I2C 4-20ma Sensor Integration

[jimpower]

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.

[timh]

@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.

[jimpower]

@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

[timh]

@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.

[robert-hh]

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

[timh]

@jimpower what do you see on a multimeter ?

[jimpower]

@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.

[timh]

@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 ?

[robert-hh]

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

[jimpower]

@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.

[timh]

@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.

[jimpower]

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

[timh]

@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 ?