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Dec 14, 2018

Basic knowledge of temperature sensor-part one

I . Basic concepts of temperature measurement

1. Definition of temperature:

Temperature is a physical quantity that represents the degree of cold and heat of an object.Temperature can only be measured indirectly by some properties of an object that vary with temperature, and the scale used to measure the value of an object's temperature is called a temperature scale.It specifies the starting point of temperature reading (zero) and the basic unit of temperature measurement.At present, the temperature scales commonly used internationally are Fahrenheit, Celsius, thermodynamic and practical.

Celsius scale (℃) regulation: under normal atmospheric pressure, the melting point of ice to 0 degrees, the boiling point of water is 100 degrees, among 100 divided into equal parts, each divided into 1 degrees Celsius, symbols for ℃.

Fahrenheit (℉) regulation: under normal atmospheric pressure, the melting point of ice is 32 degrees, the boiling point of water is 212 degrees, among 180 divided into equal parts each equal to 1 f symbols for ℉.

The thermodynamic temperature scale (symbol T), also known as the kelvin temperature scale (symbol K), or the absolute temperature scale, specifies that the temperature at which a molecule stops moving is absolute zero.

International temperature scale: the international practical temperature scale is an international agreement temperature scale, it is close to the thermodynamic temperature scale, and the repetition accuracy is high, easy to use.At present, the internationally accepted temperature scale is the international practical temperature scale 1968 -1975 revision adopted by the 15th international conference on weights in 1975, which is recorded as ipts-68 (rev-75).However, due to the lack of ipts-68 temperature, the international metrological commission, in resolution 7 of the 18th international congress of metrology, authorized the conference in 1989 to adopt the 1990 international its-90 temperature scale instead of ips-68.China has fully implemented ITS-90 international temperature scale since January 1, 1994.

International temperature scale 1990:

A. Temperature unit: thermodynamic temperature is a basic manual physical quantity, and its unit kelvin is defined as 1/273.16 of the thermodynamic temperature at the three-phase point of water. The difference between the thermodynamic temperature and 273.15k (freezing point) is used to express the temperature.By definition, the magnitude of Celsius is equal to kelvin, and the difference in temperature can be expressed in degrees Celsius or kelvin.The international temperature scale its-90 defines both the international kelvin temperature (symbol T90) and the international Celsius temperature (symbol T90).

B. General principles of its-90: its-90 is the maximum temperature actually measurable by monochromatic radiation from 0.65k up to Planck's radiation law.The its-90 is formulated as follows: at the full range, any optimal estimate of T at the time of temperature adoption is much more convenient, more precise and highly reproducible than the T90 measurement of direct thermodynamic temperature.

C. Definition of its-90:

The first temperature region is between 0.65k and 5.00k, and T90 is defined by the relation between 3He and 4He steam pressure and temperature.

The second temperature range is 3.0k to neon three-phase point (24.5661k). T90 is defined as a helium gas thermometer.

Triple point temperature range of the third for flat Heng hydrogen (13.8033 K) to the freezing point of silver (961.78 ℃), T90 is defined by platinum resistance thermometer, it USES interpolation method for dividing the definition of a set of rules.Silver (961.78 ℃) above the freezing point temperature zone, T90 is defined by the Planck radiation law emersion instruments for optical pyrometer.

Ii. Classification of temperature measuring instruments

Temperature measuring instruments can be divided into contact type and non-contact type according to the way of temperature measurement.Generally speaking, the contact thermometer is relatively simple, reliable and accurate;However, due to the fact that the temperature measuring element and the medium under test need to conduct sufficient hot alternating current, it takes a certain amount of time to achieve the thermal balance, so there is a delay phenomenon of temperature measurement, and due to the limitation of high-temperature resistant materials, it cannot be applied to very high temperature measurement.Non-contact instrument temperature measurement is through the principle of thermal radiation to measure the temperature, the measurement element does not need to contact with the measured medium, the temperature range is wide, is not limited by the upper limit of temperature measurement, will not destroy the temperature field of the measured object, the reaction speed is generally faster;However, due to the influence of external factors such as emissivity, measuring distance, smoke and gas, etc., the measurement error is large.

Iii. Selection of sensors

The national standard gb7665-87 defines a sensor as "a device or device that can feel the specified measured signal and convert it into usable signal according to a certain rule, usually composed of a sensitive element and a conversion element".Sensor is a kind of detection device, which can feel the measured information and transform the detected information into electrical signals or other required forms of information output according to a certain law, so as to meet the requirements of information transmission, processing, storage, display, recording and control.It is the first step to realize automatic detection and control.

