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How can I measure body temperature using LM35; should I touch the package body of LM35 directly to measure the temperature? How long do I need to keep touching the part before it gives a change in voltage output?
It is required for the medium to be in contact with the package of the sensor to accurately measure the temperature of the medium. The time required by the sensor to provide stable and correct output varies with medium. Attached is the snapshot from DS. So it may be expected that (my own guess ) one has to wait for more than a minute to have a stable output from LM35 for the human body temp measurement. But this can be experimented against a known surface of definitive temp.
But in my view, instead of using a LM35 for human body temp measurement, thermistor is better suited. That too with carefully selected and designed probe having the thermistor in it.
I think the standard medical devices use max of 60 sec to finish measuring the temp.
I still find some drawbacks on using analog sensors for remote sensing for that application, unless cabling is properly shielded and twisted. The LM35 provides 10mV/C degrees, and considering that the resolution for human temperature is normally assessed in the range of 0,1C degrees, this means that the noise expected should not exceed 1mV.