pt100 and meshnetic zigbit board interfacing

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Hello friends,
I have started work on a new project " wireless temperature logger".
I am using pt100( has 3 wires) as temperature sensor.
my requirements are as follows

------- desired specificatin -----

data logging --- wireless method
no. of channels -- 3
temperature range -40 deg cel to 400 deg cel
temp. accuracy - 2% of full scale.
supply volt. - 3v3
application area -- chemical plant(hazardous area.)
enclosure --- flameproof

--- pt100 interfacing---

1. Presently I am using wheatstone bridge (balanced for 100 deg cel).
2. The output of bridge is given to instrumentation amplifier (ina326 -- 3v3 operation single supply).
3. the amplified IA output is given to adc, (vref=1.25v).
4. by using formulas,I am calcualting the temperature.

5.I have build a small board with all the signal conditioning.
------------------------problems----------
I am facing following problems in this section

q1. what changes i will have to make in the circuit to measure temp in the range -40 deg cel to 400 deg cel
q2. what is the maximum wiring distance for pt100 sensor?

q3. How i can multiplex the same signal conditioning for all three channels.

q4. how to calibrate the sensor for the specified range.

--------------- wireless data logging ----

1. Presently I am using Meshnetics zigbit development kit for the wireless data logging.
2. It has 3 adc channels with 1.25v reference.
3. I can transmit the data wireless without any flameproof enclosure.
4. I don't have the clear idea about wireless technology.

------------- problems --------
1. Is it risky to use wirless/2.4ghz in hazardous area ?
2. I need to enclose the system in a flameproof housing. how it will affect the data transmisssion range?
3. what precaution i need to take so that the system will work successfully at the application area.

any help would be appreciated. Thanks in advance !!

:P

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Few hints:

For the Pt100, I suggest you to use a multiple input (can be slow) ADC, with high resolution. Three or four wires for length compensation.

If possible, avoit Pt100, and use Pt1000 instead.

RF for ZigBee is not a reliable source for industrial environments, mostly because all the metal parts that can be there can generate 'shadow areas' where you can't receive anything at all. This reduces your intallation locations.

Enclosures would reduce the data transmission range for shure. But I'm not aware about law restrictions in this field, so I recommend you to first read all legislation about this before doing anything else.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Thanks Guillem,

I can't change the sensor now since they are being used.

Can you suggest me the modifications in my wheatsone bridge for - 40 to 400 range ?

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Hi
I have some remarks.
1. The pure wheatsone bridge isn't good circuit for digital temperature measurements. The sensor current changes and formulas will be very complex. Insted good solution is to maintain constant sensor current.
2. Any bridge should be balanced at -40 deg cel. Adc converter doesn't measure negative voltage.
3. I think you should use Uref=2.5
4. Use 1% resistors in bridge.
4. Sensor wiring 10m length doesn't need three_wires .

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Current source is the most common way. No wheatstone bridges. If you search in this forums, you could find different approaches to sense Pt100.

About wiring, usually Pt100 came in trhee and four wires, and are commonly far away from the converter.

There are many other techniques possible. You can take a look at some tutorial into the academy forum about different ways to measure and linearize signals.

Guillem.
"Common sense is the least common of the senses" Anonymous.