Tag Archives: FARS

Davis FARS Upgrade to a Monitored PC Fan

Link to original Davis Fan Motor change

When the original Davis FARS motor failed I installed a new motor and kept back some spare motors, waiting for the inevitable motor failure to occur, reading online, changing from the Davis Fan to a PC varient made a lot of sense and not only is the life of motor excellent, but it is possible to monitor the motors output for operation.

I was interested to know the existing Davis fans air flow, mine was running at 2.4vDC and showed 9.7m/s:

The PC fan I bought was a Noctura NF-R8 redux -1800 PWM for £9.99 of eBay.

With the David FARS removed, the existing fan assembly slides out of the housing as a complete fan & surround, the hole left will take the 80 x 80mm fan with only a very minor filing of the fan case body to make it slide into the body of the FARS.

No modifications are made to the existing FARS body allowing reversion back to the existing fan if I wish (can’t think why, but you never know!).

To form a seal around the gaps, I used self adhesive door/window strip seal.

The finished fan after the seal is applied and checking that the fan is sucking, rather than blowing:

The oringinal fan ran on 2.4v, I replaced the voltage regulators fixed value resistor for the correct one to give 11.54vDC output and this feeds the motor and the motor monitoring Tacho.

The air flow of the PC fan when installed and connected was 5.5m/s, this is less than the original davis fan, but this is still better than static air within the sensing chamber.

The Davis weather station I have is the wired version, and I used Cat5e cable rather than the supplied 4 core cable from the ISS to the console, using the Cat5e unused cores, I fed the supply voltage to the fan and the Tacho pulse into the house where they are connected to a 52mm (2″) counter tachometer guage RPM, this was bought off eBay for £6.70 + £1.69 postage.

Tacho installed in the equipment cabinet:

The PC fan connections are:

  • Black – Negative 12v
  • Yellow – Positive 12v
  • Green – Tacho Pulse
  • Blue – PWM (Not Used)

The Tacho is set for a 4 cyclinder engine using a switch on the back of the unit, with the fan running the backlit RPM display is showing just under 1000RPM (reading slightly low due to voltage drop introducd by the distance from the ISS to the end point), to the left of the Tacho is my Meteobridge Pro  weather station server to the internet.

The motor is guaranteed for 6 years, replacement will be very easy and I’m able to remotley monitor that the fan is functioning, all for less than £20.

 

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Davis Vantage Pro2 Fan Change

I check and clean my weather station twice a year, just before Christmas I noticed the  FARS (Fan Aspirated Radiation Shieldfan which draws air past the external temperature and humidity sensor had stopped working, (write up from a previous blog on how I installed it is here –FARS Link).

As an interim measure I increased the voltage to the fan from 1.7v to 2.8v, which kept the motor working for a few more weeks before it finally wouldn’t kick into life.

In February 2014 I had bought 5 spare motors (minimum order but cheap as chips) from Jameco.com, so today I decided the weather wasn’t going to catch me out  and I made a start on replacing the fan.

Disassembly of the unit is very easy, removing the Rain Bucket exposes three cross head screws which hold the FARS in place, whilst holding the base of the FARS, undo each of the screws until you have the weight of the FARS in your hand, slowly lowering your hand the weight of the unit will be held by the interconnecting wires to the ISS.

The power to the fan is via a small plug and once removed, the fan assembly slides out of the shield as one unit.

The fan is held onto the motor by an interference fit, pulling on the fan will remove it from the motors shaft.

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The motor is held in the fan body by silicon, I used a craft drill with a tapered bit to remove this.

2016-01-23 09.41.35It didn’t take too long to get the motor out, the next job was to tidy the fan body socket and de-solder the old fan motor wires.

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New motor from Jameco compared to the one removed just in case I had ordered the wrong one!

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The motor needs packing to center it in the fan body, jerryg on the WXForum gave his solution of using insulating tape to build the diameter and this works really well, I made it so the motor was a snug, but not tight fit (as the motor uses brushes to make electrical contact to the rotating commutator, the motor will inevitably fail and need replacement).

The motor has three holes at the shaft end of the body which in the the original motor had been taped over, so I did the same, pushing the motor in the body I used a few dabs of silicon to hold it in place and pushed on the, the last part of this job was to solder wires on making sure the red wire went to the terminal marked with a + sign!

Once connections are made, a cable tie holds the wires neatly to fan body.

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While everything was apart, I checked the voltage regulator to the fan to make sure everything was ok, this is a Velleman unit with 24vAC in and I have removed the potentiometer and replaced it with a fixed value resistor giving a stable output of 2.8v DC.

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After a washing  down with soapy water the tipping bucket, Stevenson screen and rain bucket, everything was put back together, the total time taken was about an hour, now I know what I need to do, this could be done in under 30 minutes.

If you prefer to simply change the complete fan unit (Davis 7758), this can be bought from Weatherspares  in the UK who offer exceptional customer service.

Just got to see how long this motor lasts, once all the motors I’ve bought are gone, I’ll put a PC fan motor in and monitor the output to generate an alarm, I have made a a circuit based on this site, but I need to play a bit more to get it to work properly.

Link to original FARS blog..

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