T
Tim Shoppa
Guest
Digital fever temperature thermometers (wherever you stick them)
are ubiquitous these days, especially with mercury being a no-no to
the current generation. (Hey, we all played with it when we were
kids, and we're perfectly normal, right? ;-) )
What's the temperature sensor in these? Cheap LM35-style sensors
have a rated accuracy of 1 degree C, which is like 2 degrees F, and
I think for this application a thermometer would need much better
accuracy - better than half a degree F, or preferably a tenth of a
degree F (which is the display precision of most of the consumer devices).
There are better versions of these temp sensors but none rated to
be better than half a degree, as far as I can tell.
Maybe there's a special class of LM35-style sensors which can be
factory-calibrated to be known good to a tenth of a degree F near 98.6F?
I can imagine a thermistor based on some sort of phase-change near
98.6F which might do all this magic but I've never seen anything in this
range in any of the catalogs. (There are some indium and cesium alloys
with melting points in this region, that I've seen these used in certain
industrial temperature-measuring applications that require extreme
accuracy and repeatability, but I'm pretty sure that there's no such
stuff in a consumer fever thermometer).
Quartz crystal thermometers (e.g. HP 2801) are good to better than a tenth
of a degree F, but are also much more expensive (circa $1K) than the
consumer thermometers. I could imagine this technology being
mass produced though (a mass-produced crystal with a highly
temp-sensitive cut might be made for less than a $1 and the rest is
quote-unquote easy).
There are also IR-reading thermometers at the high end of the consumer
range (circa $100) and I haven't a clue how they can make measurements
so close to ambient with an accuracy of 10 degrees F, much less 0.1
degrees F! ???
Tim.
are ubiquitous these days, especially with mercury being a no-no to
the current generation. (Hey, we all played with it when we were
kids, and we're perfectly normal, right? ;-) )
What's the temperature sensor in these? Cheap LM35-style sensors
have a rated accuracy of 1 degree C, which is like 2 degrees F, and
I think for this application a thermometer would need much better
accuracy - better than half a degree F, or preferably a tenth of a
degree F (which is the display precision of most of the consumer devices).
There are better versions of these temp sensors but none rated to
be better than half a degree, as far as I can tell.
Maybe there's a special class of LM35-style sensors which can be
factory-calibrated to be known good to a tenth of a degree F near 98.6F?
I can imagine a thermistor based on some sort of phase-change near
98.6F which might do all this magic but I've never seen anything in this
range in any of the catalogs. (There are some indium and cesium alloys
with melting points in this region, that I've seen these used in certain
industrial temperature-measuring applications that require extreme
accuracy and repeatability, but I'm pretty sure that there's no such
stuff in a consumer fever thermometer).
Quartz crystal thermometers (e.g. HP 2801) are good to better than a tenth
of a degree F, but are also much more expensive (circa $1K) than the
consumer thermometers. I could imagine this technology being
mass produced though (a mass-produced crystal with a highly
temp-sensitive cut might be made for less than a $1 and the rest is
quote-unquote easy).
There are also IR-reading thermometers at the high end of the consumer
range (circa $100) and I haven't a clue how they can make measurements
so close to ambient with an accuracy of 10 degrees F, much less 0.1
degrees F! ???
Tim.