Friday, January 13, 2023

Thoughts about simplified probe build

I have been thinking about ways to further simplify the build of the probes. You may recall that the probes have a lot of hoses inside them to transfer pressure from one place to the other. The hoses, and their fittings, end up being one of the main things that take up room and require hours and hours of labor to fit. You'd think a few hoses would be easy, but these things are tiny, and they need to be super secure or else they pop off while the probes are in use. So I put my mind to seeing if we can eliminate these entirely.

The Honeywell pressure sensors I use look like this:

Note how they have little barbed fittings for hoses. Theoretically, Honeywell makes the same parts with barbless tapered fittings, which look something like this:

These would be really cool to use since they seem perfect to "slot into" some kind of one-piece manifold with no plumbing required -- only a seal of some sort, or a dab of sealant. Wouldn't it be great if I could get these?

Except no. Honeywell does not talk to small timers like me. We are left picking at table scraps from the stock that the electronics suppliers happen to have on hand. Which is primarily the ones with barbed connectors. Sigh.

The design I am tossing around in my mind looks like this:

This is a fitting onto which the pressure sensors, on a small daughterboard, are attached. Basically, the realization is: It is easier to route electrical signals around your design than it is to route pneumatic pressure. So put the pneumatic stuff in a convenient location, and run a wire to your electronics as needed.

In this case, the pressure sensor daughterboard has a tiny JST connector on it carrying the 7 signals I need. Since these are SPI sensors, the signals are 3V3, GND, MISO, SCLK, and 3 chip-select signals.

The fitting would be designed so that the barbed tips of the pressure sensors fit into it, surrounded by a soft sealing tube. In a cross section, it would look like this:


The sealing parts are basically short pieces of tubing, perhaps dabbed with sealant on the outside and then stuck into the holes. The tubing can then be trimmed flush with the surface with a razor blade, and then the daughterboard with the sensors can be mounted.

I tried this out briefly with some scrap plastic and a piece of soft silicone tubing, and it seemed to work out pretty well. If we take this to its logical conclusion, we can have a daughterboard for the barometer (a tiny chip) and the thermometer (also a tiny chip). Both of these are I2C devices, so we need a 4-pin connector:

A key part of this design is the back side of the manifold. It would contain a sort of "anthill" of small grooves going hither and yon to route the pressure:

These would in turn be sealed by layers of gaskets:

The whole assembly would be pulled tight by thru-bolts as with previous designs.

It turns out (nothing in this world is new...) that I have thought these thoughts before, in This was in the context of making probes with hoses, though; I was not thinking about using this technique to do away with hoses.

Given the relative ease of snaking wires around and moving SPI signals, it is possible -- extreme, but possible -- to put each of the 3 pressure sensors on its own separate board, and it would still not take up a lot of room, because the 1mm-pitch JST PH connectors I would use are still super small; this is what the 6-pin ones look like.

I'm still bouncing this around so all the ideas above are super preliminary, and the actual CAD work is hacky and contains a bunch of errors. That's fine. I am a very visual thinker, so I usually need to draw something in 3D to appreciate all the details and think of alternatives.

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