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Author Topic: Optocoupler for 4-20mA sensors  (Read 737 times)
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Norway
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Hello Arduino Forum!

I work at a gas station, and the boss makes me messure the gasolin and Diesel levels located in big tanks underneith the station.
This is OK at summertime, but this is a hassle at wintertime (I live in Norway...)

However.
After some Googling, I found that there exsists sensors for this exact purpose.
These will give me a report in form of 2 to 200 milli Amperes. It manipulates the voltage to make this possible.
I understand that this i the most used industry standard for this kind of sensors.

After a night of playing around with my Arduino Uno and a develpoerboard, I had made a "sensor-wanna-be" that feeds the Arduino with emulated sensor voltage. The Arduino is reading the "sensors" and displaying the levels on two 7seg. displays. It also buffers up the data, and lets a computer download the log using serial comunication.


But here comes the problem:
If this is ever going to make my work easier, then I will have to make sure that it is up to industry standards too.
That means that the inputs must be protected (with optocouplers?), the output will have to be protected with resistors, and the power will have to accept 9 to 36 volt...

Googling optocouplings, it tells me that the input side typically starts opening at 200mA. This will not work for me.
How do I get the optocouplers to "transmit" 2-200mA sensor data?


If I get this to work, my work will be a lot nicer for me and all the other employees.
And I bet my boss will love me for it ;-)
« Last Edit: April 25, 2013, 06:05:40 pm by geir_moen » Logged

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I've never heard of a 2-200ma sensor standard and I've worked (before retirement) in the petrochemical industry. I am familiar with the 4-20ma DC 'sensor standard' industrial current loops. Do you any links to the devices you are referring to?

Lefty
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Norway
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Hi RetroLefty :-)

You wrote "I am familiar with the 4-20ma DC 'sensor standard' industrial current loops.", and you are absolutely right. This is the standard I am referring to.
After some research I've come to the conclusion that it must be a touch of brilliance! 

I believe that this can be achieved in a pretty simple circuit.
It usually does. Looking back when I have the answer ...

I tried to use a opamp to boost the input signal by generating AC and use a transformer and a rectifier to have a complete circuit "outside" of the Arduino system. The noise from the AC generator rendered the inputs useless. So that didn't work ...
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In some industries, to meet certain safety standards, the interface to such sensors must be an intrinsically safe barrier. They are usually quite expensive. If that is the case in your application, it usually isn't practical for an individual to make their own barrier as it would have to be certified.
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do you simply "check it" with a visual inspection...

Maybe a little webcam and light would work? and have it transmit back...
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Norway
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To be honest, I haven't done any research if it needs any certifications to do this.

It is supposed to be placed inside the fuel station. Not inside any of the tanks, so I don't think it's that critical.
Maybe it's just me being over protective over my own design? But then again, it might be critical to have all the certifications?
I don't want to be responsible for blowing up the entire station ;-)

It might be an idea to look into this before I invest too many hours on this project?
I still think it would be nice to be able to look at a display inside of the station instead of going outside mid winter ...


But this must have a simple sollution? ... certifications asside ...
They must hade had this in mind when they made the standards (?)
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Norway
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do you simply "check it" with a visual inspection...

Maybe a little webcam and light would work? and have it transmit back...

The manual mesurements is made by dropping down a long brass pin with messurements on it.
The tank itself is about 5 metres (15 feet) under the ground. There is no natural light down there.

The idea of making making a visual inspection inide the tank would be perfect because that would show any problems with the tank too.
But after opening the inspector hole, it will expose the high explosive gas. I am sure that I'm not alowed to put a lightbulb down there.
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nope certainly not, you'd need some kind if LED light that's air tight and safety checked (like miners lights)
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Norway
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Hmmm ...
The illution that this would be an exiting 20 dollar project made up of a small circuit board in a plastic container is starting to fade away...

It is starting to look like a multi hundred (if that is a word) dollar project that would be stretching over several months.
There is no way that my boss will let me do this. Even if I do the developement in my spare time and for free.
Only the parts themselves will be way over the cost of us manually messuring the tanks.


This looked all so easy, sitting bored one night at work :-(
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