Thermal measurement concept and measurement method
This chapter describes the basic concepts of measurement and measurement errors, the general methods of measurement, the composition and type of measuring instruments, the types of measurement errors, the methods of representation and error handling, and the technical indicators for evaluating the quality of the measuring instruments. This chapter focuses on the types and methods of measurement errors, measurement error processing methods, and instrument quality standards.
"Measurement technology" is a science that studies the principle of measurement, measurement methods and measurement tools. When mankind is engaged in scientific research, engineering technology and all other production activities, it must measure in order to obtain a quantitative relationship among various things. Measurement is an indispensable means for people to understand the nature of things.
Different technology and production fields have different measurement items and measurement characteristics. Thermal measurement refers to the measurement of various thermal parameters, such as temperature, pressure, flow, level, etc., during the thermal process (in thermal power plants, component analysis, rotational speed, and vibration are sometimes included).
In thermal power generation, thermodynamic measurement can timely reflect the operating conditions of thermal equipment and thermal system, provide personnel with the basis for operation, and provide the necessary signals for thermal automation control accurately and timely. Therefore, thermal measurement is a necessary means to ensure the safety and economical operation of thermal equipment and realize automatic control.
First, the definition of measurement
The so-called measurement is the use of a measurement tool to compare the measured quantity with a standard quantity of the same nature (measurement unit) through an experiment to determine how many times the measured quantity is a standard quantity. The stated multiple is the measured value, ie L=x/b
Where x_ is measured b_ standard amount (measurement unit)
L_ says the measured value, that is, the obtained measurement result.
From the formula, we can see that the measured value is related to the unit of measurement. The unit of measurement considers it as prescribed and is recognized nationally or internationally. Before the "International System of Units" was born, the unit of measurement varies from country to country and from region to region. If the same type is measured and compared, unit conversion must be carried out. This is inconvenient, and the scientific and rigorousness of the designation of a limited measurement unit is rather poor. With the development of science and technology and the strengthening of international science and technology and economic exchanges, people are eager to formulate unified measurement units. In 1960, the 11th International Metrology Congress adopted the “International System of Unitsâ€, code-named SI, which provided uniform regulations for seven basic units: length, mass, time, current, and thermodynamic temperature. Other units of measurement can be derived from these seven basic units. Practice has proved that the international system of units has the advantages of being scientific, reasonable, accurate, and practical, which brings great convenience to production and construction and technological development. On February 27, 1984, China’s State Council, Fabry’s Order on the Uniform Implementation of Legal Measurement Units in China. The legal measurement units are based on the International System of Units, and combined with the actual situation in China has increased the number of non-SI units. In thermal measurement, it should be actively promoted.
Second, the measurement method
Measurement is an experimental task. In order to obtain accurate and reliable data in a timely manner, a reasonable measurement method must be selected according to the requirements of the industry and the characteristics of the object being measured.
According to the procedure for obtaining the measurement results, the measurement can be divided into:
(1) Direct measurement. It is to compare the measured directly with the selected standard quantity, or measure with a pre-calibrated measuring instrument to directly obtain the measured value. If the length is measured with a ruler, the water level is measured with a glass tube.
(2) Indirect measurement. By directly measuring other variables that are measured with a definite function relationship, and then substituting the resulting value into a function for calculation, the method for obtaining the measured value is called indirect measurement. For example, use a balanced container to measure the water level of the drum; measure the resistance, length, and direct resistivity of the wire.
(3) Combination measurement. Combination measurement is based on the measurement of several groups of liangzhi9d with a certain function relationship, by taking the simultaneous equations to obtain the measured method. For example, the relationship between platinum resistance and temperature over a certain temperature range is
R1=Rto(1+At+Bt2)
In the formula, the resistance value of Rto-platinum resistance at 0°C Rt-the resistance value of platinum resistance at t°C
A/B-temperature coefficient (constant), in order to find the temperature coefficient A, B, can directly measure 0°C, t1°C, t°C2 three unused temperature values ​​and corresponding temperature resistance values ​​Rto, Rt1, Rt2 , and then get the A, B values ​​through simultaneous equations.
According to the different operation principle of the detection device, the measurement can be divided into:
(1) Direct reading method. Measured to act on the instrument to compare and install, make some kind of parameter that compares and installs according to the known relation to change along with being measured, because this kind of change relation has already scaled directly on the instrument, so can read the measurement result directly from the scale of the instrument. For example, when measuring the temperature with a glass tube mercury thermometer, the height of the mercury column can be read directly.
(2) Zero value method (balance method). The measured value is compared with a known quantity. When the two are balanced, the instrument balance indicator is zero, and the known quantity is the measured value. For example, using a balance to measure the mass of an object and measuring the potential with a potentiometer is a zero method.
(3) Differential method. When the measured is not yet fully balanced with the known quantity, the difference between them is read, and the measured value is obtained from the known quantity and the difference. Measuring the resistance with an unbalanced bridge is an example of measurement using the differential method. The zero value method and the differential method measurement are very advantageous to reduce the error of the measurement system, so the measurement accuracy is high. The application is more extensive.
According to whether the instrument is in contact with the measured object, the measurement can be divided into:
(1) Contact measurement method. A part of the instrument is in contact with the object to be measured and is measured by the effect of the object being measured. For example, when using a glass tube mercury thermometer to measure temperature, the temperature of the thermometer should be placed in the measured medium to feel the temperature.
(2) Non-contact measurement method. No part of the meter can be measured directly without having to contact the measured object. For example, with an optical pyrometer to measure temperature, the temperature is measured by the effect of heat radiation from the object being measured on the meter, so the meter does not have to be in direct contact with the object.