Practical protection against dust and water (i.e. IP protection)
Recently, I was faced with a challenge to provide IP65 compliance in a product that had to have humidity and pressure sensors on it. The tricky part was to keep the cost of the unit under $15 while meeting this requirement.
Under normal circumstances, one can put all the electronics within an IP65 enclosure that is affordable and readily available off-the-shelf most of the time such as the ones shown in this link. However, given the humidity and the pressure sensor need to be exposed to the environment to make accurate measurements as needed, enclosing them within an IP65 housing was not an option at all. I was facing a "chicken and egg" dilemma.
The required solution had to provide an opening to the outside world such that dust and water exposure as per IP65 testing would not cause circuit malfunction or complete failure.
For those who are not familiar with IPxx rating, let me give a brief background information here. The IP code stands for "International Protection Marking" (also referred to as the "Ingress Protection Marking") and is covered by the IEC (International Electrotechnical Commission) 60529 standard. The IP code is all about protecting a device against solid particles and liquids and is represented by two digits (there may be additional letters too but for most applications one is likely to see only two numbers). The first digit indicates the level of solid particle protection and falls between 0 and 6 where '0' stands for "No protection against contact and ingress of objects" while '6' indicates "Dust tight". The second digit indicates the level of "liquid ingress protection" and the numbers are between 0 and 9 where '0' is for no protection and '9' indicates protection against close-range high pressure, high temperature spray downs. Because the product I work on requires IP65 level protecion, following is what it means in IPxx lingo:
6 : No ingress of dust (i.e. no dust allowed into the housing)
5: No harmful effects shall be observed when the housing is sprayed by water from any direction
There is further requirements on the water resistance part of the protection that I must note here to give you a more complete picture. During the test, the water volume the housing is exposed to is 12.5 Liters per minute through a 6.3 mm nozzle. The water pressure is 100 kPa and sprayed from a distance of 3 meters. The duration of the test is a painful 15 minutes!
As you can see, any successful solution needs to be well thought and cutting corners or a patchy attempt as far as the housing goes will not cut the mustard. I have been searching for potential solutions and talking to a few experts and manufacturers in the field and I explored applications of water tight varnish, a number of o-rings, gel type materials, polymorph plastic and mineral oil none of which I found to be satisfactory and convincing.
I was about to give up hope when I recently discovered a humidity sensor called SHT20 which is small and affordable. This sensor is actually one of the products in the SHT2x range and has the highest tolerance (4.5% max) and hence is the cheapest in that portfolio. There are a few other products with better tolerances by the same manufacturer (namely SHT21 and SHT25). The exciting thing about this sensor is it has a filter cap which can be bought separately and provides IP67 protection! You can imagine my face when I saw the string "IP67" which is more than what I needed (i.e. "only" IP65). IP67 means water proof and I could be happy with a bit of harmless water ingress!
To give you an idea of how easy it is to assemble the filter cap, have a look at this illustration that has been provided by the manufacturer. Notice how the filter cap has been used with an O-ring pressing against the housing to provide the water tight structure. Of course, the membrane on the filter cap is the critical component here as with 1.5 um pore size, it allows sufficient humidity and air flow for taking accurate measurements while leaving any excessive dust and water out to meet the IP67 specifications.
With the humidity sensor and the filter cap, half of my problems were solved. However, I still had the pressure sensor to worry about. Unfortunately, the humidity sensor manufacturer did not provide suitable pressure sensors (with filter caps) that I could use in my design. So I was online searching again. While doing this search, the fantastic filter cap was always at the back of my head though since that component provided the required solution for the humidity sensor. Therefore, it could potentially help me with the pressure sensor too. I thought if only I could find a pressure sensor that could fit under the same filter cap, I could take a sigh of relief then.
After spending a few hours and browsing tens of websites, I finally found the pressure sensor I was looking for. I had finally found the two critical components and a solution for providing IP67 protection in my design, exceeding the customer's IP65 expectation! I was a happy man.
IP protection is not something that is often requested by my clients but every now and then it is required. Therefore, I am planning to keep this tidy filter cap solution in mind for future reference in case I need to get creative once again in another project. Finding such useful components is almost like finding gold buried on a beach, don't you think?
I wanted to share this piece of information with the EmbeddedRelated.com community in case there are others who may be facing a similar challenge. I hope this short article and the solution I put together will be useful to others as well.
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