pt100 interfacing with meshnetics zibbit board

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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 signle 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 !!