Silica Gel Usage

[joewr]

Anything that uses electricity will generate heat when operating. Electrical devices aren't 100% efficent. A motor turns electrical power into mechanical motion, but if it's 80% efficent, 20% of that power is turned into heat. Lights (internal flashes) also generate heat in operation.

A simple guide line as to what camera is going to generate the most heat would be: the one that eats batteries the fastest produces the most heat. Whether this causes the camera to warm up is going to be controlled by the thermal mass of the camera and how efficently the camera transfers heat to it's surroundings.

Hi, Mark!

All true...I just want to quantify it.

When was the last time you burned your hand holding a flashlight? Now, holding in your hand a 100 watt light bulb powered by 110 volts AC...that is a different story! These little cameras have 12-24 volt batteries and I have just do not have a feel for how much heat they generate when operating.

So, my questions remain--how much heat per shot? And how many shots in what period of time to generate enough heat to vaporize surface water in/on the camera?

joewr

 

[joewr]

Hi Joe, I'm not suggesting that the humidity changes. Condensation will occur if the housing is exposed to a sudden change in temperature, such as immersing in cold water. Kind of like taking a can of beer out of the refridgerator, it begins to fog up. If you were able to slowly raise the temperature of that same can, I don't think you'd see the same level of condensation, yet the humidity in the air is still the same. As a result, I try to lessen the temperature differential of the housing by keeping it cool in between dives.

Hi, Warren,

Sure, your beer can will fog up on the outside, but the beer on the inside will not be diluted by water until you open it! Ditto the inside of your housing.

What happens to the inside of your housing during an SI depends completely on what was on the inside when you closed it. Nothing goes in or out--you hope--once it is closed up.

Now, what can happen if you let your housing overheat is that the pressure on the inside will rise--Boyle's Law, remember--but not enough to cause a leak--again, you hope. And if it did, it would leak out, not in--higher pressure on the inside. In the good old days of film, we used to keep our housings and cameras cool during SI's to protect the film. Those days are nearly gone....

joewr

 

[markr]

Hi Joe, I'm not suggesting that the humidity changes. Condensation will occur if the housing is exposed to a sudden change in temperature, such as immersing in cold water. Kind of like taking a can of beer out of the refridgerator, it begins to fog up. If you were able to slowly raise the temperature of that same can, I don't think you'd see the same level of condensation, yet the humidity in the air is still the same. As a result, I try to lessen the temperature differential of the housing by keeping it cool in between dives.

The absolute humidity doesn't change, but the relative humidity does.

Absolute humidity is a measure of the number of water molecules in a fixed volume of air.

Relative humidity is ratio of the water vapor in the air to the maximum water vapor the air can hold at it's current temperature. The maximum water vapor that air can hold decreases as the air temperature decreases.

If you put a camera in a housing and close the housing, the absolute humidity (i.e. the number of water molecules in the housing) isn't going to change, unless the housing leaks. The relative humidity however will increase as the temperature of the air in the housing decreases.

So if the water temperature of your dive is lower then the air temperature at which the camera case was closed, the relative humidity inside the case will be higher during the dive.

 

[Warren_L]

Hi Joe, yes, you are correct, but what you are forgetting is that the outside surface of the beer can in my example is akin to the inside surface of the camera housing. In both cases, you're introducing a high temperature gradient. By taking the can out of the fridge, the outside of the can is exposed to room temperature and the inside of the can is refrigerator temperature, and condensation occurs on the surface exposed to the warm air. Same thing happens when you take a housing with warm air in it and immerse it in cold water of a dive. Bottom temps of the lakes around here in the middle of summer can be as low as 37F and when taking a camera down that's been sitting in the sun or heat causes condensation to occur on surface that is exposed to warm air, which is the inside of the housing.

 

[Warren_L]

The absolute humidity doesn't change, but the relative humidity does.

Absolute humidity is a measure of the number of water molecules in a fixed volume of air.

Relative humidity is ratio of the water vapor in the air to the maximum water vapor the air can hold at it's current temperature. The maximum water vapor that air can hold decreases as the air temperature decreases.

If you put a camera in a housing and close the housing, the absolute humidity (i.e. the number of water molecules in the housing) isn't going to change, unless the housing leaks. The relative humidity however will increase as the temperature of the air in the housing decreases.

So if the water temperature of your dive is lower then the air temperature at which the camera case was closed, the relative humidity inside the case will be higher during the dive.

Yes, you are right, which is kind of my point. When you immerse a camera housing suddenly into cold water with a high enough temperature gradient, the air inside the housing does not cool immediately and uniformily. The air that is closest to the cold surface cools down quickly and with the resultant lowering of it's ability to hold water vapour, causes localized condensation which is the fogging you see on the inside of the housing.

 

[joewr]

The absolute humidity doesn't change, but the relative humidity does.

Absolute humidity is a measure of the number of water molecules in a fixed volume of air.

Relative humidity is ratio of the water vapor in the air to the maximum water vapor the air can hold at it's current temperature. The maximum water vapor that air can hold decreases as the air temperature decreases.

If you put a camera in a housing and close the housing, the absolute humidity (i.e. the number of water molecules in the housing) isn't going to change, unless the housing leaks. The relative humidity however will increase as the temperature of the air in the housing decreases.

