Off-the-shelf temperature controller modules

Robert Sneddon ha scritto:
>  Low cost is important in this project;

here i am, my 2cents

>  (liquid bath temperature control in the range 35 deg C - 85 deg C).

for such a narrow range NTC probes are useful: cheap, accurate and 
easily sampled by uC onchip ADC, without any expensive precision analog 
conditioning.

> We'd like to bring this down below 50 bucks for the controller and sensor

yes you can. looking at wall boiler controller or the like you will find 
similar product (probes+PID+front_end+actuators) well below 50 (parts cost).

>  We're not fixated on thermocouples but we do need +/- 0.5 deg C
> accuracy without drift over time.

mmmm, +- 0.5C onto K type thermocouples are +- 20uV , 0.2% for RTD.
both need precision analog conditioning.
regards

On Mon, 17 Mar 2008 14:06:35 +0100, lowcost <die.spam@invalid.com>
wrote:

>Robert Sneddon ha scritto:
>>  Low cost is important in this project;
>
>here i am, my 2cents
>
>>  (liquid bath temperature control in the range 35 deg C - 85 deg C).
>
>for such a narrow range NTC probes are useful: cheap, accurate and 
>easily sampled by uC onchip ADC, without any expensive precision analog 
>conditioning.
>
>> We'd like to bring this down below 50 bucks for the controller and sensor
>
>yes you can. looking at wall boiler controller or the like you will find 
>similar product (probes+PID+front_end+actuators) well below 50 (parts cost).
>
>>  We're not fixated on thermocouples but we do need +/- 0.5 deg C
>> accuracy without drift over time.
>
>mmmm, +- 0.5C onto K type thermocouples are +- 20uV , 0.2% for RTD.
>both need precision analog conditioning.
>regards

With thermocouples you also have to measure (and assure isothermal
conditions under operation conditions) a local temperature within that
error budget. So, if you're trying to do 0.5&#4294967295;C you might have to
measure the block temperature to within +/-0.2&#4294967295;C and have no more than
500nV/K drift and a few uV Vos to get close to your spec over a
moderate Ta range. 

RTDs are relatively easy. 
Best regards, 
Spehro Pefhany
-- 
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