In article , Percy writes
On Thu, 19 Dec 2013, "fred" writ:

Thermistors are low cost sensors and pretty standard in commercial
gear, 10k at 25degC is a typical example that would be fine on a long
line using twisted pair (phone or cat5e). I didn't bother using
shielded cable.

I already have TMP36 remote solid state sensors using STP cable and they
are pretty accurate. Certainly within 0.5C from -20C to +50C on a 5
metre cable. I have had them tested at work. You really must have STP as
any noise messes up the readings as I found out when I tried them with
UTP cable (

Useful device, thanks.

I figured the coarse and robust measurements of thermistors would suit
this app if you were rolling your own system hardware and software.

Being 2 terminal devices they lend themselves well to balanced 2 wire
measurement, noise in one limb is equally induced in the other,
resulting in cancelling of noise if measured in balanced mode.

In a single ended voltage output device though (like TMP36) you get no
benefit from twisted pairs although using a pair for the output and 0V
should give the least susceptible result.

In either case, the way to avoid noise it filter it out or integrate the
analogue output as response time is not important.

There are direct digital output devices available from Dallas that would
do the job too. Another poster here has used them in his home automation
system, search for Andrew Gabriel as poster and Dallas and you should
find the devices he used. I think they're under a fiver each and have
good application notes for interface software too. They have a large
fanbase that has resulted in quite a few apps being written for them.
It's recommended to range check your input, ignoring any results that
are dramatically out from the norm as this weeds out received data that
have been corrupted by (much heavier) noise.

A simple IP68 outdoor junction box will be fine for a housing and
sticking the sensor to the lid would give it a low thermal mass with a
response as least as fast as the weather.

Thanks for the suggestion, I hadn't thought of IP68 boxes.

Good for corrosion protection (condensation) and to protect from insect
ingress. Some view it as inevitable that condensation will occur in
outside enclosures (the boxes aren't vapur tight) so they drill a tiny
(1mm) hole at the very bottom to allow drainage, the small hole
minimising the risk of insects nesting. Another trick is to seal the
sensor in adhesive lined heatshrink or to dip the wired assembly in
thick varnish. Marginal increase in thermal lag but protects from
corrosion.
--
fred
it's a ba-na-na . . . .