So if the water temperature of your dive is lower then the air temperature at which the camera case was closed, the relative humidity inside the case will be higher during the dive.

And that brings us back to where we started: where does the condensate come from and why just from P&S cameras?

I have never had this happen to any of my old Nikonos cameras...and they are very sensative to condensed moisture...or my housed SLR's.

So, there is something about P&S housed cameras that causes this...and it appears to be water condensed on the surface(s) of the camera that is evaporated due to heating. This causes the relative humidity to go above the dew point...and condensation to occur on the port glass, etc.

I think................


joewr

 

[markr]

Hi, Mark!

All true...I just want to quantify it.

When was the last time you burned your hand holding a flashlight? Now, holding in your hand a 100 watt light bulb powered by 110 volts AC...that is a different story! These little cameras have 12-24 volt batteries and I have just do not have a feel for how much heat they generate when operating.

So, my questions remain--how much heat per shot? And how many shots in what period of time to generate enough heat to vaporize surface water in/on the camera?

joewr

If you were able to touch the filament of the bulb in a flashlight you would burn yourself. The filament on an incandescent lamp operates at around 2000 degrees Kelvin (about 1700 degrees celsius if I remember the conversion properly). You don't burn yourself on a flashlight because the outside of the casing transfers heat to the surrounding environment faster then the filament can heat the casing.

A camera in a housing is in a confined space. There isn't any airflow around the camera so it doesn't transfer heat to it's surrounding as efficently as it would outside the housing.

Also, I never claimed there was enough heat being generated by the camera to vaporize water. Someone asked where the heat in a camera came from. I was merely explaining that anything that consumes electricty generates heat and that the amount of heat generated was normally proportional to the rate at which the elctrical power was consumed.

 

[joewr]

If you were able to touch the filament of the bulb in a flashlight you would burn yourself. The filament on an incandescent lamp operates at around 2000 degrees Kelvin (about 1700 degrees celsius if I remember the conversion properly). You don't burn yourself on a flashlight because the outside of the casing transfers heat to the surrounding environment faster then the filament can heat the casing.

A camera in a housing is in a confined space. There isn't any airflow around the camera so it doesn't transfer heat to it's surrounding as efficently as it would outside the housing.

Also, I never claimed there was enough heat being generated by the camera to vaporize water. Someone asked where the heat in a camera came from. I was merely explaining that anything that consumes electricty generates heat and that the amount of heat generated was normally proportional to the rate at which the elctrical power was consumed.

Hi, again,

But that is exactly the question: where is the condensate coming from? Logically, it must come from one of two sources: water vapor that is brought below the dewpoint or condensed water that is evaporated and brings the atmosphere in the housing to the dewpoint. Since, if it were the former, all housings would fog up and that does not happen, it must be the latter.

Since the only way to evaporate the condensed water is by heating it, it must be the camera. Now, if it were my laptop in that housing, I would need no further data: it gets hot fast. However, I have never held a camera that made me feel like it was heating up. But it seems that P&S cameras do exactly that.

I must admit that I am surprised a bit by that...

Now, in the old days of flash bulbs, that would have been my immediate guess if the bulbs were in the housing...those bulbs got very hot.

At any rate, this has been fun...

joewr

 

[joewr]

Hi Joe, yes, you are correct, but what you are forgetting is that the outside surface of the beer can in my example is akin to the inside surface of the camera housing. In both cases, you're introducing a high temperature gradient. By taking the can out of the fridge, the outside of the can is exposed to room temperature and the inside of the can is refrigerator temperature, and condensation occurs on the surface exposed to the warm air. Same thing happens when you take a housing with warm air in it and immerse it in cold water of a dive. Bottom temps of the lakes around here in the middle of summer can be as low as 37F and when taking a camera down that's been sitting in the sun or heat causes condensation to occur on surface that is exposed to warm air, which is the inside of the housing.

Hi, Warren,

If you have sufficient water in your housing for condensation to occur at 37 deg F, it will happen regardless of what the temperature was during your SI...unless you opened your housing then.

Apart from that the only things that matter are the amount of water in the housing when you closed it, the dewpoint of that mixture, and the temperature at depth. If the temperature at depth is at the dewpoint or below, the water vapor will condense.

And if you have a dessicant in your housing, it will remove the water from the encased air, making the dewpoint go lower, thus avoiding condensation.

Now, let's have one of those excellent Canadian beers and make certain it does not get diluted with any condensate!

Oh, by the way, did I tell you that my father was born in Winnipeg? Eh? But he left Manitoba to come to California...and that is why I have this funny accent...and I do not spell vapour the way you do...

joewr

 

[Digger54]

I know what the silica gel does, I use some larger "rechargable" packets of it in a gun safe at home, but I am unsure of how to best apply it in underwater photography.

Do most keep silica gel in the housing during a dive?

Where do you put it in the housing? I might have enough space between the tray and housing bottom if the packet is thin and small enough.

What do you do to avoid it moving and interfering with the operation of controls?

Is it likely to produce any dust if it flops around in the housing that could coat a lens and reduce the sharpness of photos?

Thanks

Would someone please return to my original post and sort of, kind of , answer the question please instead of going off in all directions about the "why" of condensation.

So far it sounds like most do not use silica gel in their housing. For those that do, what is your souce for packaged silica gel and your technique of using it? Thanks