Re: [VAL] temperature monitor

["Roger Moseley" <rmoseley@xxxxxxxxxxx>]

I use the LM-135 (several versions give more accuracy, eg, LM-235), 
which requires only a single positive supply voltage to provide a linear 
output from zero kelvin (yes, kelvin) to whatever temp it burns up at. I 
doubt that it would actually work down near zero kelvin, but I have had 
reliable measurement from -20F to +180F (I use it to log outside 
temperatures and lots of internal house temps, like attic and gas flue 
temp in my home furnace). I've had one device dangling outside in the 
weather for 10 years, with no problems. Since it's a kelvin device, 
normal temperatures produce something in the 2.4 to 3.7 volt range. 
These devices are fantastically linear, but the slope of the temperature 
curve needs a slight bit of adjustment in the nature of y=ax+b where b 
is always zero. I take one measurement to determine what the "a" is for 
the particular device, eg, if the actual temp is 45 degrees and the 
measured temp is 46.5 degrees, then a=(1+(318.15-319.65)/318.15) or 
=0.99529 (remembering that 0C is 273.15K). Using this one correction 
will correct the entire output scale, so it's really a nice device to 
calibrate. I use a 12 bit A/D, so theoretically I can get temperature 
accurate to one part in 4096. The A/D has 8 inputs, so it's easy to 
interface and get 8 temperature channels. It also is a positive only 
device and measures inputs from 0 to 4.096 volts. I never expect to need 
hot measurements above 409.6 kelvin (180 degrees C), so I'm happy.

The above is not as complicated as it looks. I use an really archaic 
language for my controllers (Forth) and it only does integer math, so I 
have to be careful the order in which I do my multiplications and 
divisions, but other than that it's very slick.

For a somewhat easier approach, use an LM-92 two-wire device that is 
accurate to .33 degrees C and sends the data serially to your micro. It 
also has several outputs that flag exceeding or going under certain set 
temperatures. These outputs can be wire-ored together to make a very 
simple system that will detect when the temperature along a pipe, say, 
gets close to freezing and can turn on a pump or heater without any 
computer intervention once the flag temperature is set. You can gang 4 
of these together on one data line, since they have a two-bit addressing 
scheme which allows for 4 independent addresses on the same line.

Roger
Palmer Lake, CO
18' Caravel