Weather > Weather Hardware/Measurement
Using a PC case fan to aspirate your temperature/humidity probe.
elagache:
Dear WeatherCat weather station ?mods? fans,
We live in a world filled with thermometers. Alas, anyone who has put some effort into measuring the temperature accurately knows this is extremely hard to do. Top of the line stations like the Davis FARS line improve the measurement of the air temperature by actually blowing air over the temperature probe with a fan. Doing this in a self-contained solar powered unit is something of a feat and Davis makes you pay for this luxury. Worse still, the Davis unit will wear out eventually so that owners can expect to buy new batteries and/or motors down the line. Those of us without this luxury are out in the cold (or heat.) That is until we take the matter into our own hands and attempt to modify our station to improve the performance of our instruments.
Some of the guys on the WX-Forum have been using PC case fans to aspirate their temperature/humidity sensors. This write up is how I incorporated a 3? PC case fan into my Davis temperature/humidity probe. While this is a very specific application, these ideas can be used to modify a wide range of makes and models of weather stations. It is a scheme well worth considering to improve the temperature accuracy of your weather station.
The story starts back in February of 2014. As part of moving my station transmitter board from my rain gauge to the temperature/humidity sensor under our large deck, I purchased the Davis daytime only fan aspiration kit:
http://www.davisnet.com/weather/products/weather_product.asp?pnum=07747
Since there is no practical way to mount a solar panel under the deck, I planned to power my station transmitter board with AC power. With that power available, I could easily power the Davis fan using a universal AC adapter. Alas, as reported on WX-Forum, these fans do not last very long when powered at 3 volts. Within a few months, the fan motor had died.
Since I was aware of this, I had already planned to replace the motor. From other sources on the Internet, I was aware that it was no longer possible to get a quality replacement for the motors that Davis uses. So another arrangement was needed, and I had already decided to substitute a 3? PC case fan. My thinking was that having the Davis adapters would make the task easier. As it turns out, while it did make the job easier, there were other ways to incorporate the fan that would avoid this significant expense.
Back in February, I had done some research and purchased this particular fan model - the Noctua NF-A8 PWM:
http://www.noctua.at/main.php?show=productview&products_id=100&lng=en
It was more expensive, but I hoped that would make more durable in the more difficult semi-outdoor environment. I had also bought a multi-voltage AC to DC adapter:
http://www.vellemanusa.com/products/view/?country=us&lang=enu&id=350744
These devices are produced by many vendors and can be bought just about anywhere.
By in February, I had connected the Davis fan to the AC adapter using a 1/8? inline connector from Radio Shack:
This is how my station remained until the original Davis fan failed. Fast forward to this October and I really wanted to restore fan aspiration before the bad weather returned. It was time to take on the case fan ?mod.?
I took my temperature/humidity housing back apart and retrieved the portion that contained the Davis motor. This is what I was greeted with:
Clearly Davis simply packages the daytime only fan aspiration system in the same components that house the batteries for the 24/7 system. I was quite surprised at how difficult it was to attempt to extract the original motor. Replacing the original components would not be easy even if quality replacements existed.
Here is the PC case fan in the same space:
It seemed to me the easiest way to make this modification was to remove all the plastic of the lip that supported the original Davis fan assembly and secure my PC case fan using a metal plate.
So first I removed the lip. This turned out to be a bit tedious. I sawed off as much as I could. I then used a Dremel rotary tool to get close to base. Finally I carved the remaining material with a hobby knife. That left me with this:
At this point I put to work my ?secret weapon.? I?ve gotten skilled at creating templates using my favorite illustration software Canvas. In this case, it was fairly easy to make a template based on taking the dimensions of the various components. If the template is more difficult to make, there is a strategy using your camera that will help. Take a photograph of the region where the template will fit and include a ruler to provide a scale. You can then scale the photograph to actual size and then either use a computer or even draw by hand the template needed. If you want to make exactly the same modification as I have done, the template I used is available as a PDF file:
http://www.canebas.org/WeatherCat/Forum_support_documents/PC%20case%20fan%20bracket.pdf
A 1 inch scale is included so that you can confirm it has printed correctly.
