Waterproof pressure sensor

Question

I'm designing an equipment that will be in a rural area exposed to heavy rains. I already got an IP67 rated enclosure with IP67 connectors and it's working as expected.

I need to measure the ambient pressure so I found this sensor for the task. The issue is, I need to drill a hole for the pressure inside the enclosure be the same as the pressure outside. But this hole will be an entrance for water during heavy rains. I found this image from a mesh used in the iPhone 7 that allows air to pass through but not water:

But I couldn't find the name of this meshes or something similar. I'm thinking that I could protect the hole with a mesh like that to prevent water entrance.

Does this meshes has any special name? What other approach is used to measure ambient pressure in this conditions?

Answer 1

That looks like a hydrophobic membrane. These have low surface energies and the force of attraction to the water is less than the surface tension of the water. Because of this the water is able to "ball-up".

Figure 1. Surface energy measurement. Source: Rame-Hart.

Surface energy is a problem in printing on plastic substrates, for example, as water-based inks won't wet. Solutions to the problem include corona treatment to increase the surface energy by knocking out some hydrogen atoms from the polymers and replacing them with oxygen atoms. This, in simple terms, provides more molecular hooks for the water.

The strength of attraction between a material and a coating is determined by the relative surface energy/surface tension of the materials. The higher the solid’s surface energy relative to the liquid’s surface tension, the greater the molecular attraction, this draws the paint, ink or adhesive closer for high bond strength. The lower the solid’s surface energy relative to the liquid’s surface tension the weaker the attractive forces are and this will repel the coating. Source: Dyne Testing.

I would be afraid that thermal cycling will expel air from the enclosure and then, on contraction, suck water through the membrane. I would look for a sensor that could be mounted through the outside of the enclosure or read through a tube terminated on the casing.

Figure 2. A pnuematic bulkhead fitting.

Figure 3. A bellows diaphragm. Source: AP Racing

On further thought, if your pressure variation is modest a bellows diaphragm would allow the pressure to equalise while maintaining weatherproofing. Put the bellows on the inside. The photo is some sort of car part but it might get you started on a search.

Answer 2

One definition of a solution is "pressure-equalized rainscreen" but the volume of air and thermal effects affect the flow.

For pressure equalization at a slower rate, teflon caps are often used as Hydrogen can transfer thru the barrier such as for enclosed lead acid battery charging to make it explosion resistant. This is a better moisture barrier but the dynamic equalization time to solar heating or internal thermal heat source change must be designed to yield an adequate equalization time constant for a given volume and heat source.

Often for construction a RAIN SHIELD is done in a layered approach.

Answer 3

This is how I solved the problem for a weather station. The sensor is inside a cut-down pill tube with its connection pins sealed. The other end (the cap) has a stainless steel gauze with a communicating hole to the outside. The whole assembly fits tightly into a hole in the underside of the main enclosure and has a ring dripshield surrounding it. This isolates the sensor from pressure changes inside the enclosure, and has proven effective in UK rain storms. The main problem is invertebrates lodging inside the shelter.

Answer 4

First of all, thanks to all for your suggestions. I couldn't find the suggested Bellows Diaphragm here where I live neither in Mouser or DigiKey where I order almost everything I use for my projects.

I DIY approach wasn't viable in this case because this will be a production run and I needed a ready to use solution. I ended finding some caps from Gore and other companies but they were expensive.

My solution was to get a pressure sensor which is water resistant and that allows sealing the hole where it gets through. The sensor I'm using now is a MS5805, this allows me to keep the enclosure sealed and measure the external pressure, this will hopefully solve my problem.

Answer 5

A common type of microphone seal is an acoustic vents from companies like Gore.

• Venting

These are hard to source for a hobbyist but I've talked them into engineering samples professionally before.

There are readily available barometric pressure sensors that won't require building your own rain protection. Omega and other sensor manufacturers make nice units; however, they are probably cost prohibitive for your application ($500+).

I have not used it but the TE MS5837-02BA01 sensor is designed to mount through an enclosure using an o-ring and you shouldn't need any seal over it if it was pointed down and reasonably shielded from direct rain.

Answer 6

I have been using an array of sound sensors. I used a latex membrane stretched over the end of a plastic pipe to house the sensors. This has a limited life.(UV degradation) I am now investigating u bends or a topless jar inside an inverted jar to provide a rainproof maze opening