Hoses for Honeywell TruStability pressure sensors

The Honeywell TruStability brand of pressure sensors is what we have settled on for our probes -- for the time being. Followers of this blog will recall painstaking tests with different brands of sensors and our conclusion that "nobody got fired for choosing Honeywell". It's not a bold or ambitious place to find oneself, but sometimes discretion is the better part of valor.

The Honeywell sensors, for their indisputable quality, have one problem. The barbed connectors they have are nominally sized for a 1/16" ID hose, but the barbed fittings are kind of skinny:

This means that if you put a soft (low Durometer hardness) 1/16" ID hose over them, it will tend to slip off. If you use a hard hose of the same size, it will be very secure. The manufacturer, to their (dubious?) credit, in a technical note entitled Technical Note: Pneumatic Interface Recommendations for TruStability® Board Mount Pressure Sensors, HSC, SSC, TSC, NSC Series, recommend two hoses, with 85 and 95 Shore hardness respectively. The harder one is Frelin-Wade Fre-Thane® 95a-157.

So let's just be clear. A 95 Shore hose is super stiff. Stiff enough to beat a dead horse with. And that, of course, means that routing this hose around the inside of a cramped probe is super difficult. So much so in fact that, in the process of wrenching the hose this way and that, I'm worried I'm going to snap off the delicate barbed connectors on the pressure sensors.

Meanwhile, we in Airball Headquarters are pretty enamored of this other tubing. Tygon® S3™ B-44-3 has a hardness of 66 Shore, and it's a delight to work with. It doesn't kink, doesn't yank on connectors, and can be threaded around as you please.

Now of course you probably would ask: But doesn't our dream tubing also pop off every barb fitting in the universe? Well no. In fact, when mated to this Eldon James adapter or just about any other 1/16" barb fitting in the planet, it seems really quite secure. It's just the Honeywell sensors that have this trouble.

I have tried everything, including using upholstery thread and a surgeon's knot to tie a "suture" around the tubing where it fits into the Honeywell sensor and then securing the knot with cyanoacrylate glue (verdict: works, but not super well). I have tried 3D printed "clamps". I have looked for clamps on the Internet; many of these are too bulbous and would not fit between the pressure sensor and the PCB. It has seemed hopeless.

Finally, my solution as of the moment is a combination. Use a very short length of hard tubing to go from the pressure sensor to an elbow connector. Then from there, use soft tubing to go everywhere else in the probe. Whether this holds up in the field or not I don't know, but at least it seems reasonable for now.

This sort of highlights how in many ways the problems we're solving now are ones of scaling: we are looking for ways to make it easy for folks to reproduce our results, not just to have results of our own.

But in some ways, apart from the visualizations (which to our knowledge are novel), this whole project has been about scaling. Spherical alpha/beta probes have been around since time immemorial. This is not rocket surgery. But making it cheap and accessible is the challenge we have taken on from the beginning.

Stay tuned for more details of our probe builds.

Adding hoses to 3D printed parts

One of the challenges of this project has been how to attach hoses to holes in 3D printed parts.

This might seem like such a mundane problem -- surely we can get something to work. And indeed we did. We got many things to work, including:

• Gluing hoses directly into the parts
• Gluing pieces of brass tubing into the parts and pushing the hoses onto these
• Tapping the parts and screwing on threaded hose barb fittings

But the challenge is this. Given a 3D printed part that came off the machine, can you figure out an easy way to attach a hose that is not dependent on whacky techniques and skills, so the stuff can be sent to people as a user-friendly kit?

Today I'm trying the technique of 3D printing 10-32 internal threads directly into the parts, and then screwing in plastic barbed fittings. This is an example of one of the tiny parts of the probe, with a threaded hole, printed using a 0.4mm nozzle at 0.1mm layers in ABS on my Prusa i3 MK3S:

This is an Eldon James A1032-1-209BN fitting, that goes from a 10-32 UNF thread to a 1/16" hose barb:

And these are the parts screwed together:

You could imagine putting a tiny smear of silicone or glue or whatever to make the seal super tight but, for aerodynamic pressure measurement, a microscopic leak will not change the pressure reading much if at all.