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Although there is very little difference in thermal conductivity at room temperature, it's just the opposite. At all temperatures moist air has a higher thermal conductivity than dry air.

10 inches barely rates as arid; hardly drier the Stevenage's 30 inches.

An average of 10 points for the Atacama is impressively dry.

Water vapor in the atmosphere displaces nitrogen and oxygen
Humid air is less dense .

atmosphere at higher relative humidity
impairs heat exchange efficiency
by reducing the rate of evaporation .

why people feel uncomfortable
when it's humid is related to the same phenomen that slows paint solvent evaporation .

I was under the impression that, in very simple terms, materials have similar thermal conductivity and electrical conductivity properties.

Most people would be happy holding a plugged in toaster in air. Most people wouldn't be happy under a mist of water. If moist air is more electrically conductive then why wouldn't it be more thermally conductive?

John

will an air cooled engine running at a stable temperature get hotter if moisture is added to the airstream?

Is the moisture in the form of Scotch mist?

Aye. Things are much clearer with the aid of a glass.

the reason im asking is because theres more things going on than i can keep straight in my head. if the thermal conductivity goes up with temperature in the range we re interested in, then heat transfer will presumably be more efficient in moist air

but the extra mosture in the air increases the overall heat in the airstream because of the greater heat capacity of the water. does the moist air absorb more heat, faster, as it gets hotter? if so, the motor will get cooler. or does the increased heat in the moister air slow the absorption of heat by the air from the fins and permit less heat to cross the boundary? or is there a crossover? i dont understand this stuff.

its like a radiator. even if warm water transfers heat faster than cold water, a cooling system running hot water wont work as well as one running cold water. what happens with dry air versus moist air?

the owners manual fo rmy old hudson recommended using alcolhol in the cooling system, but i dont think they had scotch mist in mind

As ambient air temperature goes up, the temperature difference between your hot engine and the air becomes less, so don’t run away with the (slightly mad) idea that hot air will cool your engine better than cold air.

Water vapour is over 1000 times less dense than liquid water. It does not have the heat conduction properties of liquid water. It is less dense than air. As already said above, in the atmosphere it is displacing air.

The figure in triton thrasher's post is from a magazine article, but the following one is from an engineering publication.



Although at least one of the figures has to be wrong, both show humidity only has a 1% effect even at 40 ℃, which would mean it's negligible in the context of painting, which is the context I care about at the moment. If someone wants to look deeper into this to figure out which one is correct (or, maybe both are wrong), be my guest.

Magneto man's illustration is incorrect

the sweat on your forehead evaporates off
at lower humidity
because lower humidity has better heat conductivity .
( the system is working so well you don't even know it's a system and working )
...( its hot , but its a dry heat )

The sweat on your forehead pools and drips down into your eyes ... at higher humidity
because the atmosphere is less thermally conductive
( it takes heat transfer for water to phase transition into water vapor )
... and heat transfer , in humid air , is happening at a lesser rate
You feel hotter because water vapor in the air hampers evaporative conductivity )
( its the same hot , but now a wet heat)
... humans experience humid air as being heavier ... but it is actually lighter .

The same phenomenon is happening with the solvent in paint .
Spraying aerosoled paint through dry air and some solvent will evaporate ... on the way to the Target .
at the worse ,
This can affect how the paint wets out and lays down . (the surface may orange peel or even prill little spheres of dry paint onto the surface )
some effects are started before the the paint even hits the target
and others are after the aerosol lands .
If the evaporation rate is too high the paint may not have enough time to wet out and level
and Humidity is a key factor in evaporation rates .

if it didn't matter
no one would ever say
"man it's humid today"
or " I'm moving to Arizona ... because it's less humid "

some of today's catalyzed paints even require humidity to properly cure .

Sweat evaporating to cool your fevered brow by latent heat of evaporation and conductivity of air are two different things.

If the atmosphere is saturated with water vapour, sweat will be reluctant to evaporate. Air will still conduct heat away.

I couldn't stop myself from looking into the thermal conductivity conflict. The magazine article that has the curve triton thrasher included in his post explains:

-------------------------
Some time ago, one of my colleagues (Nelis Mies, Philips Lighting) approached me with the same question, and I sent him an old graph proving more or less my standard answer. However, he was not completely satisfied and found software that could create the desired graphs1. The software employs kinetic theory describing a mixture of two gases; in this case, atmospheric air and water vapor. Figure 1 shows some interesting results.
[graph]
The strange shapes of the curves were rather surprising. With increasing moisture content the thermal conductivity decreases, contrary to what I expected.
-----------------------

As the above quote shows, the magazine writer included someone else's software-generated curve even though he, the writer, found the result "rather surprising."

