under the specified working conditions.
2 What types of common electrical measuring instruments are there?
Usually divided into four types:
(1) Direct reading indicator. When measuring, the pointer is deflected, and the amount of electricity to be measured is directly read out, such as a voltmeter, an ammeter, a power meter, a multimeter, and the like.
(2) Compare the meter. The measurement needs to be compared with the corresponding standard quantity to read the ratio of the two. For example, the Wheatstone bridge is used to measure the resistance, and the universal bridge is used to measure the capacitance and the inductance. Often used to accurately measure some electrical quantities and to test other instruments or instruments.
(3) Graphic instrument. Specifically used to show the relationship between two related quantities. Such as an oscilloscope, this instrument has a good visual effect, but it can only be used as a rough test.
(4) Digital meters. The measured analog quantity is converted into a digital quantity and directly read out, for example, a commonly used digital voltmeter, a digital multimeter, and the like.
Lu Junfang asked: 1. How is the electrical instrument classified? What are the basic requirements for electrical instruments?
There are many types of electrical instruments, and there are usually different classification methods.
(1) According to the working principle: there are electromagnetic, magnetoelectric, electric, inductive, rectifying, thermoelectric, electrostatic, electronic and so on.
(2) According to the nature of the measurement: there are ammeter, voltmeter, power meter, ohmmeter, electricity meter, power factor meter, frequency meter, multimeter, etc.
(3) According to the way of use: switch board and portable. Switch panel meters are usually fixed on the switch board or switchboard with large errors. Portable meters generally have small errors and high accuracy.
(4) According to the working current: there are DC instruments, AC instruments, AC and DC meters.
In order to ensure the accuracy and reliability of the measurement results, there are several requirements for electrical instruments.
(1) The accuracy is high and the error is small, and the value should meet the requirements of the accuracy.
(2) The error should not change with the influence of external environmental conditions such as time, temperature, humidity, and external magnetic field.
(3) The power consumption of the meter itself should be as small as possible, otherwise it will cause a large error when measuring small power.
(4) The meter should have sufficient insulation strength and pressure resistance, and should also have the ability to withstand short-term overload to ensure safe use.
(5) There should be a good reading device and the measured value should be directly readable.
(6) The structure is sturdy and easy to use and maintain.
He Jie asked: What are the signs of electrician instruments? Types and applications of commonly used instruments?
The dial of the electrician is marked with various symbols to indicate the type of the instrument, the object to be measured, the measurement range, the accuracy, and the conditions of use.
The types and application ranges of commonly used electric meters are shown in Table 1.
The accuracy level of electrical instruments is divided into seven levels of 0.1, 0.2, 0.5, 1.0, 1.5, 2.5, and 5.0. The smaller the number, the higher the accuracy. According to the ability to resist external magnetic field or electric field interference, it is divided into I, II, III, IV, etc., among which I has the best defense ability. According to the conditions of use, it is divided into three groups: A, B, and C. Groups A and B are used indoors, and group C is used for outdoor or ship, aircraft, and vehicle.
Fang Tuguo asked: How to properly use and reasonably choose electric measuring instruments?
In order to avoid the measurement error caused by the imperfect measurement method, it is necessary to pay attention to the correct use of the instrument. The main points should be noted:
(1) Select the meter type according to the nature of the measurement object. First of all, depending on whether DC or AC is being measured, DC instruments or AC instruments can be selected. If you measure the amount of AC, you should also pay attention to whether it is a sine wave or a non-sinusoidal wave; when measuring, it is necessary to distinguish whether the measured value is the average value, the effective value, the instantaneous value, or the maximum value, and pay attention to the frequency for the AC quantity.
(2) Select the instrument level according to the actual needs of the measurement object. According to the nature of the project, as long as the error of the measurement results is within the practical allowable range of the project. For example, in commonly used standards and some precision measurements, instruments with an accuracy of 0.1 to 0.2 can be used; in the experimental measurement, instruments of 0.5 to 1.5 can be used; in the factory production, instruments of 1.0 to 5.0 can be used.
(3) Select the internal resistance of the meter according to the resistance of the measuring object and the measuring line. For the voltmeter that measures the voltage, the larger the internal resistance is, the better the resistance inside the voltmeter is greater than 100 times the measured object. For an ammeter that measures current, the smaller the internal resistance, the better. It is often required that the internal resistance of the ammeter is less than 100 times that of the object under test.
(4) Select the allowable rating of the meter according to the measurement object. Do not use a large range of meters to measure small values ​​to avoid readings. Of course, it is even more difficult to measure the large amount of power with a small-range meter to avoid damage to the meter. Therefore, when selecting the instrument, you must carefully observe the rated voltage, rated current and rated power that the instrument and equipment are allowed to withstand.
Table 1 Types and application scope of commonly used indicating instruments
Name Mark Symbol Maximum Readiness Level Measurement Range Consumption Power Overload Capability Made Instrument Type Application Range Current, A Voltage, V
Magnetoelectric system 0.1 10-11～102 10-2～103 <100mW Small A, V, n, Mn
The galvanometer clamp meter DC meter and with a variety of converters to expand the range of use, for the ratio meter electromagnetic system 0.1 10-3 ~ 102 1 ~ 103 is larger than the magnetoelectric system, slightly smaller electric system A, V, HZ,
COSφ synchronization table,
Clamp meter for 50Hz-5kHz mounted electric meter and general laboratory AC (straight) flow meter electric system 0.1 10-3～102 1～103 Larger small A, V, W, Hz,
COSφ synchronization table for 50Hz-10kHz
AC and DC standard meter and general laboratory table ferroelectric system 0.2 10-3～102 10-1～103 Smaller A, V, W, Hz, COSφ For power frequency, mainly for installed meter induction system 0.5 10-1～102 10-3～103 Smaller A, V, W, 1h,
Mainly used for electric energy meter for power frequency, measuring electric energy rectification system in AC circuit 1.0 10-5～10 5×10-3～5×102 small A, V, n,
COSφ, 1h,
Multimeter as multimeter, frequency from 50Hz to 5kHz
Electronics 1.0 Smaller A, V, Ω, Hz, COSφ Applied in weak current lines, frequency <108Hz
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