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".


  • \$\begingroup\$ you are right to assume a linear transfer function, unless otherwise specified in the datasheet of your sensor. \$\endgroup\$
    – Sclrx
    Jul 9, 2020 at 12:22
  • \$\begingroup\$ Life could be so easy with IO-Link.... \$\endgroup\$
    – Mike
    Jul 9, 2020 at 12:50
  • 1
    \$\begingroup\$ 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. \$\endgroup\$ Jul 9, 2020 at 13:26

1 Answer 1


You need to search for a better data sheet such as this one for the OS136A.

enter image description here

So, for the OS136-1 device, 4 mA represents 0 degF and 20 mA represents 400 degF. You can assume that the reading will be linear between these two points but, if in doubt, contact their sales team.

Here's the PDF for the pressure sensor so I'd tak a look through it to check.

  • \$\begingroup\$ their datasheet does not show anything about the conversion I am refering to. \$\endgroup\$
    – abomin3v3l
    Jul 9, 2020 at 12:35
  • 1
    \$\begingroup\$ Who's data sheet? If it doesn't show stuff you need to know you need to speak to the supplier. \$\endgroup\$
    – Andy aka
    Jul 9, 2020 at 13:12

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