Submerging myself deeper, I have access to the following recent article through my university's library (but you, too, can download a pdf of it from the publisher for a mere $45):



I highlighted an interesting section from the Conclusions:



Surprisingly, even though both air and water have been around for quite a while (unverified reports suggest maybe for even more than 500 years...), what we know about the thermal properties of humid air is based on theoretical models that have yet to be experimentally verified.

Unwanted moisture that might be added to paint would be "chemically bad," so the humidity inside the compressed air line has to be low enough that droplets can't form when the air emerges from the gun. However, even 100% relative humidity changes the thermal conductivity of air by at most 1%, one way or the other, in the temperature range where we paint so the humidity of air is "thermally irrelevant" to the task of painting.

Everyone I know, myself included, hates warm humid air, so now I'm really done with the topic.

Getting back on topic:

That would be a fine idea and, at ~5 cfm, the gas in one cylinder would paint quite a few motorcycles. However, at ~$250 for a filled cylinder and ~$225 for a pressure regulator, there wouldn't be a lot of difference in price if I were to have done it that way. Plus, the Ar, Ar/He, O2, and acetylene tanks I already have take up a lot of room so another tank, with the strap to hold it against the wall, wouldn't be a welcome addition.

I'm not so sure about that.

According to this engineering article, the Mason and Saxena approximation formula was experimentally validated by Cheung et al 60 years ago.

Engineers have quite an interest in working fluids, so engineering departments tend to do lots of experiments to empirically derive tables and graphs. To be fair, the focus is on enthalpy.

They also tend to be more content with lower precision than experimental physicists, so there is much less ongoing work to refine the figures.

You don't have a plasma arc cutter? You'll fall behind in the tooling wars.

The bottles are at least twice that much here

Not much chance of him falling behind, but I’ll take solace in the fact that I could slice and dice a Caterpillar D11T CD in a matter of hours and he can’t.

MM… don’t you think you should be incorporating a data logger?

The screeching sound you heard was, once again, this thread lurching from one off-topic to another...

I bought a plasma cutter for my lab, but in 20 years I never used it and I can't remember my technician doing so either. Other than making engine plates, I can't think of any motorcycle-related uses for a plasma cutter, and it wouldn't even be essential for engine plates. I'd be interested if someone who owns one has some examples they could share.

Pretending to get back on topic, you could use it to cut out the pieces to make a new UFM

Now that you mention it, if I had needed to cut open my UFM to clean out stubborn deposits that couldn't be removed any other way, which I didn't need to do, a plasma cutter would be one way to cut a "manhole cover" into it.

I dont have one of my own but a close friend has one that I use occasionally. Its something on my own list of tools to get in the not too distant future, in fact if money were no barrier a CNC plasma cutter would be near the top of my list (as would a CNC router).

However, I am pretty sure old motorcycle related tasks would be uncommon. I could see a use if one were building jigs or maybe a wheel or crank truing stand or similar

I like to make stuff and often have to repair stuff. For example I have some repairs and some replacement parts to make for my old telehandler and a plasma cutter would be very handy. However, for motorcycle work, there is a lot of stuff that would be much more useful than a plasma cutter.

TBH, you have an oxy torch so any thicker metal cutting you need to do on rare occasions can be easily done by flame cutting rather than plasma.

John

As luck would have it, the only thing even vaguely interesting while vegging out in front of the TV tonight is "Richard Hammond's Big", about the VW factory.

Apparently the paint shop puts a positive charge on the car bodies and a negative charge on the paint robots / guns. This reduces overspray (attraction) and helps the paint layer to even out (repulsion).

I don't know if it will be any use in the TARDIS, but it seemed to be worth mentioning.

I used the plasma cutter for some motorcycle related work as recently as 4 years ago. I made engine plates to mount that Comet engine into the Honda GL500I frame. I may have also used it to cut out the inner primary cover, but don’t remember for sure. The M/C tool/equipment cost benefit ratio puts it squarely in last place, but the fact remains that I have one and you don’t.
Your attempt to apply logic to a tool purchase in a British motorcycle forum (of all things) will not dampen my spirits and the Comet/Honda should make it crystal clear that trying to do so is totally fruitless.

Damn, Cyborg saw straight through my attempt to justify not having a plasma cutter. This is a major setback in the Tool Wars®.