Medical transistor thermometer. Scheme, description

Medical transistor thermometer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electronics in medicine

This device allows you to measure the patient's temperature in no more than 5 seconds with an error of ± 0,05 ° C. The measurement limits are from 34 to 42 ° C. The device is powered by two FMC-type elements, consuming a current of not more than 1 mA.

The device (see diagram) is a bridge, one of the arms of which includes a sensor - the KMT-14 thermistor with a DC amplifier based on two transistors T1 and T2 of the P13-P16 type. The indicator is a measuring device of the M-24 type for a current of 200 μA, the scale of which is calibrated in degrees Celsius.

(click to enlarge)

When in contact with the human body, the resistance of the thermistor changes, the balance of the bridge is disturbed, and the current flowing along the diagonal of the bridge is amplified by transistors 20-50 times and deflects the arrow of the device. The shift in the range of the measured temperature is achieved by selecting the resistor R2, and expanding or narrowing this range - by selecting the resistors R5 and R6.

Switches Vk 1a and Vk 1b (tumbler) are used to simultaneously turn on both batteries. Calibration of the device to the beginning of the scale (at 34 ° C) is carried out by the potentiometer R9 with the contacts of the switch Vk2 open, and the patient's temperature is measured with the contacts Vk2 closed.

All parts and batteries of the device are placed in a duralumin case with dimensions of 130x130x35 mm.

The thermistor sensor is connected to the device using a two-wire cord.

The instrument is calibrated as follows: the thermistor is placed in a thermostat with water stirred by an impeller. The temperature of the water is controlled by a laboratory thermometer, and the resistance of the thermistor in the desired temperature range is fixed by a DC bridge of the MO-62 type. Then, instead of a thermistor, the same bridge is included in the bridge arm (as a resistance box) and resistors R2, R5 and R6, set the desired range of measured temperatures.

Author: A. Manzyuk; Publication: N. Bolshakov, rf.atnn.ru

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