Waterproof Microphones? - Page 1
Forum section: electronics / projects

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 07:33:05 pm
content: I'm involved with a project that needs to make some fairly low range pressure measurements in a water tank.The area if interest is very small (8mm ?). I've tried using a pressure tap to bring the pressure out though a small tube and to a large (proper) pressure sensor, but there are a lot of issues with getting the tap to work right -- dynamic response, clear of air bubbles... etc.As a replacement sensor, I've thought a small microphone like this would work:(4.5mm dia).However I am having trouble finding any that are compatible with water. And by compatible, I mean will give me a reliable week of operation -- for $3/ea and in the situation I am working in, I can just replace them every week if need be.I've tested most of the 4mm and 4.5mm dia microphones that are sold on Digi-Key. Attached is a copy of the excel chart of their nominal impedances measured with time after they were submerged in distilled water. You can see where they failed when the impeadances suddenly dropped out. I didn't bother to include the microphones that gave less than one day of performance.I'm hopeful that 2 or 3 of them may meet the 1 week performance mark in a larger statistical sampling. However, I am wondering if anyone here has experience with this kind of thing? Any suggestions on hacking a microphone (or whatever) to work underwater for a week or more?PS: The one 4mm dia microphone that are sold on digi-key as "waterproof" (http://www.digikey.com/product-detail/en/CME-1538-100LB/102-2190-ND/2364631) is anything but. It fails within the first 10mins of being submerged -- its redeaming value is it tends to 'dry out' and function after a few hours of being pulled out.

author: Yago
author_rank: Frequent Contributor
timestamp: March 26, 2014, 07:48:42 pm
content: I think the transducer you want might be a hydrophone.Had a look and a couple are based on similar condenser you pictured, probably a matter of sealing.It also depends on what you wish to measure, is it static pressure or dynamic, what range and frequency etc.What is the tank like?Might be worth a try bonding a little condenser mic to the outside of the tank.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 07:59:25 pm
content: Quote from: Yago on March 26, 2014, 07:48:42 pmCan you link in a ref to those hydrophones? I wasn't able to find any on the order of 4mm dia.I need both static head and dynamic. 0-100Pa / DC-100Hz. But higher would always be nice.Modification / calibration is not an issue. Just a roughly linear response is all that is required so I can calibrate them in situ.Outside won't work. It needs to be in contact with the water.

author: Fraser
author_rank: Super Contributor
timestamp: March 26, 2014, 08:13:08 pm
content: I just posted but it appaears to have got lost in Cyberspace.Try sealing the microphone in a condom. No joke. Give it a go.Note that if you are using electret Mic elements, they use electrostatic charge across plates and so are not compatible with direct contact with water ! A dynamic (speech coil around magnet) based mic may work in distilled water without sealing. You just need to be sure that both sides of the diaphragm can flood to equalise pressure across it. Failure to equalise the pressure will just crush the diaphragm making its useless.Another possibility is the use of a piezo electric element as a microphone (crystal microphone) A crystal earpiece may work but I am uncertain of the frequency response.Unsealed microphones would need to be operating in an insulator. Have you considered using very thin oil instead of water as the liquid medium ?

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 08:15:20 pm
content: Quote from: 3roomlab on March 26, 2014, 08:06:21 pmLike place a membrane across the opening of the microphone? I've thought about it, and its what some manufactures do to make a waterproof mic (like Knowles -- good mics, just too expensive). I haven't tested anything like that yet. I was hoping to find ones that didn't require that extra step. But it may be where this project is headed. Maybe a small bit of Kapton tape over the inlet? (suggested welcomed here!!!)

author: Fraser
author_rank: Super Contributor
timestamp: March 26, 2014, 08:19:11 pm
content: The mic would need complete sealing, not just the acoustic aperture as the rear is not watertight. Also consider pressure differencial across the diaphragm and the effect on performance. I use Knowles mics and at GBP55 each they are expensive if killed during experiments !

