resistance temperature detector RTD

Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

Hi All,

anyone into Brewing beer here?

I have a friend with a couple of temperature monitors model: "PXR4RAS1-4V0A1".

The sensing element is a: "Reotemp S1TPX1259.5X"

Its a 3 wire RTD type sensor 100ohm @ 0.0 C.

I want to log the temperatures but the existing monitor does not have a serial output or anything other than a dry contact.

I do have physical access to all 3 RTD wires... but i dont think this is going to be a straight patch to the ADC jejeje... I do have some hi Quality op amps laying around though i dread analog projects.

Experiences and "advices" are appreciated.

G.
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CCS PCM 5.078 & CCS PCH 5.093

temtronic · Posted: Mon Jul 13, 2015 2:01 pm

Try maxim or TI or Microchip for an RTD interface chip. It'll allow the PIC to properly read the RTD sensor ! probably a couple bucks or see Microchips application notes pages.. I know they have 3 or 4 'aps' about using them.

Jay

ezflyr · Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA

Hi,

An 'RTD' is fundamentally a resistance that varies as a function of temperature. So, why three terminals? Have you made any preliminary measurements? A DVM and a soldering iron (heat source) could potentially tell you a lot!

John

Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

The 3 wires are to compensate for "wire resistance".
I read with my DVM and all i got where static voltages... i thought i would learn somthing.... i did not.

I did read an aplication note by MCHIP but it was mostly on building the hardware to drive one....

Im trying to piggyback on an existing system so i dont know if the RTD driver chips will interfere with the existing exitation currents and voltages.

Ive been reading alot about these RTD and i think im either dumb or forgot how to electronics.

G.
_________________
CCS PCM 5.078 & CCS PCH 5.093

Ttelmah · Joined: 11 Mar 2010 Posts: 19338

The Wikipedia entry for RTD's, is a very good place to start.
<https://en.wikipedia.org/wiki/Resistance_thermometer>

The three wire configuration is shown in this.
The 'power supply' across the bridge, will normally be a constant current source.

You need to be measuring Vb, so the difference in voltage between the two points at each side of the bridge. Your op-amp experience will come in here, allowing this to be read, and fed to the PIC input with suitable scaling. You can adjust this to give better resolution, depending on the actual temperature range your design needs.
Remember the resistors used in the rest of the bridge, need to be super stable types or your accuracy vanishes (and you need to be looking at instrumentation op-amps - possibly chopper stabilised types). Obviously the AD, should be using an external Vref. If you want good range/accuracy, look at PIC's with 12bit ADC's.