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I am trying to read an industrial pressure sensor (datasheet) with range 0-10 bar and output 4-20 mA, with an Arduino Opta RS485, powered from an external 12 VDC source.

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Leaving the sensor in the air, not connected in any process pipe (-> 1 bar) and using a 470 Ω resistor in order to convert 4-20 mA into a 0-10 V signal that the Opta can handle, I expected a reading of ~2.6 V (-> 5.6 mA.) I am instead seeing a reading of ~ 1.6 V which would land below the 4-20 mA range.

Is expecting 1 bar out of the pressure sensor in plain air wrong, or is there a problem with the connections? Does anyone have experience with reading 4-20 mA pressure sensors and had similar problem?

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  • \$\begingroup\$ It's more common to use a 24V loop supply, in which case the compliance issue that Marcus identified won't be an issue for you. \$\endgroup\$ Dec 4, 2023 at 12:23

3 Answers 3

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According to the data sheet, your load resistor must be \$R\le(\text{V}_{\text{CC}} - 8 \text{ V})\cdot 0.02\text{ A}\$, which in your case would be 200 Ω.

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  • \$\begingroup\$ Wow, missed that. Will test it again and accept your comment as solution in case it works, thank you \$\endgroup\$
    – p_a321
    Dec 4, 2023 at 11:28
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Further to @MarcusMüller's answer ...

The sensor is "loop powered" since it only has two connections. That means that it needs to always have enough voltage dropped across it to power the internal circuitry. As Marcus pointed out, the datasheet implies that the sensor needs at least 8 V to operate.

If you need to generate a 0 - 10 V signal then you need at least an 18 V supply: 8 V for the sensor and up to 10 V for the resistor. If you can use a suitable supply (and 24 V would be the standard for industry) then you can use a 500 Ω resistor for the conversion. TE make these and are available through RS for < €4 at one-off prices in 0.1%, 0.5 W. See here. That should make calibration very simple.

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@MarcusMüller and @Transistor are both right, I chose the wrong resistor for the purpose, just noticed something else that might help someone working with Opta.

I found out that Opta has inside a soldered resistor network ( with values 4.7, 2.7k and 1.5K ) in all of its 8 inputs, in order to bring 0-10V to 0-3V range for STM32, which is not documented as it is not open hardware, so that makes '4-20 mA to Voltage' conversion even more cumbersome and in need of tinkering.

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    \$\begingroup\$ 4.7, 2.7k and 1.5K seem very low. The Opta looks very like the Siemens LOGO! It's interesting to see Finder get behind the project. It's a pity they don't publish the schematic. \$\endgroup\$
    – Transistor
    Dec 5, 2023 at 10:15

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