Question

Why is the range of clinical thermometer is from 36 degree Celsius to 42 degree Celcius

Answer 1

Because that is the normal operating range needed to measure living human’s body temperature.

When you build a measuring device such as a thermometer, you try to limit the range of reading (Span), so that you get the most accurate reading possible. Since sensors are normally rated in percent of span, (in this case span = 45 C–32 C = 13 deg C). One percent of 13 is 0.13 deg C, so if your sensor has an accuracy of 1% (typical) then your temperature measurement is accurate to around 0.1 deg C.

If you doubled the span, then your accuracy would be 0.2 deg C. Now the accuracy of the reading is getting unacceptable.

Another reason for analog and mercury clinical thermometer is readability. If a mercury thermometer had 10 times the range (span), then the marks for 0.1 deg C would be ten times closer together. There is a limit to the readability of the device.