I2C 4-20ma Sensor Integration



  • I have a project where I need to connect to a 24V 2 wire loop water flow sensor that outputs 4-20ma, I was wondering if anyone out there has already successfully been able to do this. I am currently trying to achieve this with a INA219 but not having much luck.



  • @jimpower Oh, didn't realize you had changed devices.

    So do you get a value equivalent to 4ma through the INA device (may need to be scaled) ? when not in the water ?



  • @timh I don't have access to the flow meter only a submersible level transducer which doesn't have a calibration mode



  • @jimpower does the flow meter have a calibration/simulation mode. The ones we have do, so you can set them up to output a specific flow rate - and you will get the appropriate ma fed to you input.



  • @timh I don't have the sensor deployed when it is connected it is showing 4mA which is to be expected but I dont have a tank to test depth with so I have connected the following cct.

    0_1527139164626_b8667737-2047-4982-b1a9-5c108465ef85-image.png



  • @jimpower Does the device have different connections for the current loop output and power supply?



  • @jimpower so either somethings very wrong or I don't understand what you are trying to do.

    If you have 4-20ma current loop device you should only be seeing between 4-20ma (less than 4ma is used to indicate sensor fail).

    So why are you seeing 150ma ?



  • @timh I read the same within a 1mA. I have it attached in line with an Access point and connect devices to test the rise in current. I have the current limited to 150mA @ 12V.



  • @jimpower what do you see on a multimeter ?



  • I managed to get this print I found a DC regulated power supply in the workshop.

    0_1527125037716_a70d5d18-0cb4-4c99-b2d1-b5d0ed90c58f-image.png



  • @jimpower What is the range you get?
    The INA219 sensor measures the current and returns a numeric value for that. The default configuration of the adafruit board with it's 0.1 Ohm shunt is 1 digit per 0.8 mA. For 4-20 mA th eraw return values are in the range of 5-25. It is possible to increase the sensitivy
    a) by configuration, increasing the resolution to 0.1 mA per digit, resulting in a range of 40-200
    b) replacing the shunt resistor with a higer value, like 1 Ohm or 10 Ohm.
    For calibration, insert a amperemeter and scale the returned values accordingly.



  • @jimpower I haven't used the INA219 so can't comment specifically about it. But I have worked with Flow meters in the past.

    You probably have this already covered but thought I would ask a few things.

    Most flow meters have rated max flow, but the specific process may not need such a high flow. ie a 300mm Krohne might have a max flow of 300LPS, but the process is only going to hit a max of 80lps. So you would normally set the output scale of the flow meter so that 100lps would match 20ma.

    This catches out a lot of people when dealing with controllers (industrial, COMAP for instance), so the values you get aren't expected.

    Most flow meters have a simulation mode so you can set 80lps and then measure 20ma with a multi-meter (given the above scenario).

    Is this the library you are using ?

    https://github.com/chrisb2/pi_ina219/blob/master/ina219.py

    Its current reading isn't using voltage. Its reading a current register.

    Other thing to note, most flow meters need a chassis ground, especially when there is no water as the flow meter is often connected to non metallic pipe (ie rubber suction/discharge hose), and without the water to provide a ground on the chassis of the flow meter, we see random values produced.



  • @timh I can talk with INA219 however results aren't what I expect, the library I'm using is this it does current calculation using bus voltage so not sure how accurate it will be. ina219.py



  • @jimpower I am using MCP3424 based device https://www.ereshop.com/shop/index.php?main_page=product_info&products_id=805 and this is working well.

    What sort of problem are you having, talking to the device over i2c or not getting a value ?

    There are python libraries for the INA219.


 

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