Dew Prevention From the Inside?

[Carbon Brush]

Last night the rapid drop in temperature and clear sky, coupled with rain earlier in the day, meant it was a 'dewy' night.

Unusually I suffered dewing on a finderscope. Or more accurately IN the finderscope. Inside the objective.

My usual dew removal is the 'Vidal Sassoon' quick blast on the glass. But of course it takes a long time for the heat to work through to the inside glass.

The idea of a tangle of wires to feed heaters on the finder doesn't appeal. So I wondered about filling my finder with a dry gas. In the garage I have a cylinder of argon for welding. From what I remember of BOC specifying the contents, water vapour is not mentioned. So perhaps a small tapped hole near each end of the tube and let the argon run through for a minute or two before closing the holes?

Daft idea?

Anyone done this? Anyone done anything simpler?

[rfdesigner]

Get a dessicant bag and fix it to the inside of the tube. (out of the light path obviosly)

Dessicant can be recharged by putting it in the oven at just above boiling point for several hours. In a sealed unit like a finerscope I think I'd want to recharge it annually.

I have a dry box for my DSLR/guider/OAG etc. and have a large dessicant bag and humidity indicator in it. That bag last a week but the box gets opened about 3 times a week.

Derek

[steelfixer]

I think you will find if you run Argon through it you will cause a pressure difference inside the viewfinder as the Argon will dissapate the air. This could cause you even greater problems.

[Carbon Brush]

Thanks for your comments.

Steelfixer.

I was thinking of two holes, at distant points of the tube. That way most of the (wet) air gets displaced. Running the gas at a couple of litres/minute means there is no danger of pressurising the tube.

The benefit of the 'argon' approach is that I have to dismantle the finder once to drill/tap for the screws. After this, I only have to remove the screws and sealing wshers to change the gas.

I realise that the finder is not perfectly sealed. Therefore as the air pressure changes (980-1040mb) there will be some gas exchange with the outside. So allowing moist air to enter. This is in addition to the temperature expansion/contraction volume changes.

RFdesigner.

A nice simple approach and the best choice to anyone who does not have a source of dry gas. I just have to locate a suitable place in the tube and make a small bracket to hold it the bag in place. The disadvantage of this method is that it involves dismantling the finder annually, or whenever dewing becomes a problem.

Again, many thanks for your comments.

[steelfixer]

David - go for it. It will be interesting to know how you get on with it.

The other advantage of this is if you do get problems you only have to open the seal and you are back to square one.

[dmahon]

Is the refraction at a glass/air interface different to that at s glass/argon interface?

[AndyH]

Could you seal either end of the tube with rubber 'o' rings then pump in nitrogen through a valve in the body?

Some top end scopes n eyepieces are nitrogen(I think) purged.

Andy.

[steelfixer]

Nitrogen would be my preferred choice as well, trouble is bottles of pure Nitrogen come with a very expensive price tag.

[AndyH]

Ahhh. Nuff said:)

Andy.

[m.tweedy]

go down the easy route and make a small dew shield from thin camping mat. It is easy to attach with an elastic band. Mine has never needed either a dew heater or blasts of warm air since.

[m.tweedy]

I have some spare and if you want to PM me I will send it to you FOC (with a free elastic band).

[Carbon Brush]

Thanks everyone.

For gas filling, nitrogen is a good low cost choice being 78% of the air. As I only have access to argon, that was the reason for my suggestion. Refractive index of argon and nitrogen are virtually the same as air at visible wavelengths. So I won't have to make spacers, or cut down the tube to focus.

As for refractive index variation with wavelength. Not an issue from the sources I have checked. Especially as this is a low magnification finder.

Until I take the tube apart, I won't really know how well it is sealed. I am though hoping for a good seal. The grease around the focusser will be the largest potential leak. Once filled, the tube pressure will not be a lot above or below atmosphere on any given day, so there is no requirement for venting. The better the seal, the longer the argon will stay in there.

This particular finder, like many doesn't have a long dew shield. Thanks for the camping mat offer. I did consider this type of approach. But as the condensation was more troublesome inside, I thought I ought to address the tube contents first.

Probably this weekend I will take the finder off and dismantle. So you have notice of a clear sky while I have my kit in bits!

[dmahon]

Nitrogen would be my preferred choice as well, trouble is bottles of pure Nitrogen come with a very expensive price tag.

Tyres are routinely filled with it these days aren't hey?

[steelfixer]

Tyres are routinely filled with it these days aren't hey?

interesting point i suppose you take it to a garage and ask them to 'fill her up please'

[Grunthos]

make a small dew shield from thin camping mat.

Where can one buy a (black) camping mat at this time of year?

Can't seem to source one anywhere (and get some odd looks when asking for one during the christmas shopping madness)

[m.tweedy]

Have you tried go outdoors

[Carbon Brush]

Dry nitrogen is used for tyre fillling. I read somewhere there are several reasons. Keeping water and oxygen out means no wheel rim rusting and rubber degradation.

Thinking about old cars I have owned, any rims that have been rusty inside have been down to proper water ingress with deflated tyres, rather than water in the inflation air.

There is also apparently less pressure loss over time as air molecules diffuse through the rubber. Nitrogen atoms I think bind in 3s so are quite large compared to oxygen in 2s. Presumably if this was a major issue, over time, your air filled tyres would lose their oxygen and gradually become nitrogen filled anyway.

As I understand things, when BOC and the like produce gases, they usually start by refridgerating air until it is liquid. Then it is allowed to warm. As it warms the different gases nitrogen, oxygen, carbon dioxide, argon, etc boil off at different temperatures and are captured that way. They have to find a use for the nitrogen which comprises 78% of the raw material.

So perhaps there is a bit of sales hype in nitrogen filling car tyres?

Or am I just beinbg cynical?

[steelfixer]

Nitrogen was originaly use in tyres for racing.

Less mass (no water vapour) so less centrifugal force generated, less pressure difference as the tyre heats up ect.

The use of it in general day to day motoring is pretty much a sales hype and serves no useful purpose just more expense.

As most wheels are Aluminium now a days the rust issues do not exsist.

[rfdesigner]

Nitrogen was originaly use in tyres for racing.

Less mass (no water vapour) so less centrifugal force generated, less pressure difference as the tyre heats up ect.

The use of it in general day to day motoring is pretty much a sales hype and serves no useful purpose just more expense.

As most wheels are Aluminium now a days the rust issues do not exsist.

Bang on.

First unsed in the aero industry to cope with the staggeringly severe conditions (-50C to +40C in half an hour or less, then several tons load possibly as fast as 200mph with heavy braking).. getting water out of the equation makes a noticable improvement and reduces blow outs.

With motor racing they run everything at tight as they can, a 0.01% improvement in consistancy due to nitrogen is worth doing when hundredths of a second mean poll position or second row of the grid.

For road use there is no point unless you expect to push your tyres up and down past freezing point regularly, so Canadians with heated garages might concider it, nobody else need bother.

Derek