Prefix for measuring temperature with a digital multimeter. Scheme, description

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Prefix for measuring temperature with a digital multimeter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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A simple attachment containing six resistors (the circuit is shown in the figure) allows you to use a digital voltmeter (or multimeter) to measure temperature with a resolution of 0,1 ° C and a thermal inertia of 10 ... 15 s. With such speed, it can also be used to measure body temperature. No changes are required to the measuring device, and the manufacture of the attachment is also available to novice radio amateurs.

Prefix for measuring temperature with a digital multimeter

A semiconductor thermistor ST3-19 with a nominal resistance of 10 kOhm at t = 20°C was used as a sensor. Together with an additional resistor R3, it forms one half of the measuring bridge. The second half of the bridge is a voltage divider of resistors R4 and R5, the last one during calibration is the initial value of the output voltage. The multimeter is used in the DC voltage measurement mode within 200 or 2000 mV. An appropriate choice of the resistance of the resistor R2 changes the sensitivity of the measuring bridge.

Immediately before measuring the temperature with a variable resistor R1, the supply voltage of the measuring circuit is set equal to that at which the initial calibration was performed. The attachment for reading the measured temperature is turned on with the SB1 push-button switch, and the transfer from the measurement mode to the voltage setting mode is switched on with the SB2 switch.

The calculation of an additional resistor R3 connected in series with the thermistor is carried out according to the formula R3 = RTM (B - 2TM) / (B + 2TM), where RTM is the resistance of the thermistor in the middle of the temperature range; B is the thermistor constant; TM - absolute temperature in the middle of the measuring range Т = t° + 273.

This value of R3 ensures the minimum deviation of the characteristic from linear.

The thermistor constant is determined by measuring the resistances RT1 and RT2 of the thermistor at two temperatures T1 and T2 and then calculating by the formula B = ln(RT1/RT2)/(1/T1-1/T2).

On the contrary, with known parameters of a thermistor with a negative TСC, its resistance for a certain temperature Т can be determined by the formula RT = RT20.e(В/Т-В/293), where RT20 is the thermistor resistance at a temperature of 20°С

The attachment is calibrated at two points: TK1 = TM + 0,707 (T2-T1) / 2 and TK2 = TM-0,707 (T2-T1) / 2, where TM = (T1 + T2) / 2, T1 and T2 - the beginning and end of temperature range.

During the initial calibration with a fresh battery, the resistance of the variable resistor R1 is set to the maximum so that as the capacitance is lost and the cell voltage decreases, the voltage on the bridge can be kept unchanged (the prefix consumes a current of about 8 mA). By adjusting the trimmer resistors R2, R5, the readings of the digital indicator of the multimeter are matched in three digits to the temperature values of the thermistor TK1 and TK2, controlled by an accurate thermometer. In its absence, use, for example, a medical thermometer to control the temperature within its scale and a stable melting temperature of ice - 0 ° C.

The author used M-830 from Mastech as a multimeter. Resistors R2, R5 are better to use multi-turn (SP5-1V, SP5-14), and R1 - single-turn, for example PPB; resistors R3 and R4 - MLT-0,125. To turn on the power and switch the set-top box mode, you can take the P2K pushbutton switches without fixing.

In the manufactured attachment, the boundaries of the measured temperature range were set - Т1 = 15°С; Т2 = 45°С. In the case of measurements in the range of positive and negative temperature values on the Celsius scale, the sign indication is obtained automatically.

Author: V. Ratnovsky, Samara

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