Phidgets temperature sensor formula and conversion

  phidgets, raspberry-pi

https://www.phidgets.com/?tier=3&catid=14&pcid=12&prodid=95

Given formula:

Temperature C = ( VoltageRatio×222.2 ) - 61.111
  • VoltageRatio is the raw voltage you read divided by 5.0 VDC.
  • Example: raw reading of 2.0145 VDC
  • Voltage Ratio would then be: 2.0145 / 5.0 = 0.4029

Let’s re-write formula using raw voltage reading instead of ratio:

C = R / 5 * 222.2 - 61.111

where C = Celcius and R = raw voltage reading

Fahrenheit to Celsius:

C = (F - 32) * 5 / 9

Celsius to Fahrenheit:

F = 9/5 * C + 32

Substitute C with 

R / 5 * 222.2 - 61.111

and we have:

F = 9/5 * C + 32

F = 9/5 * (R / 5 * 222.2 - 61.111) + 32
  = 79.992 R - 109.9998 + 32
  = 79.992 R - 77.9998

Testing with raw voltage value of 2.0145 V DC:

F = 79.992 * 2.0145 - 77.9998
  = 83.144084

Thus, 2.0145VDC translates to 83 degrees Fahrenheit. Correct.