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Author Topic: Control Air Compressor for filling scuba tanks  (Read 882 times)
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Hello All,

Looking for some advice and input about building a controller for our dive clubs air compressor.
What I need to do is to cut power to the compressor when it reaches a certain pressure.

  • 200 bar
  • 232 bar
  • 300 bar

It should be very easy so I'm thinking of using three buttons one for each pressure.
(and put a display showing set pressure and current pressure)

Anyone know If anyone already have done this? Any tips on what kind of sensor I
should get etc?

Cheers, Fredrik

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I don't think you connected the grounds, Dave.
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Don't.
Please, don't.
Are you going to automate reading the tanks test status too?

Way, way too dangerous.
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Don't.
Please, don't.
Are you going to automate reading the tanks test status too?

Way, way too dangerous.

Ok, little more information about the compressor:

The compressor is connected to an air fill ramp that got two outputs, 232bar and a 300bar.

The 232 side got a pressure regulator that makes sure that you never overfill (starts to spew out air)
The 300 bar side is monitored with the compressors guard which stops it at 320bar.

 So there should not be any real danger of overfilling (as long as you place the 200 tanks on
 the 200 bar side!).

I want is to stop the compressor when it reaches these pressures, cut the power (especially when
filling 200 bar tanks, the compressor is still pumping and the pressure regulator is letting go of
the pressure).

Cheers, Fredrik
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My old compressor had a pressure switch on it that would turn it off. It was adjustable.

You will either need a separate switch for each pressure level or something like a digital gauge with a serial output hooked up the the arduino. ( or just a pressure sensor instead of 3 pressure switches).

If you use the 3 pressure switches then just use a 3 way switch to decide on which switch to activate by. A 3 way switch is way easier and cheaper to implement than an Arduino.

But if you do use an Arduino you may as well add in a display and have it act as an hour meter too. Go a bit further and chuck a Nitrox/Tx blending stick on it with the O2 sensor feeding the Arduino too.
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My old compressor had a pressure switch on it that would turn it off. It was adjustable.

You will either need a separate switch for each pressure level or something like a digital gauge with a serial output hooked up the the arduino. ( or just a pressure sensor instead of 3 pressure switches).

If you use the 3 pressure switches then just use a 3 way switch to decide on which switch to activate by. A 3 way switch is way easier and cheaper to implement than an Arduino.

But if you do use an Arduino you may as well add in a display and have it act as an hour meter too. Go a bit further and chuck a Nitrox/Tx blending stick on it with the O2 sensor feeding the Arduino too.

Hour meter is a great idea!

The buttons is just for clarity, press button 1 with a big sign that says 200bar for 200bar and
should be lit.. I was thinking of only using one sensor and set the cut off pressure reading which
switch is active.

Any tips on sensors?

Cheers, Fredrik
« Last Edit: October 30, 2011, 03:09:26 pm by lnxrkthkr » Logged

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It sounds as if you are trying to use a computer to emulate a pressure switch. Would it not be simpler, cheaper and safer to use a plain old pressure switch for each pressure you want to support, with a selector to enable the switch corresponding to the pressure you want?
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No idea about what sensor to use but you could also set it up to cool down for 10 minutes after switching off. Then turn back on to top up the tanks to desired pressure.
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If the supplier of the compressor system states it can operate unattended then adding something extra to shut it down should not be a problem. The system has to be able to operate unattended because your arduino system may fail and may not activate the shutdown. Any additions you make, and failures of the additions, must not be able to affect the original safety function of the equipment.

Having systems that automatically start or restart is a much bigger issue and best avoided. It is a good idea  to charge slowly and to have the bottles in a cold water butt or tank.

Presumably there are no gauges already on the existing system that provide serial or 4-20mA readings of the bottle pressure? You really just need to search the net for pressure  gauges or switches that can be interfaced to the arduino. Pressure switches will be cheaper and also easier to interface as you will be able to get them with simple Normally Open or Normally Closed contacts. If you opt for switches check that their maximum pressure, adjustment range and tolerance suits the application as the tolerance may be quite wide.

Perhaps in addition to the pressure gauge or switches  you could make the arduino perform some other checks e.g.
IF pressure high OR system venting OR  system noisy OR bottle hot THEN shut down.

If the compressor is electric then you might be able to detect the extra noise of venting or something going wrong.
You could probably also rig something to detect that air is being released by venting but you should not directly attach a gauge or switch to the vent line as it might affect venting particularly if it fails – don’t modify the original safety design in any way.
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