Thermometer structure

1. "Thermometer"

Commonly known as "Hot and Cold Table". In China's meteorology, instruments that can directly read values without automatic recording devices are collectively called thermometers. There are many types, such as wet and dry bulb thermometer, minimum thermometer, maximum thermometer, ground thermometer, etc. Household thermometers are common capillary glass tubes with closed inner diameters at both ends. The closed lower end is round or cylindrical, filled with mercury, alcohol or kerosene. Due to temperature changes, the liquid column moves up and down and expands and contracts. According to the position of the top of the liquid column, the scale value can be directly read out.

2. "Mercury Thermometer"

It is a kind of thermometer that uses the properties of mercury's thermal expansion and contraction. High temperature can measure more than 300 degrees Celsius. Due to the melting point relationship, it cannot be used when measuring low temperatures below -30 ° C. To manufacture a mercury thermometer, first, a glass tube with a thick wall, a thin hole, and a uniform inner diameter should be selected, and the inside of the tube should be cleaned by pickling and other processes. One end is heated and blown into a thin-walled spherical or cylindrical container. Mercury is injected into the spherical vessel and glass tube at a certain temperature, and the temperature of mercury should be higher than the highest temperature measured later. Then the top of the glass tube filled with mercury is closed with flame. When the mercury temperature decreases, it begins to shrink, so a vacuum appears in the upper tube of the mercury column. The calibration scale of the thermometer must first determine two fixed punctuation points as a mark that never changes. Insert the liquid bubble part of the thermometer into the melting ice cube at a standard atmospheric pressure. When the mercury column drops to a certain point, a mark is used as the lower fixing point. Then place the whole thermometer in the water vapor at a standard atmospheric pressure. When the mercury column stops at a certain position, make a mark as the upper fixed point. The distance between these two fixed points is called the basic gauge distance. The length of this gauge distance is related to the diameter of the thermometer and the volume of the bubble. Divide this gauge length into 100 equal divisions, and each equal division is one degree. Mark the lower fixed point with a 0 ° mark and the upper fixed point with a 100 ° mark. The same long scale can be engraved below the melting point and above the boiling point. A scale engraved below 0 ° is called coldness, and a scale engraved above 0 ° is called heatness. Because the basic scale of the thermometer is evenly divided into 100 equal parts, it is called a percent thermometer, also called a Celsius thermometer. In addition to the Celsius temperature scale, there are also Fahrenheit temperature scales. This temperature scale uses 32 ° as the freezing point and 212 ° as the boiling point, of which 180 divisions are equally divided. The Fahrenheit thermometer is indicated by the letter F. The relationship between the two temperature scales is

F = 95 + 32 C = F 32 ° C, (-). 5 9

Mercury thermometers have certain disadvantages. For example, the inner diameter of a glass tube cannot be exactly the same. Although the distance between each scale and each scale is equal, because the inner diameter of the tube is different, the volume of the mercury liquid column between each scale is not Not equal, thus causing errors. When the volume of mercury in the glass tube expands when heated, the glass tube of the thermometer and the glass of the liquid bubble part also expand by heat. The result reads only the difference between the mercury expansion value and the glass expansion value. Due to the relationship between the freezing point (-38.87 ° C) and the boiling point (356.7 ° C) of mercury, its measurement can only be within this range and high temperature can be measured. If it is used to measure low temperature, it must be restricted.

3. "Alcohol thermometer"

The structure is the same as the mercury thermometer, except that alcohol containing red dye is contained. Easy to observe, this type of thermometer uses alcohol as the working substance. Because the boiling point (78 ° C) of alcohol is low and the freezing point is -117 ° C, an alcohol thermometer is often used as a low-temperature substance.

4. "kerosene thermometer"

The working substance of kerosene thermometer is kerosene, its boiling point is generally higher than 150 ℃, freezing point is lower than -30 ℃. Therefore, the measurement range of the kerosene thermometer is about -30 ℃ ~ 150 ℃. Because the boiling point of alcohol is 78 ℃, the freezing point is -114 ℃. An alcohol thermometer can measure a lower temperature than a kerosene thermometer, but it cannot be measured at a temperature higher than 78 ° C. Judging from the temperature range often measured by middle school physics laboratories, kerosene thermometers are more suitable than alcohol thermometers. When a student sees the scale of the thermometer at 100 ° C, it is wrong to say that the thermometer is an alcohol thermometer without analysis (alcohol temperature has reached 78 ° C and it has boiled, how can there be a temperature scale of 100 ° C). The thermometers with red working substances currently used in the middle school laboratory are generally kerosene thermometers, not alcohol thermometers.

5. "Full radiation pyrometer"

It is an instrument for measuring high-temperature radiation sources. The radiation from the radiation source is collected by a concave mirror onto a black-coated foil, which is attached to the thermocouple. According to the measured temperature difference electromotive force, the temperature of the foil can be known. Therefore, the temperature of the radiation source is known from the temperature reflection on the foil.

6. "Highest and lowest thermometer"

Namely "Xikes Thermometer". It can indicate the maximum temperature and the minimum temperature reached during the measurement time, but cannot indicate the exact time. The tube is filled with mercury and colorless alcohol. Due to the disparity between the expansion coefficients of alcohol and mercury, when the temperature rises, the alcohol expands, which forces the mercury to squeeze into the capillary and rise, and the upper pointer also rises accordingly, indicating that the maximum temperature has been reached; When the temperature drops, the mercury returns to the other tube, pushing the lower pointer to the lowest temperature.

7. "Beckman Thermometer"

It is also a thermometer with mercury stored in a glass tube. Its structural feature is that on the upper part of the thin tube containing mercury, there is a cavitation bubble that can adjust the amount of mercury. By adjusting the inflow of mercury, the height of the measurable temperature can be changed, but the entire tube length is only allowed to have a temperature of a few degrees. Variety. Its purpose is to be able to measure the temperature difference more accurately. The scale can directly read 0.01 ℃, can be estimated to 0.0001 ℃, quite accurate. The whole temperature range of this thermometer is only 5 ℃ or 6 ℃, so it can measure the temperature difference of 5 ℃ or 6 ℃