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 08:19:21 pm
content: Quote from: Aurora on March 26, 2014, 08:13:08 pmIt seems that ones I have tried have some ability to reject water from the electrostatic area. Perhaps its "sealed"?It would seem from my testing that the seal is only good for ones to several days.The voice coil sensing method is hard to find in such a small package... and it wouldn't give any static pressure indication.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 08:20:34 pm
content: Quote from: Aurora on March 26, 2014, 08:19:11 pmHeh... yeah, plus you got my shaky hands soldering them.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 08:21:19 pm
content: Here's another idea following up on the Kapton tape... I could back fill the port using a viscous silicone gel.

author: Yago
author_rank: Frequent Contributor
timestamp: March 26, 2014, 08:25:06 pm
content: I didn't think the mics mentioned went down to DC.No Volts generated without the coil or plate moving?How about something like this?http://www.omega.co.uk/pptst/PXCPC.html

author: Fraser
author_rank: Super Contributor
timestamp: March 26, 2014, 08:32:34 pm
content: Is it possible to provide a little more detail of the test that is being carried out? Members of this forum may have different approaches to offer ?

author: oldway
author_rank: Super Contributor
timestamp: March 26, 2014, 08:40:28 pm
content: Look for "pressure transducers" on Farnell....there are some for low pressure H²O. (Honeywell and Amphenol)

author: Fraser
author_rank: Super Contributor
timestamp: March 26, 2014, 08:43:40 pm
content: Another thought...... have you considered a probe tube with passive diaphragm driving it to trasfer pressure changes to an external sensing element ?

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 08:48:24 pm
content: The experiment deals with erosion control. My group is studying hydrodynamic forces incident on small features of models.You got your lasers and your doppler systems that can give flow velocity. But the incident pressure is hard to determine. And that's where I'd like to instrument these models with pressure sensors.Drilling pressure taps and attaching them to a bank of large (a few inches wide) pressure sensors is what is in place now. Basically the same way you'd think of looking at pressure on an airfoil in a wind tunnel. But here we have this nasty "water" stuff to deal with. And it is a real pain in the butt to flush these pressure taps between experiments. It can take hours... and one stray bubble can ruin a set up. Not to mention the un-calibratable change in frequency response the fairly long tube (5-10 inches) of water adds. And have you ever tried to drill around a curve? I could go on and on...So enter the local sensor. These microphones are just about the only thing I've found that 1) measures pressure and 2) is small enough.I've looked at these DIP-8 sensors that honeywell, et al make (barometers, HVAC sensors, etc). But they are just too big. I've even dissected a few and tried to rip out the chip that's housed inside... fun times were had by none.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 08:50:22 pm
content: Quote from: Aurora on March 26, 2014, 08:43:40 pmIf its a water / water diaphragm we're basically in the same boat we were before. If its a water / air diaphragm the calibration of the change in material impedance is an issue.I can calibrate the sensors in another machine, but I cant calibrate a long loop.

author: Fraser
author_rank: Super Contributor
timestamp: March 26, 2014, 08:58:55 pm
content: Thank you for the full and interesting description. It helped me understand your situation. Sorry I have not got experience in this area. I only operate in air environments and not fluid.

author: oldway
author_rank: Super Contributor
timestamp: March 26, 2014, 09:02:32 pm
content: http://www.ge-mcs.com/download/pressure-mems/920-303B_LR.pdf

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 09:06:57 pm
content: Quote from: oldway on March 26, 2014, 09:02:32 pmTotally.... but its too big. The microphones I'm using could almost fit completely inside of just the pressure on that bad boy.

author: oldway
author_rank: Super Contributor
timestamp: March 26, 2014, 09:19:42 pm
content: Are you sure ?Diameter of the tube of this sensor is 0.127 inch or 3.23 mm.You have show pictures of microphone of 4.5 mm of diameter.How deep will the pressure sensor stay?

