🚀 go-pugleaf

"Michael F. Coyle" <mNOfSPAMcoyle@erolsPLEASE.com> wrote in message news:<i1c4a.386520$HG.69411585@news4.srv.hcvlny.cv.net>...
> "David Lesher" <wb8foz@panix.com> wrote in message
> news:b2r75d$951$1@reader1.panix.com...
> > "Antony N. Lord" <antony_lordX@hotmail.com> writes:
> >
> >
> > >>   Actually, the LM35 and variants are more accurate than most
>  commercial
> > >> thermometers; one has to shell out a fairly good chunk of change to get
> > >> a calibrated thermometer as good as or better than these ICs.
> > >>   No calculations, lines of best fit, or any other trash like that.
>  
> > >I suppose that is true - but surely there has to be calculations /
> > >adjustments made due to voltage loss etc. in the cable run between the
> > >sensor and the interface.
> >
> > I'm missing the original post, but I installed about a dozen LM35's
> > several years ago. The data sheet had a spiffy current loop scheme
> > that solved that issue. The price was some fixed error that you
> > could easily offset in software if need be. (We ignored it as we were
> > concerned about delta, not absolute, data.)
> 
> The AD590 (from Analog Devices and Intersil) operate by regulating the
> current passing through them, rather than outputting a voltage.  This means
> you could have a wire run of thousands of feet and not have to worry about
> the voltage drops along the way.  In the simplest case, the current can be
> converted to a voltage with a precision 10K resistor.

<small snip>

The down-side of the AD590 approach is that the self-heating is
relatively high.

If you monitor the output voltage from an LM35 with a relatively high
impedance circuit (nothing fancy) you can have pretty low
self-heating.

The noise level is higher than you'd get with an "interchangeable"
thermistor or a platinum resitance sensor, and the long term stability
isn't as good, but the LM35 is relatively easy to use and adequate for
all but the most demanding applications. Nijmegen University used them
for years to control milli-degree calibration baths.

My current employers hate calibrating temperature sensors, despite
having invested quite a lot in a good calibration bath (not
millidgree, though), and we tend to buy pre-calibrated thermistors.

-----
Bill Sloman, Nijmegen