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Chapter: Medical Electronics - Bio-Chemical and Non Electrical Parameter Measurement

Temperature Measurement

Temperature is one of the inductor of the general well being. Two types of temperature measurements can be obtained from the body. These are · Systematic temperature · Surface temperature

TEMPERATURE MEASUREMENT:

 

Temperature is one of the inductor of the general well being. Two types of temperature measurements can be obtained from the body. These are

 

·        Systematic temperature

·        Surface temperature

 

Systematic temperature is the temperature of the internal region of the body. Usually, the heat is generated by the active tissues of the body and heat is lost by the body to the environment. But, the temperature of the body is maintained carefully. The normal mouth temperature is 370C. the normal underarm temperature is about 10C lower than the above temperature. Systemic temperature is measured accurately at tympanic membrane of ear. The brain contains the temperature control centre for the body.

 

Systemic body temperature measurement

 

Mercury thermometer is used to measure the systemic temperature. It is inexpensive to use.. when continuous temperature recording is necessary, then, thermocouple or thermistor can be used. In thermocouple, a junction of two dissimilar metals produces an output voltage. This is proportional to the temperature at that junction.

 

Thermistor is a temperature sensing device. Its resistance varies with the temperature. This is mostly preferred in the biomedical field compared with thermocouple. Thermistor can be manufactured in a various shapes. In this thermistor, the relationship between resistance change and temperature is nonlinear. The resistance of the thermistor is given by using the expression,


Special circuits are used to overcome the nonlinear characteristics of thermistors. This special circuit consists of 2 matched thermistors.

 

Problems associated with thermistor

 

Self heating is very important problem in thermistor. This problem can be overcome by limiting the current used in measurement. The power dissipation of thermistor is to be maintained in milliwatts range to overcome this problem.

 

Thermistor probe should be chosen correctly based on

 

·        Resistance range

·        Sensitivity

 

Skin temperature measurement

Skin temperature is not constant throughout the body. It is varied from 300C to 350C. Various factors affect the skin temperature are given below

 

·        How fat covers over capillary area

·        How the skin portion is exposed to ambient temperature

 

·        Blood circulation pattern beneath the skin

 

Probe used for measurement

A small flat thermistor probe is used to measure the skin temperature.

 

Infrared thermometer

 

It is a device used to measure skin surface temperature. It is used to locate breast cancer. It is also used to identify the spots in which blood circulation is poor.

 

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