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 09:27:06 pm
content: Let me back up for a moment. The over-all size is important too. Not just the frontal area. The sensors have to fit 'inside' features that are on the order of 8mm. So this 15mm wide chip wont cut it, sadly. Thanks for the suggestion though.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 09:31:05 pm
content: Quote from: Yago on March 26, 2014, 08:25:06 pmSorry, I missed this reply from before....The sensor is just too big.... 9.9mm to fit into an 8mm space...Electret microphones change their nominal impedance with respect to DC offset. So there's a way you can back-out the DC component if you interrogate it correctly.

author: oldway
author_rank: Super Contributor
timestamp: March 26, 2014, 09:46:49 pm
content: http://sensing.honeywell.com/index.php/ci_id/142179/la_id/1/document/1/re_id/0

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 26, 2014, 11:19:14 pm
content: Piezo discs with the ceramic side painted after soldering might work.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 11:27:21 pm
content: Quote from: NiHaoMike on March 26, 2014, 11:19:14 pmI looked at PVD film. You can get close to DC with some clever guarding and DSP... but not DC. Unless I'm mistaken?

author: qno
author_rank: Frequent Contributor
timestamp: March 27, 2014, 07:18:39 am
content: You can try a MPXV5004DP.If the environment temperature is reasonably constant you can see 100 Pa.This is a $10 sensor.There are better ones but far more expensive, $100 buy's you a Sensirion.SDP1000-L025 -62 to 62 Pa -/+ 0.25" H2O 0.5% FS / 1.5% m.v. linearSDP1000-L05 0 to 125 Pa 0.5" H2O 0.2% FS / 1.5% m.v. linearSDP1000-L 0 to 500 Pa + 2" H2O 0.1% FS / 1.0% m.v. linearSDP1000-R 0 to 500 Pa + 2" H2O 0.2% FS / 1.0% m.v. root squareSDP2000-L 0 to 3500 Pa +14" H2O 0.1% FS / 1.0% m.v. linear

author: envisionelec
author_rank: Frequent Contributor
timestamp: March 28, 2014, 03:32:56 am
content: PUI makes some waterproof microphones tested to IP57.POW-2242P-C3310-B-RDigi-Key Part Number 668-1315-NDDisclaimer: I work for PUI. ** Edited, removed incorrect technical content.**

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 28, 2014, 04:13:38 pm
content: Quote from: envisionelec on March 28, 2014, 03:32:56 amOh wow. I'm glad you work for them. Time to get some grade A technical support.I tried two of the closely related (?) POW-1644L in my initial experiment:I was really disappointed they didn't work because everything else about them is perfect for my application.Now I just took the mics and dropped them in a plastic cup of distilled water. But you're saying the rubber shroud seals the *back* of the mic. Like as if it were going to be placed tightly up against a PCB board?Now if I epoxied or silicone sealed the back of the unit. Would that make it waterproof(ish) if I wanted to use it with wire leads?

author: envisionelec
author_rank: Frequent Contributor
timestamp: March 29, 2014, 04:43:17 am
content: Quote from: CaptnYellowShirt on March 28, 2014, 04:13:38 pmThe waterproof (designated by a 'W' as the third letter in the part number for our viewers playing along at home) is designed to be fully dunked, rear exposed and all for 30 minutes at 1m per the x7 in IP57. However (and as I learned after some internal discussions) our process of sealing the rear is unique and supposed to eliminate leakage completely. We're a bit surprised you're seeing a performance drop if the issue is indeed water penetration as both the Goretex 'felt' screen and the rear seal should prevent water ingress.I would like to know more about your application. Please send an email with as many details as you care to share, including photos, to 'hammett(at)puiaudio(dot)com' and we'll tackle this issue head-on.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 29, 2014, 04:56:24 am
content: Can do. Thanks for your help.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: March 29, 2014, 08:59:52 am
content: In case that doesn't work out for some reason, I think carbon mics or fiber optic mics might be easy enough to waterproof. I'm pretty sure they can both respond to DC as well. A carbon mic might be harder to miniaturize, though.Wikipedia describes them.If you want to go in a very different direction, you could try using fine-pitch bubble wrap as the transducer, with the volume of each bubble measured by an external (i.e. dry) optical device.

author: Fraser
author_rank: Super Contributor
timestamp: March 29, 2014, 09:54:59 am
content: The input from envisionelec just goes to prove how useful this forum is in connecting clever people with those with expertise in a particular area who can assist them....... well done PUI (envisionelec)

