Magnetic Levels Probe

[Penri]

Hello

Just to record a conversation with Hamish, I have asked him to review the practicality of inserting new functionality in the sump control software to ON/OFF control the main circulation pump via a SSR.

We do plan to implement this functionality through control of the speed / flow rate of the pump but as yet have not had time to implement it. In the interim I'd like to implement a "quick and dirty" solution so that we can protect the pump when the water level goes low and then turn it back on, automatically, when the water level rises to an acceptable level.

Why this solution now?, when the sump runs dry the standing instruction is to turn it off, when that happens the water head in the bog and river system drains into the sump and unless one is vigilant it overflows into the soak away.

I'm thinking that the low water triggering level for turning the pump off is 200mm and back on again 400mm.

I'll make a start on building the SSR circuit and enclosure, we'll need to agree on the appropriate I/O from the Pi, assuming there's one available.


Hwyl

Penri

[TerryJC]

Just to record a conversation with Hamish, I have asked him to review the practicality of inserting new functionality in the sump control software to ON/OFF control the main circulation pump via a SSR.

I'm presuming that it is the intention that this switch is located at the input to the existing Controller, so that it is simply a fail-safe control?

[Penri]

That was my thought yes.

The controller retains its previous setting if switched off and back on again, so yes fail-safe.

[TerryJC]

Hamish and I were discussing the problem with the float on the probe getting stuck this morning and we came up with the idea illustrated below:

Hall_Effect_Probe_Protection.gif (11.91 KiB) Viewed 944 times

In effect we encapsulate the probe in a cylindrical mesh screen with the probe rod constrained so that the are no problems with the float coming into contact with the inside wall of the screen. The idea is that water is allowed into the volume contained by the screen, but not debris.

There are of course several compromises here:

• If the mesh in the screen is fine enough to trap debris, it may be too fine to see the position of the float (for sanity checks).
• If the mesh in the screen is fine enough to trap debris, it may slow down the passage of water sufficiently to introduce a significant lag into the system.

Other methods could be used:

• If we assume that the debris is mainly trapped when it floats into the top of the float (eg, the debris is not present beneath the surface), then we could use a perspex tube with holes near the bottom to let the water in and near the top to let the air out. This may suffer from a build-up of algae on the outside of the tube, necessitating frequent cleaning.
• For simplicity, the screen could be a fairly large box structure, using very fine mesh, forming a 'pool' within a 'pool'. In this case, the position of the float would be viewed from above.
• An even simpler solution, we could position the probe directly in front of the waterwheel (underneath the 'penstock' in 'hydrography' parlance). This will cause the float to be agitated, which may in itself cause problems, but could also help to prevent debris staying in the area at the top of the float long enough to get jammed.

Just some ideas.

[Penri]

Hello

Nice ideas and interesting thoughts.


Hwyl

Penri

[Penri]

The current magnetic probe as it exists at the end of the summer season 2018 works well and has proved robust and reliable but has some shortcomings:

• The probes are not easy to manufacture.

• The interfacing is crude and involves a lot of wire.

• The level resolution, while adequate, is in fairly large steps.

Some potential solutions have been kicked around and a decision taken to try a new interfacing method. The basic floating magnet / Hall effect switch measurement technique has been kept but the way it is used changed. Details will be posted in a new topic "Magnetic Level Probe Mk2".