speedo and tacho face colours ?

While we're on the subject of speedo and tacho dials, does anyone else's gauges fog up when riding?

Both of mine fog up on the inside of the glass in cool weather. It only happens while the bike is moving. When I stop, it clears itself in a couple of minutes. When I get moving again, it slowly fogs up again and stays fogged as long as the bike is moving. The colder the temperature, the more fog. It got that bad today that I could barely see the speedo. The tacho isn't so bad.

I know a bit about psychrometrics but it's got me stumped as to what's going on here.

Cam
 
I can't help with what is right for an early bike but hopefully picture below will show the difference between the green and blue faces as we all seem to view colours differently. (at least this variation)
Green speedo is a speedo I bought from Wolfgang some time ago as mine hadn't worked since about 1994. Obviously from an earlier model however don't know how early (220km? - not sure when changed to 250km). Blue face clock back on bike is original from my bike which I have just had Otto's refurbish. Note the Tacho has not been worked on and still has very similar blue face. This is on a '80 Jota ('81 model)
Clocks004.jpg


Edit - Meant to say, the clocks on the bike do not look as blue as in the photo. Without the green one around probably even think they were a dark green........
 
Sod it I will have green and risk being attacked by the rivet counters  :D Not as easy these clocks as the Smiths ones I do They are mostly just black and white. They never seem to fade you would think they would go grey over a period of time.
 
Cam, the moisture trapped inside your instruments probably get a higher wind chill when riding, and so start to fog up as the moisture condenses out.
When you stop, the temperature increases and the condensation disappears from the glass.
Maybe a sort of dew-point thing going on?
 
G'day Grant

A common misconception there. Wind chill factor only affects wet or moist surfaces, like human skin or a wet bulb thermometer. So unless it's raining and the instruments are wet, the temperature of the speedo/tacho glass cannot fall below ambient temperature.

The only explanation I can think of is increased pressure inside the instruments from air ramming into the backs of them and getting in through various openings (wire, globe and trip meter knob penetrations). So the total pressure inside the instrument has a velocity pressure component (as well as static pressure). Something like a pitot tube airspeed indicator on an aircraft. An increase in pressure also increases the saturation temperature. If the saturation temperature is pushed towards ambient it will causes water to condense out of the air. That's why air compressors need condensate drains and aircraft pitot tubes need de-icing heaters.

If the moisture in the air is close to condensing anyway (cold day) then the increase in pressure might be enough to create the observed effect. A lot of "if's" there.

Maybe I should test the theory by shielding the backs of the instruments from the wind.

Cam
 
Hi Cam,

I'll have a stab at your issue.

I think your velocity pressure actually DROPS the static pressure inside the instrument while on the move. When you stop the pressure returns to normal - so moisture 'disappears'.
We know that cold air cannot hold moisture as well as warm air, so this is why it's worse in cold weather.
Some compressed air driers use chilled air to create dry 'instrument' grade air, the moisture is trapped in a vessel (in your case the instrument casing).

As an example if pressure suddenly drops then moisture in the atmosphere will condense out (vapour trail from plane wing upper surface - higher velocity thus lower pressure), also much like a condenser on a steam turbine exhaust, the condenser runs lower than atmospheric pressure to fully condense all of the low pressure steam to liquid.
 
Maybe I should fit a fairing to shelter the instruments from the wind.Only problem then is that it'll also shelter me from the wind so I'll ride faster, get booked for speeding, lose my license. No licence - no job. So I'll have to sell my house, become a homeless bum, and die from hypothermia while living in a cardboard box.
 
This is another example of a faded face plate. Although it's from a Honda, this is a ND clock which had the same original green colour as the Laverda clocks had. The edges are still green!

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