A high temperature thermometer is an instrument that measures higher temperatures. Two types of optical high temperature thermometer and radiation high temperature thermometer are the most important.

In an optical pyrometer, the filament is heated to the calibrated heat, and compared with the color of the high-heat object, adjust the heat of the filament to make the color consistent with the high-heat object. The temperature of the filament is the temperature of the high-heat object. In the radiation pyrometer, the radiation from the object is concentrated on the thermopile, and the corresponding voltage is generated according to the amount of thermal radiation intercepted, and then the voltage is converted to the temperature of the object.

There are two types of high temperature thermometers:

1) A device for measuring the temperature of the molten pool, including: a refractory mounting sleeve and an optical pyrometer;

2) Generally refers to the thermometer used to measure the temperature higher than 500℃. Commonly used pyrometers include optical pyrometers, colorimetric pyrometers and radiation pyrometers.

In addition, thermocouple high temperature thermometers can also be used to measure high temperatures below 3000°C. But because the measuring range of a thermocouple thermometer can be as low as -200℃, it is generally not classified as a pyrometer.

1, Hidden wire type high temperature thermometer:

Hidden wire pyrometer is an optical pyrometer. There are two measurement methods: one is to adjust the resistance R to change the brightness of the filament. When it is equal to the brightness of the image of the light source to be measured, the filament disappears from the image of the light source. At this time, the brightness and temperature of the object are read on the meter G; or Use the compensation method to measure the precise value of the current by the potentiometer, and then calculate the brightness temperature. The latter method is suitable for precise temperature measurement.

The second is to keep the brightness of the filament at a constant value, and rotate a piece of absorption glass whose thickness changes with the angle. When the brightness of the object image is the same as the brightness of the filament, the brightness and temperature value of the object can be read from the corner of the absorption glass.

The temperature indication value on the optical high temperature thermometer can be determined with a standard temperature lamp. The standard temperature lamp is shown in Figure 1b. It can reproduce the brightness temperature in the range of 900～2500℃ and can also be used as a light source. The temperature of 700～3000℃ can be measured by using the hidden wire optical pyrometer. It can also measure higher temperatures by replacing the bulb and the additional absorption glass of the objective lens.

2, photoelectric high temperature thermometer:

Photoelectric high temperature thermometer is also a commonly used high temperature thermometer, and its basic principle is the same as that of optical pyrometer. The main difference between them is that photoelectric pyrometers use photoelectric conversion devices instead of human eyes to compare brightness, so the measurement results are not affected by human subjective factors. Some photoelectric pyrometers also use a photomultiplier tube as a conversion device, which greatly improves the sensitivity of the instrument, and can perform continuous measurement and automatic recording. Some photoelectric pyrometers also use interference filters instead of red filters to improve the accuracy of the working wavelength λ. The precision photoelectric pyrometer has been used as a reference pyrometer, using it to reproduce the temperature higher than 1064.43℃ in the "1968 International Practical Temperature Scale".

3, radiation type high temperature thermometer:

Radiation pyrometer receiver is usually composed of thermopile, the hot end collects radiant energy, and the cold end is at room temperature. When measuring, aim at the object to be measured through the eyepiece, so that the image of the object falls on the "+"-shaped platinum plate of the receiver.

The electric meter indicates the temperature rise of the receiver, and the usual graduation is marked as the black body radiation temperature value Tp. The main disadvantage of this kind of pyrometer is: due to the chromatic aberration when the objective lens is focused, only a part of the radiant energy can be focused on the receiver to cause errors. The reflection type pyrometer uses a concave mirror to focus the radiation energy on the receiver for measurement. The structure is roughly the same as that of a refraction high temperature thermometer. Although this kind of pyrometer avoids the error caused by the chromatic aberration of the lens, because the cavity is open, dust easily enters the cavity and requires better maintenance.