Absolute Humidity vs Relative Humidity

Dichromated Gelatin.
Tony

Absolute Humidity vs Relative Humidity

Post by Tony »

Can anyone explain how Absolute Humidity and Relative Humidity effect DCG?
In other words when we shoot we sometime raise the room temp and the humidity. Our RH meter reads a number (say 50%) if I raise the room temp a bit more does that meter reflect this?
Same question if we drop out humidity for say film drying. If I raise my temp and I will also lower my RH, what is hapenning to the water? If all I am doing is warming the air, the water is still there, how does that effect my film? Is it drying?

Thanks
Tony

Absolute Humidity vs Relative Humidity
Relative humidity is another way of measuring the amount of water vapor present in the air. Relative humidity is the amount of moisture present in the atmosphere, at any given point in time, compared with the amount of moisture that would be in the air if the moisture was completely saturated, at the same temperature. So while absolute humidity is an exact measure of the amount of moisture, relative humidity is the comparison of this amount with another amount.

What this means is that if the actual temperature at a point is 27 degrees Celsius, and the relative humidity is 75%, the temperature would feel like 29 degrees Celsius. It is this relative humidity that weather forecasters display on their reports. Absolute humidity is the measurement that they use in order to derive the percentage of relative humidity in the air. Forecasters thus create an absolute humidity table for one entire day, and arrive at an average figure in order to calculate the relative humidity in the form of a percentage.
Dinesh

Absolute Humidity vs Relative Humidity

Post by Dinesh »

Tony wrote:Can anyone explain how Absolute Humidity and Relative Humidity effect DCG?
It depends on the dcg, but you have several effects going on. When the gelatin is in a liquid state, as in the beaker where you're preparing the gel, there is water in the beaker and water in the atmosphere; water is leaving the beaker to the air and water is entering the beaker from the air. There will be an equilibrium point where there is a steady state. This depends on the vapour pressure above the beaker as well as the temperature of the beaker. Usually, at this stage, all that happens is that you dilute or strengthen the gel solution by a small amount. As the plate is drying, the same thing occurs. But this time, the gelatin is rapidly drying, and so there will be more water going in than coming out. This also depends on the vapour pressure.

As to your question about relative vs absolute, if it's anything at all, it's probably the absolute humidity. The relative humidity is a measure of how much the air can hold, as opposed to how much it does hold. Think in terms of the fact that a glass can be 75% full or it can hold 2 cups of water. In the end, the actual quantity is the same, but the previous figure (75% full) gives you a measure of how much the container can hold. However, the relative humidity determines the vapour pressure and the actual vapour pressure controls how much is going into the gel, or coming out of it.So, you can get a good handle on the situation by using the relative humidity as a guide. As an example, we had about 75% humidity recently at the lab. At these high levels the air was pretty well saturated and the holograms were disappearing right in front of our eyes!

Perhaps another analogy. Consider a sponge; and consider that the sponge is pressing on your dcg with a certain pressure. For example, you have a sponge on the dcg and a book on the sponge. The more water the sponge holds, as a function of how much it can hold - how "soggy" the sponge is - determines how much water is being pressed into the dcg (relative humidity), but the absolute amount of water transferred from the sponge to the dcg determines how much the emulsion swells. Now consider that the book is a relatively light book (in the sense of weight, not readability!); in this case, the amount of water that penetrates into the dcg will depend on how saturated the sponge is, but the water penetration is slow. However, if the book is heavy, then again, water penetration is dependent on how "soggy" the sponge is, but the penetration will be a lot faster. However, this book has magical qualities: it's weight is dependent on the temperature!
Tony

Absolute Humidity vs Relative Humidity

Post by Tony »

Dinesh wrote:The relative humidity is a measure of how much the air can hold, as opposed to how much it does hold.
So I think I understand this. Increasing my room temp lowers my RH since there is more capacity to hold water. That state will cause the DCG plate to want to reach equalibrum and therefore release water hence drying. And when we leave it in a room to swell, better to be in a warm room as well. And for storage, cool is the rule.

Absolute might be inportant but I guess knowing there is a delta change in environment is good enough.

Thanks as always Dinesh

Tony
kyodai

Absolute Humidity vs Relative Humidity

Post by kyodai »

Yes the hotter the air the more water it can hold, so the typical living room humidity meter would display Relative humidity at "room temperature" Unless you turn your room into a sauna the change will be rather small. A warmer room for drying will be better to dry plates or film faster, but the effect is not based on the fact that the air can take "More" humidity, this does not affect the rate at which plates dry. It is rather the increased temperatureof the plate that increases the molecular motion of the water and thus raises the rate of water molecules vaporizing. The higher the temperature of the plate (or object to dry) the faster this will happen.

With drying plates or film the amount of water is so small you will not be able to raise the RH significantly unless you have a hermetic sealed room.

If you would reach 100% RH or close to that there would still be water vaporizing from the plate, but at one point the amount of condensating water and vaporizing water would reach a balance.

Absolute humidity would be a more practical value to calculate with (In g water/m3 air) but most household humidity meters will only show RH with a rather bad precision and latency.


I would not think too much about it - as mentioned you'd need to have kinda sealed room to keep it from exchanging air (and thus the airs humidity) with other rooms.


In chemistry you use a Desiccator to dry stuff faster. It works by removing (nearly) all air with a vacuum pump and adding a hygroscopic agent (Like silica gel). Through the low pressure the water is more likely to vaporize and as it gets to the drying agent it gets bonded as it can not condensate back to the material to be dried. Of course this works faster if you heat it up.
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