13 May 2013

Thermometers

The Human body temperature indicates the health of a person. The thermometer is a diagnostic tool and is used to measure the temperatures from 35˚C to 43˚C (95˚F to 110˚F). The normal temperature of the human body is 37˚C or 98.4˚F.

The thermometer is placed under either tongue or the arm pit for about two or three minutes. Due to temperature rise, the mercury in the bulb expands. The temperature can be noted. On removing the thermometer, the pellet of mercury may be sent back into the bulb by vigorous jerking. Since the high degree of accuracy is not always important, this is still used in all hospitals even though it has slow response.

Electronic thermometer:

Figure (a) shows the Electronic thermometer circuit.

* Whenever continuous recording of the temperature and greater accuracy are needed, electronic thermometers should be used.

* The voltage drop developed across a forward biased silicon diode is temperature dependent.

* It varies at the rate of 2mV/˚C.

* A diode is supplied with a constant current via a series resistor from a 12V source.

* And also the voltage drop across the diode is sensed with a d.c operational amplifier.

* In backing off circuit, the voltmeter is connected at the O/P of the amplifier.

* It can be calibrated to measure the temperature from 35˚C to 43˚C.

* The thermistor can also be used as a temperature sensor after connecting a parallel linearising resistor with it.

Digital Electronic thermometer:

Figure (b) shows the Digital Electronic thermometer circuit using thermistor as a sensor.

* FET is controlled two switches S1 & S2.

* The thermistor RT, RRef and two equal resistors R1 form the arms of a bridge.

* VRef serves as the bridge excitation voltage.

* The voltage across RRef is connected to the noninverting input of op amp A1. The output of A1 is applied to both the noninverting and inverting inputs of A2 such that noninverting input directly receives and inverting input indirectly receives through the parallel combination of R2 & C. So that output of A2 is balanced. But the capacitor C charges to the voltage equal to VRef.

Working:

* At time T1, the position of the switches S1 & S2 are changed simultaneously.

* Op amp A1 receives its input at the noninverting terminal alone from the thermistor RT which is in contact with the human body.

* Thus A2 receives a less positive voltage than the voltage across RRef at the noniverting terminal and no voltage externally at the inverting terminal.

* The differential between inverting and noninverting inputs causes the output of A2 to go negative.

* Meanwhile the capacitor is discharged through R2. It continues to go negatively till the voltage across C equals the voltage generated by RT.

* After that the output of A2 becomes positive and delivers a negatively going square pulse whose width is equal to the duration from T1 to T2.

* The duration from T1 to T2 is proportional to the measured temperature and high frequency clock pulses are counted during the duration from T1 to T2 . Thus the digital display shows the temperature reading with the help of counter and scalar circuits.