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Analogue Pressure Gauge Monitor

Posted: Fri Dec 20, 2013 1:28 pm
by talkeasy
Looking for suggestions from you inventor types out there!

We have 4-bar analogue pressure gauge to show the water pressure level in our central heating system. And, yes, we have a leak somewhere. Despite years of searching (big house) we cannot find the source.The pressure drops from one bar to zero in a week or two - sometimes more quickly.

While on-going checks around the house for leaks continue, I want to be able to remotely monitor the pressure - so if it drops by, say 20%, I can be notified by email or SMS. Busy lives mean that more often that not I forget to check the level and top up the pressure.

It is a standard Caleffi 4-bar water pressure gauge with a removable plastic front cover. Ideally I would like a non-invasive solution (e.g. proximity switch/magnetic maybe?) but all Pi-based ideas welcomed and considered!

Re: Analogue Pressure Gauge Monitor

Posted: Sat Dec 21, 2013 2:00 am
by DougieLawson
A neodymium magnet on the pointer and a reed switch (on the face plate) is probably least invasive. There is a limit of 80°C for neodymium magnets and they're brittle.

Re: Analogue Pressure Gauge Monitor

Posted: Sat Dec 21, 2013 1:55 pm
Sometihng like that:
- remove front glass from the analogue gauge,
- take photo interrupter as a detector of gauge's pointer warning position,
- make the pointer's shape proper (high enough) for the interrupter -> by a piece of isolating tape you can easily create proper barrier shape,
- connect interrupter's output to RasPi, add usb dongle for SMS sending, add/compose software bits, etc...

P.S. take care about hysteresis, to avoid a bunch of SMSs / emails each time. This can be solved simply by a flag set by the first alarm trigger and reset after a period of valid state.
I'd avoid manual flag reset requirement... it could easily be forgot to reset the flag after the alert, and then you have disabled system you're relying on.

Best wishes, Ivan Zilic.

Re: Analogue Pressure Gauge Monitor

Posted: Sat Dec 21, 2013 2:30 pm
by talkeasy
Thanks very much for the suggestion, Dougie. I have ordered a small neodymium magnet and reed switch off ebay this morning and look forward to experimenting.

In the meantime I have been trying another approach: I have attached an LED to a strand of fibre optic which I will replace behind the gauge dial. When the dial moves, the change in light level will be picked up by an ldr. That's the plan, at least.

So far I have tested this idea on a breadboard and have detected a workable light level change in a darkened room - about 15-20%, surprisingly effective for one very thin strand of light. I have ordered an ultra bright LED to see if this solution will work with the dim ambient light expected near the heating tank and gauge. A problem has been the fragility of the fibre glass strand - but in principle I'm rather pleased with the concept so far.

Ivan - I will investigate photo interrupters. Thanks for the idea.

Re: Analogue Pressure Gauge Monitor

Posted: Sat Dec 21, 2013 2:40 pm
talkeasy wrote:Ivan - I will investigate photo interrupters. Thanks for the idea.
They are basically LED + PhotoTransistor elements. :-)
Their main advantage is IR light they are using, so ambient light does not disturb them...

Best wishes, Ivan Zilic.

Re: Analogue Pressure Gauge Monitor

Posted: Fri Jan 10, 2014 2:25 pm
by talkeasy
I have managed to pull together a solution for monitoring the water pressure in my heating system based on Dougie Lawson's suggestion of a neodymium magnet on the pointer of the pressure gauge triggering a miniature glass magnetic reed switch.

Positioning and securing the magnet was probably the trickiest part. The gauge pointer proved too narrow to support a magnet, especially since the pointer itself was not magnetic so would have involved gluing a magnet to the tip. A better approach was to attach the magnet to the pointer’s wider counter-weight by placing two minute 0.5mm magnets on the underside which holds the front 3mm magnet in position above. A magnetic sandwich (thanks to my 10 year old daughter for that idea)! The plastic cover of the gauge can then be put back so no permanent modification is required.

A further advantage of placing the magnet away from the pointer itself is that the pressure level reading is not obscured by the glass reed switch which needs to be placed on the face of the dial. A disadvantage on the other hand, and this was a tricky one, was the travel distance of the counter-weight was much shorter than for the pointer so accurate positioning the magnetic reed switch became a real challenge.

Meanwhile a cable was run down to the Raspberry Pi conveniently located in the room below and hooked up to 3.3v, Ground and GPIO Pin 17. I like PHP so modified some python alarm code that I had found.

Eventually though trial and error (snapping 3 glass switches in the process) a good position for the switch was located on the gauge face. This was found by monitoring a test script on the same daughter’s Nexus 7 as I tried placing the switch in different positions while raising and lowering the water pressure manually.

The script is run hourly during the day through crontab and is set to trigger an email when the bar pressure drops to about 50%.