Friday, December 26, 2014

No Pressure - I Got This

Earlier this year, My Lovely Wife and I enjoyed a visit from My Evil Brother-In-Law, who also just so happens to work for Environment Canada.  Naturally he was interested in my Davis Weather Station and some of the hacking I've done on it.  But he was more interested in its barometric pressure reading of somewhere around mid-800 kPa.  That is a really low number that could mean only one of two things:
  1. My barometer was not working.
  2. We were in the eye of a hurricane.
The weather seemed pretty nice out, but I know from what little TV I watch that the weather can be deceptively nice in the eye of a hurricane, only to worsen when the eye moves on and you're subjected to the hurricane's nastier bits.  So we waited a while and the weather stubbornly continued to be nice.  Broken barometer it was.  Later my suspicions were confirmed when the barometric pressure would just show as a series of dashes.  No need to take cover under the stairs, for sure.
Broken Barometer Badness
Fast forward a little bit and My Evil Brother-In-Law decides to take up the invitation from My Lovely Wife and myself to visit us for Christmas.  My barometer is still broken, but we've offered our invitation a couple months in advance.  I've got time.  I would fix my barometer before the weatherman's arrival.

The first part of the process was figuring out how the hell I was going to do this.  Everything else on the station was working well, so I figured it was just a dead sensor.  But exactly what sensor are we talking about?  I figured this much out a while ago and documented it in this post.
The part is a Measurement Specialties 5534CM.  Applications include "Weather control systems" according to its product page (man, I'd love to be able to control the weather). Here is its product page, here is where you can buy it, and here is some Arduino code to drive it!
The cool thing about this part is that it has a digital interface rather than an analog one I had originally guessed at.  It also is factory calibrated.  Davis was smart and kept things simple: the amount of A/D they have to do in the console is exactly zero.  But there seems to be a price to pay for this simplicity: $27.27 for a single piece, to be exact.  Good thing they are cheaper by the 1,000.
But there was a problem.  This picture was taken in my old console that has since failed me.  I got a new one and U84 above is not populated.  U85 is.
What is U85?
Good question.  What is U85???  I figured after a bit of research that it must be a minified version of the 5534CM: the 5540CM.  Here is where you can buy it.
You can, but...
Note I said you "can buy it" there, but I didn't say you "should buy it" there.  The price of this little bastard is $32 plus shipping that was going to put me out over $40 total.  Looking good in front of My Evil Brother-In-Law was one thing, but $40 is another.  What to do?  Well, Digikey is a well known name in the electronics components biz, but so is Shenzhen Kesun Electronics Co., Ltd.  Actually, no they aren't.  But their price and their shipping was about a third that of Digikey's.  Here is where you should buy it.  Take my money!  All $12 of it.
The Price is Definitely Right
The only price to be paid for not paying much of a price is time: I ordered this on November 12th and got it December 19th.  It was obviously on the slowest of the slow boats from China, but I do feel Canada Post was at least somewhat to blame.  The good folks at Shenzhen Kesun Electronics Co., Ltd. shipped within two days of my order and gave me a tracking number to boot.  However, that tracking number was never updated again by Canada Post.  Even after I received it, the order showed that it was still inbound from China.  I think my little sensor somehow got lost in the Christmas parcel rush.

So I had the What part of the equation covered.  Equally important is the How.  How do you get the old part off and the new one on?  Any time I had ever tried to take a multi-leaded part off a board like this with just a soldering iron and desoldering wick (a fine copper braid that soaks up solder like a sponge), it always ended in heartache.  You never get all of the solder, and you can't get enough heat around all the part at once to keep the solder molten to have the whole thing come off.  In the end, I would end up just wrecking what I was trying to remove.

That's why I got me a hot air rework station.  Which one?  This one.
Dave Jones Has Many Fine Traits.  Brevity is not One of Them 

Actually, mine is a Yihua 858D.  Same difference.  There are a lot of combo units that combine a soldering station with hot air rework, but I already have a decent iron.  This unit is small, it is quiet, the temperature is well regulated, it comes up to temperature very fast, and it has a neat cooldown function.  Best of all it is cheap.  Better of all, it works great.  I'm not sure how I lived without it.  I got it from here for $79.99 with Free Shipping and a spare heater element.  Nice.  This is my new favorite toy: you can take my hot air gun from my cold dead hands.

