The temperature measuring element of the forehead temperature gun is a thermopile. As an infrared temperature sensor, the thermopile can directly sense infrared heat radiation and convert the heat into electrical signals. It can quickly measure the surface temperature of the object without directly touching the object to be measured. The NTC thermistor packaged in the thermopile is used to measure the ambient temperature of the thermopile and use it as a temperature reference.

The thermopile is composed of multiple thermocouples in series. The receiving surface that receives infrared heat radiation is divided into several blocks, each block is connected to a thermocouple, and the thermoelectric potentials output by each thermocouple are superimposed on each other. When measuring temperature, the thermocouple must have A reference temperature or cold junction temperature. In general, the reference temperature of a thermopile is 25°C. When measuring human body temperature with a forehead thermometer, first write the output voltage measured under standard test conditions—that is, when the temperature of the measured object is 37°C and the reference temperature of the thermopile (ambient temperature) is 25°C, into the memory of the microcontroller. In the receptacle, the actual temperature of the measured object is obtained by calculating the actual measured output voltage of the thermopile and the ambient temperature of the thermopile.

The following approximate relationship exists between the temperature of the measured object and the ambient temperature of the thermopile:

Tb=Ts+Vout/A

Tb is the temperature of the measured object (K)

Ts is the ambient temperature of the thermopile measured by the NTC thermistor (K)

Vout is the output voltage of the thermopile (V)

A is the scale factor

Ts in the above formula is the ambient temperature measured by the NTC thermistor packaged in the thermopile. The temperature measurement accuracy of the forehead temperature gun is closely related to the accuracy of the reference temperature, that is, the ambient temperature of the thermopile, except for the conversion error of the thermopile system. , The higher the accuracy of the NTC thermistor’s measurement of the ambient temperature, the higher the accuracy of the final temperature displayed by the forehead thermometer, so the NTC thermistor plays an important role in the temperature measurement accuracy of the forehead thermometer.

The outstanding advantages of using NTC thermistors for temperature measurement are very high sensitivity, high measurement accuracy, good consistency and low price. Moreover, since the size of the NTC thermistor can be made very small, the response speed is also very fast.

The manufacturer of the forehead thermometer uses standard test conditions to read and store the output voltage of the thermopile of the forehead gun and the resistance value of the NTC thermistor in the thermopile one by one before leaving the factory, which skillfully eliminates the NTC heat in the thermopile. The accuracy error of the resistance value of the thermistor at 25°C, and the accuracy of the B value of the NTC thermistor (the thermal index of the thermistor is proportional to its resistance temperature coefficient), the temperature measurement accuracy of the forehead temperature gun when the ambient temperature deviates from 25°C appears very important.

The measurement errors of various B value accuracy of NTC thermistors at different ambient temperatures are shown in the following table: (take R25: 100KΩ±3%, B25/50: 3950K as an example)

Assuming that the calibration error of the two standard temperature points and the system error of the device circuit that cause the temperature measurement error of the forehead thermometer are not considered for the time being, the temperature measurement accuracy error simply caused by the B value accuracy error of the NTC thermistor is shown in the following table: : 100KΩ±3%, B25/50: 3950K as an example)

There are clear requirements for the temperature measurement accuracy of the forehead thermometer in the national standard, see the following table:

Considering the calibration error of the two standard temperature points of the forehead temperature gun and the systematic error of the circuit device, the error caused by the performance of the NTC thermistor has to be considered. It can be seen from the above table that the products with a B value accuracy of ±2% on the market cannot meet the accuracy requirements of the forehead temperature gun, and the NTC thermistor with a B value accuracy of ±1% can barely meet the national standard. Therefore, in order to meet the temperature measurement accuracy requirements of the forehead thermometer, the B value accuracy of the NTC thermistor should be selected as high as possible.

The NTC thermistor mass-produced by Nanjing Shiheng Electronic Technology Co., Ltd., the governing unit of China Electronics Element Association (CECA), not only has a resistance value accuracy of ±0.5%, but also a B value accuracy of ±0.2%. Using this NTC thermistor Based on the thermopile as the benchmark, the temperature measurement error caused by the accuracy error of the NTC thermistor is within ±0.1°C in the temperature measurement range of the ambient temperature from 0°C to 50°C.

As the temperature measurement benchmark of the thermopile in the forehead temperature gun, the NTC thermistor not only requires high B value accuracy and good batch consistency, but also requires high reliability and stability. The annual drift rate of the NTC thermistor must generally be controlled within ±0.4%, so that the temperature error caused by its drift can be less than 0.1°C/year. The annual drift rate of NTC thermistors produced by Shiheng Electronics is less than 0.04%, the resulting temperature error is less than 0.01°C/year, and the cumulative 10-year period will not exceed 0.1°C.

It is understood that the MF58 thermistor product made by this chip produced by Shiheng Electronics has passed the CQC certification, UL and C-UL certification many years ago, and passed the 100,000-time durability in the extremely severe UL standard. It has passed the AEC-Q200 full performance test, and the environmental RoHS test, which is outstanding not only in China, but also internationally.

This kind of small-sized NTC thermistor is packaged in blue film, suitable for bonding, electronic welding, plug-in dipping soldering and other processes, not only in the thermopile for forehead guns, but also in IGBT modules, information control modules, and heat pipes. It has a wide range of applications such as reactor controllers, optical communications, semiconductor lasers/detectors, etc.

For more information, please visit www.shiheng.com.cn.

Leave a Reply

Your email address will not be published.