Medical thermometer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Medicine
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The medical thermometer is designed to measure the temperature of the human body and can operate in two measurement subranges.
The first of them measures the temperature in the range of +20...44°С, and the second one controls the temperature deviation from the preset value in the range of -2°С. The temperature-sensitive element is the thermistor Rt, included in one of the arms of the bridge formed by resistors R1, R2, R3 and R4. The unbalance voltage of the bridge is amplified by the operational amplifier A1 of the K153UD1 type, in the feedback circuit of which a microammeter with a full deviation current of 0,1 mA is included. The bridge can be balanced by resistors R1 and R4 at a temperature of 20°C on the first subrange. On the second subrange, the resistor R5 is short-circuited and the sensitivity of the device increases by 10 times. In this case, the full deviation of the instrument pointer corresponds to a temperature change of 4°C.
With a variable resistor R3, you can adjust the voltage at the non-inverting input of the operational amplifier by setting the instrument arrow to the middle of the scale to monitor the temperature deviation by 2 ° C.
The resistances of the resistors that make up the bridge circuit can be selected taking into account the resistance of the thermistor at a temperature of 20 ° C:
R1=R4=R2+R3=Rt
The device is powered by a Krona battery.
It is advisable to choose a thermistor of small size with a resistance at 20¦С in the range of 500 ... 5000 Ohm. It is covered with a thin layer of epoxy glue, and the leads are carefully insulated and connected with a shielded cable to the input of the device.
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