I got the What.  I got the How.  The Who is me.  The Where is here.  The When is now.  I got a two week Christmas holiday.  Time to get busy.  Open the case.  Set up on something that could take an accidental blast of heat (in this case, my stove), and put some aluminum foil around the surrounding parts to keep the heat off them to prevent their inadvertant removal.
Preparing for Battle
I set my trusty hot air gun to 340°F and held the nozzle just over the part.  The solder melted after maybe ten seconds and the part was off.  No fuss, no muss.  I freaking love this thing.

The next part of the operation did not go smoothly.  At all.  The plan was to use my soldering iron to solder the new part into place.  I took the new sensor out of its package, applied a bit of flux to the circuit board with a flux pen (always use flux when soldering), and placed the part on the board.  The first problem I had was the part did not want to line up well on the board: the part had some solder on each of its eight pads on its underside.  The circuit board also had a bit of solder on the eight copper pads receiving the part.  Placing the part on the board invariably caused the part to slip out of place.  My first attempts to solder the part in place were met with total failure: the part would be misaligned, I'd get a solder bridge, I'd get a missing connection... and all of my attempts had the part seemingly sitting on top of the board rather than being fastened to it.

A good night's sleep made the answer clear.  As so many instruction manuals say, installation is the reverse of removal.  I was using my soldering iron to attach the new part when I should really have been using the hot air station.  I had been leery to try this because I thought the heat might damage the new part.  But thinking about it some more, these boards are usually built using some kind of wave soldering machine that basically heats the whole board with all parts attached in an oven brought to temperatures above that of the solder's melting point.  That is basically what the hot air station does.

So, I laid down the aluminum foil in the area again and hand placed the part as best I could after having put a little liquid flux in the area.  I set the fan speed of the air gun about halfway and again set the temperature to 340°F.  I slowly lowered the nozzle of the gun directly over the part, trying hard not to blow the part off to one side.

What happened next was beautiful.  The solder around the part melted and the resulting surface tension moved the part perfectly into position.  The part also visibly pulled itself toward the circuit board and was no longer just kind of sitting on top of it.  Huzzah!

I let the area cool down a bit and then used a Q-tip and some isopropyl alchohol to clean the residual flux away.  I put it all together and...
Victory Is Mine!!!
The console display is 1020 hPa, or 102.0 kPa, a value that checked out nicely with the value reported from the nearest airport.  Better yet, the repair passes muster with My Evil Brother-In-Law.  Now all I've got to do is keep him on a steady diet of good food and good booze for the rest of his visit.  It's all good.

And by the way, Merry Christmas and a Happy New Year from all of us here at Mad Scientist Labs.


  1. My VP2 ISS is over 10 yrs. old and has already needed a couple of things on it replaced. This post got me to thinking about future sensor failures. Since both the old and new boards had both U84 and U85 pads, wouldn't that seem to indicate that the new sensor would also work on an old board ?


    1. You would certainly think so. My understanding is that the two sensors are software compatible. Someone would have to try it first to know for sure :-)

    2. Im not sure if that last post when through.
      Im wondering if you would be interested in using your hot gun to replace my Baro sensor. I ordered the sensor you had above (yes it WAS on a slow boat from china) I cant really justify buying a hot gun even as cool as it sounds. I could take the board out and send it and the sensor if your interested.

      Please email me at and let me know.

      Thanks, Tony

    3. Tony, I'd feel pretty bad if I said yes and then ended up wrecking your console. Consider this an opportunity to convince yourself that that hot air gun is worth it!!!

  2. OK I had the same Faulty Baro so went out and bought a Hot air Air / solder station from Ali Express (YIHUA 995D+ LCD Rework Station With Hot Air Gun,soldering station, ordered the part recommended (The new smaller one) and installed it, now works perfectly as I had never tried this before I did practice on an old motherboard first. Davis Service was not great they just said "Buy a new board we can,t repair them ...." Idiots.


    Steve, NZ

  3. PS: I used 347F for the Baro reinstall. when I was practicing on the PC motherboard it was taking 750 F to remove items within 10-15 secs but when I removed the faulty Baro at that temp it fried it so backed it down to just over what DeKay posted and it worked fine, my advice if you decide to get your own station is have a good practice on old Boards of different types before the real install. Trust me. I bought to Sensors and did not encounter any problems so have a spare now as well.

  4. Oh yea and use flux (I had a flux Pen) and have solder wick handy ....... :-)