Raspberry pi OHM Meter VEC No6

import spidev
import time
analog_ch = 1
spi = spidev.SpiDev()
spi.open(0,0)
Vin=3.3
R1=1000
def readADC(adcnum):
    if adcnum > 7 or adcnum < 0:
            return -1;
    r = spi.xfer2([4 | 2 | (adcnum >> 2), (adcnum & 3) << 6, 0])
    adcout = ((r[1] & 15) << 8) + r[2]
    return adcout
while True:
    value = readADC(analog_ch)
    voltage = value*3.3/4096
    if voltage > 0:
        ohm = (R1 * (Vin-voltage))/voltage
        #ohm = ((Vin*R1)/voltage)-R1
        print("R? = %d"  % ohm)
    time.sleep(0.3)