The era of use of mercury thermometers for measuring human body temperature is coming to an end. The fragility of mercury thermometers makes them unsuitable for children and unconscious patients. Also, mercury is an ecologically hazardous
I. B. Fogel, S. I. Berdnikov · 1996
material. Electrical thermometers (both analog and digital) have been designed to replace mercury thermometers. We have developed electrical thermometers based on thermotransistors as thermal sensors: two models of electrical thermometers for special purposes (TET-7 and TET-BM) and two universal models (TET-2 and TET-Tsll), the accuracy of the latter two models being sufficient for measuring human body temperature. An axillary sensor was developed to measure human body temperature. The TET-7 electrical thermometer was developed to the order of the Institute of Biomedical Problems. The measuring range of the thermometer is 35-41C; measuring error, __.0.05~ scale division, 0.01~ The time interval of one measurement, 1 min. The thermometer is designed to operate under acceleration of up to 20g and do not fail under acceleration of up to 100g. The volume of the device does not exceed 0.5 dm 3. To measure body temperature, the thermometer is put under the patient's tongue. The diameter of the sensor is less than 10 mm; thickness, 3 nun. High requirements for measuring accuracy and mechanical strength (vibration survival, impact resistance) did not allow analog circuitry to be used. The device was developed on the basis of the pulse bridge circuit with asymmetric power supply and a thermotransistor described in [5, 6]. This circuit (Fig. 1) includes a pulse shaper (PS) powered by a voltage stabilizer (VS). Rectangular pulses of the PS are applied to the measuring arm (resistor R 3 and thermotransistor t) of the bridge. Linearly increasing (decreasing) pulses are applied to the reference arm (Rt, R2) of the bridge. The PS output voltage during the gap between the pulses is zero. A signal of variable polarity appears at the output diagonal of the bridge. The moment when the polarity of the signal is changed linearly depends on the sensor temperature. After amplification and detection, the length of the output pulse is proportional to temperature. The output signal is transformed and applied to indicator lamps. The TET-BM household device of medical purpose is based on the circuit shown in Fig. 2. The thermotransistor (t) of the circuit operates in an amplifying mode. This a simple and inexpensive device. Its scale is nonlinear, and the maximum sensitivity range is from 35 to 40C. The TET-2 universal transistor electrical thermometer is an analog device. It consists of a battery (1.5 V), a voltage stabilizer, and a bridge with asymmetric DC power source. A microammeter is connected to the diagonal of the bridge. The scale division of the thermometer is 0.2~ [6]. The TET-Tsll universal transistor electrical thermometer (Fig. 3) contains a voltage stabilizer VS and a bridge measuring circuit with a transistor t in one of its arms. The output signal from the diagonal of the bridge is applied to the input of an analog-to-digital ADC. The ADC output is connected to a liquid-crystal or LED (model TET-Ts21) indicator. The TET-2 and TET-Tsll devices were rather inexpensive, and they have been manufactured over the last 20 years (1000 units per year). The developed medical thermometers should provide minimum measuring time and sufficiently high measuring accuracy. A technique of double approach was employed to reduce the thermal inertia of the sensor. The technique consists in approaching the measured temperature from two sides: from below and from above. The temperature curves are plotted on a graph, and measuring time is equal to the time interval when the difference between the curves does not exceed measuring accuracy (Fig. 4).