One huge advantage of making templates like this is that you can make prototypes to confirm the fit. I routinely print out a template and glue it to posterboard using rubber cement:
In this case I was quite lucky the template fit with very little modification:
At this point I could now repeat the process using metal. I decided to use some 0.032? anodized aluminum sheet that I had successfully used on some projects for my trusty wagon. I glue the paper template to the metal with rubber cement - this works just fine for the brief period the paper needs to be there:
To cut the aluminum, I used aviator tin snips. They have a mechanical advantage so that it is closer to using scissors than cutting metal. First I cut out a broad region containing the template and then drilled the holes for mounting the case fan:
As you can see my holes departed from my original template. I was concerned that my template could have been a bit off on the holes and so I used the fan directly. Alas, the fan pivoted while I was attempting to get the hole positions. This could come back to haunt me.
To cut the hole for the air to blow through I did the old fashioned trick of drilling a few holes in a line so that I could start cutting. I then followed a spiral pattern until I reached the desired diameter:
Finally, I cut the outer shape of the template. The best way to get accuracy is to be willing to cut out small chunks rather than trying to cut shapes in one step. With a little filing I had a reasonably good fit:
I then glued the aluminum plate to the Davis part using 2-part epoxy:
I used the fan itself to transfer weight to the plate and insure a good contact between the plastic and the aluminum.
Another task that was required was to solder the case fan wires to the power source. This can be confusing because modern case fans have 4 wires so that they can be controlled by the computer. It turns out that the black wire corresponds to ground and the yellow wire should be at +12 Volts DC. For now I just soldered the wires directly:
I then just applied old-fashioned electrical tape:
If these fans can last 5 years or more, that might be good enough. It isn?t that difficult to redo the connection if it is that infrequent a replacement. If the fans last only a year, then some sort of connector would be the way to go.
At last it was time to assemble everything. Here is how the gluing job turned out:
Mounting the fan on the plate was more of a chore than I wished because of the slip of the mounting holes. I had to file some of the screws because their was so little clearance in the Davis part. However, after those adjustments were made, the fan was secured and wires connected:
After that, I secured the temperature/humidity assembly underneath the fan assembly:
Finally, the entire assembly was mounted next to the station transmitter board. Here is the view with the board visible:
Here is what the area under the deck looks like with the transmitter housing closed:
Overall this has been a real improvement over the original Davis fan. This fan is much quieter and the temperature/humidity probe seems much more responsive to changes in temperature.
This project may seem limited to a very unique application, but there is no reason to limit yourself to AC to DC adaptors and without that limitation then many more options become possible. For example, you could buy the following 12 Volt 5 Watt solar array and use that to power your PC case fan:
http://www.amazon.com/gp/product/B00QRHDIPY/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687642&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B006Q7C7L6&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1E6BFRD4Y8H6K0M3KF1T
That would provide daytime aspiration at a modest cost and it is during the daytime that a fan is most beneficial. With a little more digging around, there might a 12 volt solar system out there with a rechargeable battery at a moderate cost. That would make it possible to create a 24/7 case fan station aspiration system for any make and model of weather station and at any location with sun exposure.
In retrospect, I wasn?t as bold as I could have been. It would have actually been easier to create a circular aluminum panel that could have been sandwiched between the plates of the radiation shield and supported by spacers and longer mounting screws. That approach could be tried for any weather station where it is possible to dismantle the radiation shield. The solar panel need not be located nearby the temperature sensor, so there is a lot of flexibility in creating a fan aspirated temperature system for any weather station.
So I hope this write up has given a number of WeatherCat station caregivers some fresh inspiration on what they can do with their station. It isn?t only classic cars that can get ?mods!?
Cheers, Edouard
wurzelmac:
Cool description, you will make it to ifixit.com!
Two weeks too late for me, I replaced the Davis fan with ... a new Davis fan. :-[
Cheers,
Reinhard
SeanA:
As solutions go that is top... [tup]
Blicj11:
Fascinating and thorough description accompanied with easy to understand photos! Nice job MacGyver.
WCDev:
Impressive work!
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