Thank you so much Brian and Andrew. I was struggling with my PT 100 , perhaps due to collective error of voltage drop and error due to variation in the resistance of devider circuit I was getting an error of 5 degrees F.

finally my sensors are giving accurate readings code is following:-

#!/usr/bin/python3

from ABE_ExpanderPi import ADC

import time

import math

"""

===============================================

Success_read_PT100-1.0

===============================================

this demo reads the voltage from channel 1 on the ADC inputs

"""

from math import sqrt as sqrt

adc = ADC() # create an instance of the ADC

# set the reference voltage. this should be set to the exact voltage

# measured on the Expander Pi Vref pin.

adc.set_adc_refvoltage(3.5)

alpha = 0.003851

a = 0.0039083

b = -0.0000005775

c = -0.000000000004183

ro = 100

vs = 3.5

r1 = 270

D1 = adc.read_adc_voltage(1)

D2 = adc.read_adc_voltage(2)

D3 = adc.read_adc_voltage(3)

D4 = adc.read_adc_voltage(4)

D5 = adc.read_adc_voltage(5)

D6 = adc.read_adc_voltage(6)

D7 = adc.read_adc_voltage(7)

D8 = adc.read_adc_voltage(8)

def getTemp(voltage):

voltage = adc.read_adc_voltage(voltage)

Z = (r1/((vs/voltage-1)))

X = a*a*ro*ro-4*ro*b*(ro-Z)

Y = math.sqrt(X)

T = ((-(a*ro)+Y)/(2*b*ro))*(9/5)+32-5

return T

def tempAvg(voltage):

temperature = getTemp(voltage)

time.sleep(1)

temperature += getTemp(voltage)

time.sleep(1)

temperature += getTemp(voltage)

time.sleep(1)

temperature += getTemp(voltage)

time.sleep(1)

temperature += getTemp(voltage)

temperature = temperature/5

print(temperature)

#def getTemperature(voltage):

#temperature = (((a*ro*-1)+math.sqrt(((ro*ro*a*a)-4*ro*b*(ro-(r1/((vs/voltage)-1)))))/(2*b*ro)))*(9/5)+32

#temperature = ((-(a)+math.sqrt(a*a-4*b*(1-outputRes(voltage)/ro))/(2*b)))*1.8+32 # I like this one :P

# read the voltage from channel 1 and display on the screen

while True:

print (str(D1))

print (str(D2))

print (str(D3))

print (str(D4))

print (str(D5))

print (str(D6))

print (str(D7))

print (str(D8))

Z1 = (r1/((vs/D2-1)))

Z2 = (r1/((vs/D3-1)))

print (str(Z1))

print (str(Z2))

# print (getTemp(2)) # Temperature 1

# print (getTemp(3)) # Temperature 2

tempAvg(2)

tempAvg(3)

time.sleep(2)

print("\n" * 100)