Temperature sensor refers to a sensor that can sense temperature and convert
it into available output signals. Temperature sensors are the core part of
temperature measurement instruments, with a wide variety. According to the
measurement method, it can be divided into two categories: contact and
non-contact, and according to the characteristics of sensor materials and
electronic components, it can be divided into two categories: thermal resistance
and thermocouple.
The following matters should be noted when installing and using temperature
sensors:
1. Errors introduced by improper installation
For example, the installation position of the thermocouple and the depth of
insertion cannot reflect the real temperature of the furnace, etc. In other
words, the thermocouple should not be installed too close to the door and
heating, and the depth of insertion should be at least 8 to 10 times the
diameter of the protective tube; the gap between the protective sleeve of the
thermocouple and the wall is not filled with thermal insulating substances,
resulting in heat overflow or cold in the furnace. Air intrusion, so the gap
between the thermocouple protection tube and the hole of the furnace wall is
blocked by thermal insulating substances such as refractory mud or asbestos rope
to avoid cold and hot air convection and affecting the accuracy of temperature
measurement; the cold end of the thermocouple is too close to the furnace body,
causing the temperature to exceed 100℃; the installation of thermocouples should
avoid strong magnetic fields and strong electric fields as much as possible, so
it should not The thermocouple and power cable are installed in the same duct to
avoid interference and error; the thermocouple cannot be installed in the area
where the measured medium flows little. When the thermocouple is used to measure
the gas temperature in the tube, the thermocouple must be installed in the
direction of the flow velocity and fully in contact with the gas.
2. The error introduced by the deterioration of insulation
If the thermocouple is insulated, too much dirt or salt slag on the
protective tube and wire plate will cause poor insulation between the
thermocouples and the furnace wall, which is more serious at high temperatures.
This will not only cause loss of thermoelectric potential but also introduce
interference, and the error caused by it can sometimes reach Baidu.
3. The error introduced by thermal inertia
Due to the thermal inertia of the thermocouple, the indicated value of the
instrument lags behind the change of the measured temperature, and this effect
is particularly prominent in rapid measurement. Therefore, a thermocouple with a
thinner thermal electrode and a small protective tube diameter should be used as
much as possible. When the temperature measurement environment is allowed, the
protective tube can even be removed. Due to the measurement delay, the amplitude
of the temperature fluctuation detected by the thermocouple is smaller than that
of the furnace temperature fluctuation. The greater the measurement delay, the
smaller the amplitude of thermocouple fluctuations, and the greater the
difference from the actual furnace temperature. When using thermocouples with
large time constants to measure or control the temperature, although the
temperature displayed by the instrument fluctuates very little, the fluctuation
of the actual furnace temperature may be large. In order to accurately measure
the temperature, a thermocouple with a small time constant should be selected.
The time constant is inversely proportional to the heat transfer coefficient,
and directly proportional to the diameter, material density and specific heat of
the thermocouple hot end. If you want to reduce the time constant, in addition
to increasing the heat transfer coefficient, the most effective way is to
minimize the size of the hot end. In use, materials with good thermal
conductivity are usually used, and protective sleeves with thin walls and small
inner diameters are used. In more precise temperature measurement, bare thread
thermocouples without protective sleeves are used, but thermocouples are easy to
damage and should be corrected and replaced in time.
What problems should be paid attention to when installing and using
temperature sensors?
4. Thermal resistance error
At high temperature, if there is a layer of coal ash on the protective tube
and dust is attached to it, the thermal resistance will increase, which hinders
the conduction of heat. At this time, the temperature indication is lower than
the true value of the measured temperature. Therefore, the outside of the
thermocouple protection tube should be kept clean to reduce the error.
Of course, Kunshan Danrui sensing and control has rich industry application
cases on temperature sensors, and has many years of practical experience in
products and industries. If you can't solve the sensor problems you have
encountered at present, please consult the online customer service on the right
or call the customer service hotline. Danrui will provide free answers.
www.danruisensor.com
Danrui Sensors (Suzhou) Co., Ltd
