RTD sensor (pt100/pt1000) input card?

[meurig]

Hello,

Is anyone working on an RTD sensor input card?

It's my understanding that these probes can be more accurate than thermocouples, though they are tricky to interface to - an open source interfacing card seems to be in order!

The arduino forum links to the following design (the multiplexer might be overkill):
http://openenergymonitor.org/emon/node/75

Regards,
Meurig

[rocketscream]

Hi Meurig,

We are working on it but will go on a slightly different route from that link (saw that some time ago).
It'll be a 3-wire RTD card (can be used with 2 wire also) with proper micro current biasing on the RTD sensor.
And yes, they are trickier to interface with.

[meurig]

Hi Rocketstream,

That is great news, I thoroughly look forward to seeing the design.

Will you be making the cards available individually?

Many thanks,
Meurig

[rocketscream]

Yes, the cards will be available individually.

[meurig]

Page five of the following whitepaper gives a circuit suitable for 3-wire RTDs:
http://ww1.microchip.com/downloads/en/a ... 00687c.pdf

It looks straight forward enough (though I don't claim to understand it). Is that the sort of thing you had in mind?

They have suggested a 12bit ADC and note that this gives a resolution of 0.22 degrees C per LSB over the -200 to 600 degree range.
I note the ADC in the ATMega is only 10bit, so the resolution would be less (around 1 degree?).

The whitepaper mentions that with proper calibration, such sensors/circuits can achieve 0.01 degree accuracy, is the plan to include a high resolution ADC to take advantage of some of this accuracy? (assuming the calibration problem can be solved).

The following post links to a few ADCs, though 100k SPS is certainly overkill for anything I could imagine using a pid for:
http://www.arduino.cc/cgi-bin/yabb2/YaB ... 1287074482

The MPC342x (first link on that post) certainly looks interesting, 18bit resolution, i2c connection for around $4. Is this considered an acceptable price for components on an input shield? The 3.75 SPS would be plenty for any application I'd consider.

[rocketscream]

That is some papers that we went through earlier on.
But, we are going with an external ADC thingy.
Resolution wise is still not decided yet though we have few of them lying around (16, 20, & even 24 (overkill)), but definitely not the on-board ADC on ATMega. A pretty slow sampling rate pretty enough for these type of application.

[rocketscream]

The MPC342x (first link on that post) certainly looks interesting, 18bit resolution, i2c connection for around $4. Is this considered an acceptable price for components on an input shield? The 3.75 SPS would be plenty for any application I'd consider.

They are going for $3.02 for above 100 pieces (4 differential input version). Pretty cost effective compared to other similar product in the same range.

[samward]

I am interested in using the osPID with the RTD card for my brewing rig I am currently building. Do you have an ETA on when you think the RTD cards might be available for purchase??

Cheers
Sam

[rocketscream]

I am interested in using the osPID with the RTD card for my brewing rig I am currently building. Do you have an ETA on when you think the RTD cards might be available for purchase??

We were slightly side tracked to release something special for osPID.
The RTD PCB design is completed but haven't do a last round check before sending them off for fabrication.
No ETA at the moment.

[meurig]

The RTD PCB design is completed but haven't do a last round check before sending them off for fabrication.

That's great news, really looking forward to finding out what design you went for, and getting my hands on an ospid with the new card!