# Sensors - How to convert 4-20mA output to temperature and pressure values?

I'm dealing for the first time with two sensors, of pressure and temperature. Both have output of 4-20mA, then I'm using a 47 ohms 1% to convert to voltage. When flows 20mA, 20mA * 47 ohms = 940mV, and my ADC has a positive reference of 1V.

My doubt is about how to convert the mA value (or voltage on the resistor) to temperature (degrees) and pressure values (psi). I just didn't found documentation about that on both manufacturer's webpage.

Do they have linear relationship? For example for the temperature sensor, 4mA would mean 0°F, 20mA would mean 400°F and 12mA would mean 200°F?

These are the sensors:

TEMPERATURE SENSOR: OMEGA OS136A-1-MA Range is from -18 to 202°C (0 a 400°F).

PRESSURE SENSOR: KELLER PA-21Y Its body show "range of 0 to 200 psi".

Regards.

• you are right to assume a linear transfer function, unless otherwise specified in the datasheet of your sensor. Commented Jul 9, 2020 at 12:22
• Life could be so easy with IO-Link....
– Mike
Commented Jul 9, 2020 at 12:50
• Ugh. If you are using those decent industrial sensors with the dismally crappy ADC in the ESP8266, I would worry a lot more about the accuracy and linearity of the ADC than the sensors. Just assume they are linear with 12mA mid scale etc. Commented Jul 9, 2020 at 13:26