Re: Off-the-shelf temperature controller modules

"Robert Sneddon" <fred@xxxxxxxxxxxxxxxxxx> wrote in message
news:sQKMmTGw6E3HFwaa@xxxxxxxxxxxxxxxxxxxxx

or b) a pre-built single-board computer (SBC) with on-board circuitry
for interfacing a K-type thermocouple.

Unlikely.

Low cost is important in this project; our current proof-of-concept
prototype cost us about 300 bucks in parts and is too big physically
to
do the job we want (liquid bath temperature control in the range 35
deg
C - 85 deg C). It's also too complex to use in the real world. We'd
like
to bring this down below 50 bucks for the controller and sensor (which
must be submersible) and simplify the front-end controls a lot.

We're not fixated on thermocouples but we do need +/- 0.5 deg C
accuracy without drift over time. The info on the Dallas DS1620
suggests
it could do the job as a sensor/digitiser but we'd have to encapsulate
it to make it submersible in fluid (water, mineral oil etc.).
Thermocouples are available off-the-shelf already packaged and proven
for this sort of application.

The temperature range you quote is the so-called "organic" range, and
you may find it easier to use an RTD (aka PT100) sensor. These sensors
*may* be a tad pricier than a type K t/c, but they are easier to read,
either in 3-wire or 4-wire configurations, and you don't need a cold
junction (but you do need a stable current source). Also the accuracy
you're after may well be easier with an RTD.

Re cost: I've designed temperature controllers (albeit for high-volume
production) with way tighter budgets than that ;). Have you considered
some of the 1/16th DIN controllers from e.g. CAL or West Instruments?
Also, have you considered how you'll amortise the development costs? A
PID algorithm is not rocket science, but there are a number of wrinkles
(e.g. integral wind-up etc etc) that newcomers tend to have to learn the
hard way...

Steve
--
http://www.fivetrees.com


.

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