These are two alloy wires. Solder them together and plug in a voltmeter. Insert the ice-water mixture at one end, and heat the other end with an alcohol lamp.
The temperature difference between the two ends will generate thermoelectric potential in the loop. The higher the temperature difference, the greater the thermoelectric potential. Therefore, even in the absence of an external power supply, the reading of the voltmeter will gradually increase.
When the reading shows 1.48 mV, we can know that the temperature of the working end is 37℃ according to such a standard index table. In this way, we have made a simple thermocouple thermometer.
Since it can measure 37°C, can it be used to measure the temperature of the human body? The answer is yes, just replace the alcohol lamp with the armpit of the human body. After a few minutes, you can get the approximate human body temperature based on the reading of the voltmeter.
But the disadvantages of this scheme are obvious. Compared with the same contact type mercury and digital thermometers, the thermocouple thermometers are cumbersome to operate and have low accuracy.
However, it can achieve non-contact temperature measurement. The premise is that we have to make some transformations.
The principle of non-contact temperature measurement is to use the infrared rays radiated by the human body to heat the working end of the thermocouple thermometer, and then measure the body temperature according to the change in the voltmeter reading.
However, in actual use, the low human body radiation power makes the temperature difference and thermoelectric potential in the thermocouple not obvious enough.
So, how should we modify it to make it more sensitive to temperature changes?
First, the volume of the thermocouple is reduced, and then a number of them are connected in series to superimpose the output potentials to form a thermopile.
Next, we need to add a silicon substrate under the thermopile. A layer of silicon nitride support layer is deposited between the silicon substrate and the thermopile by deposition coating technology.
By etching a thermally isolated cavity on the bottom of the silicon substrate, we can ensure that the silicon nitride coating is suspended. The edge is the cold junction area, and the center is the hot junction area.
A layer of film material is covered above the hot junction area as an absorption zone, which is used to absorb the heat radiation of the human body and heat the hot junction area. The temperature of the cold junction area is consistent with the ambient temperature, and the temperature can be determined by measuring the resistance change of the built-in NTC thermistor.
The temperature difference between the heated hot junction area and the cold junction area generates thermoelectric potential and output voltage value.
After repeated experiments, we can establish the relationship function between the target object temperature, ambient temperature, and output voltage. In the actual measurement, we only need to substitute the ambient temperature and output voltage into the formula to get the temperature of the target object.
However, the resulting thermopile sensor is not accurate enough. When we hold it to the forehead, it is not only the infrared radiation of the human body that enters the sensor’s field of view, but also visible light, near infrared and other clutter.
Therefore, we need to install a filter on the temperature sensor to select electromagnetic waves of specific wavelengths.
The temperature of the human body is mainly in the range of 30℃~40℃. According to the Wien displacement law, the peak wavelength of human body radiation is mainly concentrated in the far-infrared band between 9200 and 9600 nm.
Therefore, we can use this 8~14 μm bandpass filter to effectively filter out unnecessary electromagnetic waves.
The thermopile, NTC thermistor and filter are packaged together to form a temperature sensor. And this is the core component of the current infrared forehead thermometer.
Behind the temperature sensor is an operational amplifier circuit to amplify the weak output voltage of microvolts to millivolts. The amplified voltage is input into the single-chip microcomputer through AD analog-to-digital conversion, and the target object temperature indication value is output through the built-in algorithm.
Finally, we can place a Fresnel lens in front of the temperature sensor to make the sensor field of view narrower and farther.
With the buzzer and power supply, we assembled an infrared forehead thermometer that costs about 25 yuan. With its non-contact and fast response characteristics, it has now become the new crownOne of the most important medical devices in the epidemic.
The question is, is this infrared forehead thermometer accurate?
Regular infrared forehead thermometers need to be verified and calibrated before they leave the factory. According to this calibration specification: the forehead thermometer should be placed in a constant temperature and humidity environment for at least 30 minutes before calibration, and then the forehead thermometer should be converted from the body temperature mode to the calibration mode, and the indication and value of the calibrated infrared forehead thermometer should be recorded Standard temperature.
In this way, we get the calibration value of the infrared forehead thermometer and the laboratory error.With the indication correction value, the infrared forehead thermometer can be corrected so that the measured temperature is consistent with the standard temperature and does not exceed the laboratory error of ±0.3°C.
Therefore, a regular infrared forehead thermometer can be considered “quasi” in the calibration mode. How many degrees a person’s forehead is, the infrared forehead thermometer can measure how many degrees.
The problem is that the forehead temperature of the human body is the body surface temperature, not the body temperature in the medical sense.
Medical and clinical body temperature refers to the average temperature of the core temperature of the human body, and there is a difference between the body surface temperature such as forehead temperature.
If you want the infrared forehead thermometer to measure the body temperature, you need to let it “calculate” the body temperature from the forehead temperature based on the conversion relationship.
But it is not so easy to find the exact conversion relationship. In this paper published in 2017, the researchers determined the functional relationship between human core temperature, forehead temperature, and ambient temperature.
Experimental tests show that the body temperature obtained by function conversion from forehead temperature is basically close to the core temperature of the human body, but there is still a certain deviation. The reason for this deviation is that the body surface temperature and core temperature are affected by different factors.
For the body surface temperature, an increase in the ambient temperature or a decrease in the convective heat transfer coefficient will cause its value to increase. The core temperature also fluctuates with the difference between day and night and the female physiological period.
Such fluctuations make it almost impossible to correspond to the body surface temperature and core temperature one by one.
Therefore, the body temperature displayed by the current infrared forehead thermometer is the true temperature value of the forehead temperature plus the statistical correction value converted from forehead temperature to human body temperature. Although this estimated value is close to the core temperature of the human body, it is not the true temperature.
The statistical correction value is generally set by the manufacturer, so when two different brands of forehead thermometers are used to measure the same target object, it is likely to show different indications.
Therefore, the infrared forehead thermometer is not a tool for accurate measurement in clinical practice, but it is still important in large-scale fever screening.
So, when you are pushed against your forehead by it, please feel free to be close to it. Don’t worry, it won’t hurt you.