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: June 04, 2014, 04:40:36 pm
content: After several weeks of looking for a truly waterproof* subminiature microphone, I've concluded they don't really exist on the market. Many microphones exhibit some level of water resistance -- ones of minutes to hundreds of hours. However, for my application, I need a microphone that can resist water on the time scale of weeks to months.For others who are interested, I've developed a method to waterproof these subminiature microphones. My tests have used the cheapest generic brands that DigiKey and Newark sell (e.g. $0.50/ea) and have achieved resistance to water, lightly salt water, IPA (20%), and soapy water.Contact me for details.* Here I mean IP68, not IP67, when using the term waterproof (http://en.wikipedia.org/wiki/IP_Code). Personally, I'd consider IP67 (30mins) to be water resistant.

author: envisionelec
author_rank: Frequent Contributor
timestamp: June 04, 2014, 04:56:02 pm
content: We can make an IP68 mic, but it's custom so you'll be looking at tooling costs for the felt and rubber boot. All things are possible...for a price.Just putting that out there in case somebody comes across this thread and feels there isn't truly a solution to the issue.

author: retrolefty
author_rank: Super Contributor
timestamp: June 04, 2014, 05:05:29 pm
content: Maybe hyrophones would be more appropriate?https://www.google.com/search?q=hydrophone+frequency+range&rlz=1C1CHMO_enUS581US581&espv=2&tbm=isch&tbo=u&source=univ&sa=X&ei=ZlGPU4a0GdCiqAaVsILoBQ&ved=0CDQQsAQ&biw=1686&bih=900

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: June 05, 2014, 03:07:31 am
content: Quote from: retrolefty on June 04, 2014, 05:05:29 pmSub-miniature? 4mm or less?

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: June 10, 2014, 04:27:52 am
content: Quote from: subolg123 on June 07, 2014, 05:21:49 amWill you design either for under $5/ea?

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 10, 2014, 04:58:58 am
content: Microphones are inherently designed to NOT be sensitive down to DC! No conventional electret microphone, waterproof or not, will measure the steady-state pressure. Perhaps a review of a couple of the basic fundamentals of microphones videos would be in order here.You need a pressure transducer. Something that is designed to work down to DC.For example: Freescale MPX4115 series transducers.http://www.freescale.com/files/sensors/doc/data_sheet/MPX4115.pdfI have seen examples of using these things with a flexible plastic tube attached and going down into the water (or whatever liquid) to measure the pressure at the end of the tube. That way the transducer itself is never exposed to the liquid.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: June 10, 2014, 10:16:43 pm
content: I've been obtaining good results by modulating the current to the microphone -- on / off / on / off.You can't resolve a typical spectrum from these types of measurements, as you're blanking out a good portion of your time history in periodic fashion. However, it does appear to remove the RC decay one would typically see when applying a static pressure to a microphone.They didn't cover this method in those 'basic' videos you suggested.But hey, if you're an expert, I'd love to have you weigh in on why this may be working.And as far as the tubes are concerned. The water/air/transducer interface that you're suggesting is horrible for measurement integrity and stability. You really need a 100% water/transducer interface that's been completely purged of air bubbles for good data.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 11, 2014, 01:27:04 am
content: Quote from: CaptnYellowShirt on June 10, 2014, 10:16:43 pmCongratulations, you have re-invented "chopping"!That was a technique used back a generation or two ago to handle DC values with circuits that otherwise worked only with AC signals. By turning the microphone on and off, and as long as you can "calibrate" the results, then you can use that method as a crude pressure detector. You may get even better results sealing the back of the electret capsule (as with RTV from a squeeze-tube) .QuoteMr. Ford mentioned putting a small hole in the rear of the capsule to equalize the pressure. He didn't make a big deal out of it, so it was probably easy to miss.. By sealing up this hole, the capsule becomes more sensitive to absolute pressure. However, the diaphragm itself is still capacitively coupled to the internal amplifier, so it is not capable of sensing a steady-state pressure.QuoteIt works in many cases. But not all cases. Since there are dozens of factors you didn't reveal here, we would have no way of knowing whether a specific solution would work in your (undefined) situation.