(1) modern sensors vary greatly in principle and structure. How to select sensors reasonably according to specific measurement purposes, measurement objects and measurement environment is the first problem to be solved when carrying out a certain amount.When the sensor is determined, the corresponding measurement method and equipment can also be determined.The success or failure of measurement results depends largely on whether the sensor selection is reasonable or not.

1. Determine the type of sensor according to the measurement object and the measurement environment: to carry out a specific measurement work, what kind of principle of sensor should be considered first, which needs to analyze various factors before determining.Because, even if the same physical quantity is measured, there are also a variety of principles of the sensor to choose, which principle of the sensor is more appropriate, it is necessary to consider the following specific issues according to the characteristics of the measurement and the use of the sensor conditions: the size of the range;The volume requirements of the sensor for the measured position;The measuring method is contact or non-contact;Method of signal extraction, wired or non-contact measurement;The source of sensors, is the import or domestic, the price is acceptable, or self-developed.

2. Selection of sensitivity: generally, within the linear range of the sensor, the higher the sensitivity of the sensor is expected to be, the better, because only when the sensitivity is high, the output signal corresponding to the measured change is relatively large for signal processing.However, it should be noted that the sensitivity of the sensor is high, and external noise unrelated to the measurement is also easy to mix, and will be amplified by the amplification system, affecting the measurement accuracy, so the sensor itself is required to have a high signal-to-noise ratio, as far as possible to reduce the factory anxiety signal introduced from the outside.The sensitivity of the sensor is directional.If the sensor is measured as a single vector, and its directivity is required to be high, the sensor with low sensitivity in other directions should be selected. If the sensor is measured as a multi-dimensional vector, the smaller the cross-sensitivity of the sensor, the better.

3. Frequency response characteristics: the frequency response characteristics of the sensor determine the frequency range to be measured, and the measurement condition must be kept within the allowable frequency range. In fact, there is always a certain delay in the response of the sensor, and the shorter the delay, the better.The frequency response of the sensor is high, and the frequency range of the measurable signal is wide. Due to the influence of the structural characteristics, the inertia of the mechanical system is large, and the frequency of the measurable signal of the sensor with low frequency is low.In dynamic measurement, the response characteristics should be based on the characteristics of the signal (steady state, random, etc.), so as to avoid excessive errors.

4. Linear range: the linear range of the sensor refers to the range in which the output is proportional to the input.Theoretically speaking, within this range, the sensitivity remains constant, and the wider the linear range of the sensor, the larger its range and the certain measurement accuracy can be guaranteed.When selecting a sensor, when the type of sensor is determined, the first thing to see if its range meets the requirements.But in fact, no sensor can guarantee absolute linearity, its linearity is also relative.When the required measurement accuracy is relatively low, the sensor with small nonlinear error can be regarded as linear approximately within a certain range, which will bring great convenience to the measurement.

5. Stability: the ability of the sensor's performance to remain unchanged after a period of use is called stability.In addition to the structure of the sensor itself, the main factor affecting the long-term stability of the sensor is the use environment of the sensor.Therefore, in order to make the sensor have good stability, the sensor must have a strong ability to adapt to the environment.Before selecting a sensor, it is necessary to investigate its operating environment and select the appropriate sensor or take appropriate measures to reduce the environmental impact according to the specific operating environment.In some requirements sensor can be used for a long time and easily replaced or calibrated occasions, the selected sensor stability requirements are more stringent, to be able to withstand the test of a long time.

6. Accuracy: accuracy is an important performance index of the sensor, and it is an important link related to the measurement accuracy of the whole measurement system.The higher the precision of the sensor, the more expensive it is. Therefore, as long as the accuracy of the sensor meets the accuracy requirements of the whole measurement system, it is ok, and it is not necessary to choose too high, so that the cheaper and simple sensor can be selected from many sensors that meet the same measurement.If the purpose of measurement is qualitative analysis, select the sensor with high repeatability accuracy, not the one with high absolute value accuracy;If it is for quantitative analysis, the accurate measurement value must be obtained, and the sensor whose accuracy level can meet the requirements must be selected.For some special applications, if the appropriate sensor cannot be selected, the sensor shall be designed and manufactured by ourselves, and the performance of the self-made sensor shall meet the requirements for use.