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Liquid-in-glass Thermometers

Dec 25, 2018

Liquid-in-glass Thermometers

Liquid-in-glass Thermometers,An apparatus for measuring the temperature by changing the position of the liquid column by using a temperature-measuring liquid contained in a glass container to cause a change in volume as the temperature changes.

Measuring principle of Liquid-in-glass Thermometers

The glass liquid thermometer adopts the temperature measurement principle of thermal expansion and contraction effect: when the temperature changes, the volume of the liquid in the glass ball expands or contracts, so that the height of the liquid column entering the capillary changes, and the temperature can be indicated from the scale. Variety.

The scale resolution of the thermometer is related to the sensitivity of the thermometer. If the sensitivity is large, the scale resolution of the thermometer is high. To increase the sensitivity of the thermometer, increase the volume of the temperature measurement fluid or reduce the diameter of the capillary. However, increasing the volume of the temperature measuring liquid does not easily achieve thermal equilibrium with the substance to be tested, resulting in a large hysteresis error and easy deformation of the ball portion; and reducing the capillary diameter makes the capillary tube difficult to process evenly, causing the liquid column to rise. Uneven, affecting measurement accuracy. Therefore, appropriate sensitivity should be taken.

In addition, the sensitivity of the thermometer is also related to the difference between the thermal expansion coefficients of the temperature measuring liquid and the glass, and is proportional. Generally, a liquid having a large coefficient of thermal expansion is selected as the temperature measuring liquid, and the coefficient of thermal expansion of the glass should be as small as possible. Commonly used temperature measuring fluids are mercury and alcohol.


Basic structure of Liquid-in-glass Thermometers:

The structure of the glass liquid thermometer is basically composed of a temperature sensitive bubble containing a temperature sensitive liquid (or a temperature measuring medium), a glass capillary tube and a scale scale. The temperature sensitive bubble is located at the lower end of the thermometer and is the temperature sensitive part of the glass liquid thermometer. It can accommodate most of the temperature sensitive liquid, so it is also called the liquid storage bubble. The temperature sensitive bubble is either directly processed by a glass capillary tube (called a bubble) or made by welding a thin-walled glass tube (called a bubble). The temperature sensing liquid is a temperature measuring medium encapsulated in the temperature sensitive bubble of the thermometer, and has the characteristics of large body expansion coefficient, small viscosity, low vapor pressure at high temperature, stable chemical property, no deterioration, and liquid state in a wide temperature range. Commonly used are organic liquids such as mercury, toluene, ethanol and kerosene. The glass capillary is a central thin glass tube connected to the temperature sensitive bubble, and the temperature sensitive liquid moves inside with changes in temperature. The ruler is to engrave the index line directly on the surface of the capillary, and the scale is marked with a number and temperature unit symbol to indicate the temperature of the measured temperature.

(1) ordinary thermometer

A glass liquid thermometer for measuring air temperature. It is mainly composed of glass balls, capillaries, scales and outer sleeves. The glass ball is filled with mercury, and one end of the capillary has a small opening connected to the glass ball, the other end is sealed, and part of the liquid enters the capillary. The capillary is lined with a white porcelain plate with a temperature scale and a thick glass tube on the outside of the capillary to protect the capillary.

The sensing part of the ordinary thermometer is usually made into a spherical shape with a scale of 0.2 °C. The actual reading can be estimated to 0.1 °C.

(2) Maximum temperature gauge

Refers to the thermometer used to measure the highest temperature in a certain period of time. Mercury is also used as the temperature measuring solution. The scale division is generally 0.5 ° C, and the reading is also estimated to 0.1 ° C. Unlike ordinary thermometers, the highest temperature gauge has a narrow passage at the ball and capillary junction. This narrow path is formed by welding a long conical glass needle to the capillary opening in the bottom wall of the ball.

When the temperature rises, the mercury expands, forcing the mercury to squeeze through the narrow channel into the capillary; and when the temperature drops, because the mercury cohesion is not enough to overcome the frictional resistance at the narrow channel, the mercury column in the capillary cannot be retracted to the ball, the highest temperature The indication value is retained. When measuring, the highest temperature gauge should be placed horizontally and the ball is slightly lower. In order to prevent the top gas pressure from forcing the mercury column back to the ball, the upper space of the liquid column in the highest temperature gauge capillary is vacuum.

(3) Minimum temperature table

The temperature measuring liquid is generally made of alcohol. Since the thermal conductivity of alcohol is small, in order to increase the surface area of contact with air, the ball portion is usually formed in a fork shape or a cylindrical shape. The scale division is generally 0.5 ° C and can be estimated to be 0.1 ° C when reading. Unlike a normal thermometer, a dark, dumbbell-shaped cursor is placed in the capillary column.

The cursor can move in the liquid column. When the temperature is lowered, the liquid level of the alcohol reaches the top of the cursor. The surface tension of the liquid film overcomes the friction between the cursor and the capillary wall and pulls the cursor toward the direction of the ball; when the temperature rises, the alcohol It will escape through the slit between the cursor and the capillary, and the friction between the cursor and the capillary is enough to keep it in place. The segment of the cursor away from the ball maintains the lowest temperature indication.

When measuring, the lowest temperature gauge should also be placed horizontally.

(4) Curved pipe temperature gauge

There are two sets of geothermal thermometers, one is a curved pipe temperature gauge for measuring the soil temperature of 5 cm, 10 cm, 15 cm and 20 cm depth below the ground; the other is a straight pipe geothermal thermometer for measuring 40 cm, Soil temperature of 80 cm, 160 cm, and 320 cm depth.

A curved pipe thermometer is a glass mercury thermometer that is bent at a right angle or any other suitable angle below the lowest scale line, usually bent to 135°, as shown in Figure 4. The curved tube thermometer is used to measure soil temperature with a depth of 20 cm or less. The ball of the thermometer is buried underground to the required depth, and the indication is read directly on the thermometer.

Main source of error  of Liquid-in-glass Thermometers

(1) Zero point forever displacement

(2) Temporary deformation of the ball

(3) Pressure changes

(4) The scale is not accurate

(5) The reading method is incorrect

(6) Thermal hysteresis effect

(7) Special reasons for errors in alcohol thermometers

(8) The special cause of the error caused by the highest temperature meter

The errors caused by the above series of reasons are some systematic errors, and some are random errors. For system errors, the measurement results can be corrected by verifying the difference.