Comparative Relative Humidity Table

[grisell]

[---]
Does it explain why drying to Adam Marianski's regime (higher temps and RH than you lads would use) leads to more chance of mould? I'd hate to suggest so as he's so much more experienced than we are.
[---]

I doubt that he is more experienced than we are if you add all our experience together.

[wheels]

According to him, only the relative and not the absolute humidity (the absolute amount of water in the air) plays a role in the drying rate...

...We can thus establish that drying at e.g. 75% R.H. is as efficient at 1 C as at 20 C and concentrate on the temperature instead. Temperarure will affect the rate of chemical and microbiological changes.

Whilst I can't argue with this, as he is obviously better qualified than I am, it's not what I have experienced in practice. In my experience drying is faster at higher temps and (bad) mould formation more problematic.

I assumed that this was related to the increased amount of water in the air at higher temperatures even if the RH% remains constant.

Phil

[grisell]

As for the mould, I have no idea. I only asked him about the drying process.

[Yannis]

Phil your table was very useful as it helped me to understand what RH really is.

I am testing my curing chamber (I finished it at last) and I tried to lower RH by taking fresh air inside as I had read that other people use this technique. Conditions inside were 11 oC, RH 80% and outside 24 oC and RH 50%. So I expected that "dry" air would lower RH inside but to my surprise RH skyrocketed to 99% within 2 minutes !!!!

After looking at Phil's table I searched a little and I found that Dew Point is the measurement I needed. Dew Point is the temperature at which water vapor will condense. Here is a usefull calculator http://hyperphysics.phy-astr.gsu.edu/hb ... um.html#c3 for my case 24 oC and 50% gives Dew Point of 12.5 oC meaning that if you chill the air at that temperature (or below) you get 100% RH, exactly what happened to me.

So if you want to lower RH you must insert air with Dew Point lower than the Dew Point of desired conditions. Checking with a nearby meteo station I found that this is not possible for me for most of the year so I decided to use a dehydrator.
I was searching for a small unit to fit inside refrigerator when I realized that I could just use refrigerator as a dehydrator. When I want to lower the RH I turn on the heater to make refrigerator kick in and lower RH by condensing water vapor, that simple !!!

I think Phil that this could be the solution to your problems lowering RH.

[wheels]

I think Phil that this could be the solution to your problems lowering RH.

Not a problem Yannis, that's why I have both a humidifier and a dehumidifier in my setup.

However, I still maintain that there is less chance of bad mould at the lower end of the recommended temperature scale than the higher.

Phil

[Yannis]

However, I still maintain that there is less chance of bad mould at the lower end of the recommended temperature scale than the higher.

I agree with you Phil, that's why I programmed my controller to keep temperature at 11 oC.

I posted my findings here as it was your table that helped me understand what is going on. Maybe this was not the correct thread to do so

Not a problem Yannis, that's why I have both a humidifier and a dehumidifier in my setup.

If I remember correctly you use dehydrator outside chamber. If this is the case you could try parallel the output relays of heating and dehudifying to turn on heater instead. I think that the response of the system would be faster and probably less energy consuming than lowering RH of entire room.

Yannis

[wheels]

Ah, but that's brought us back to what this thread's about; by heating it'll lower the RH, but it won't actually change the amount of moisture in the air. That'll be the same.

Phil

[grisell]

Correct, but it is the relative humidity and not the absolute amount of moisture in the air that matters when drying.

[Yannis]

Ah, but that's brought us back to what this thread's about; by heating it'll lower the RH, but it won't actually change the amount of moisture in the air. That'll be the same.

Phil

Heater will turn on when RH is above humidity controller threshold. Then temperature will rise until is above temp threshold and refrigerator will turn on. Heater and refrigerator will be on at the same time. If you use a reasonable heater let's say 100W then the heat that produces will be transfered to ambient by refrigerator without raising temperature. At the same time water vapor will condense lowering RH. When RH reach desired value heater will turn off.

I am trying this setup now with a large wet towel in my chamber and it seems to regulate well but the outside temp is quite high and refrigerator turns on often anyway.

Correct, but it is the relative humidity and not the absolute amount of moisture in the air that matters when drying.

We want to reduce RH keeping temperature constant so we must reduce absolute moisture.

[Yannis]

I improved the program of my controller, when RH is high heater is turned on until refrigerator is turned on. Then heater is turned off and refrigerator reduces RH. This way I just use heater only as long is required to turn on refrigerator.

[wheels]

Correct, but it is the relative humidity and not the absolute amount of moisture in the air that matters when drying.

But is it the complete answer? We not only want drying, but to avoid bad mould and case hardening etc. Could they be affected by the amount of moisture? There's certainly anecdotal evidence around that this may be so.

Yannis

Apologies, I misunderstood your use of the heater. Yes, that's a good idea.

Phil

[Ianinfrance]

Hi Phil,
Can I preface this by saying that I'm totally ignorant about the procxesses, but I do understand the physics a bit.

Correct, but it is the relative humidity and not the absolute amount of moisture in the air that matters when drying.

But is it the complete answer? We not only want drying, but to avoid bad mould and case hardening etc. Could they be affected by the amount of moisture? There's certainly anecdotal evidence around that this may be so.
Phil

Drying and mould formation are surely going to be effected both by the temperature and the rH. Actually, I guess the movement of air in the chamber and how fast it is exhausted into the atmosphere are almost more important than a measured rH, distant from the drying meats)So I could well imagine a case where the rH could be near 100%, and yet the temperature was very high too. Now clearly this would encourage moulds, but there's be almost no evaporation from the surface, so case hardening woulds be unlikely.

Equally one could imagine a case where the temperature was very low and an equally high rH. In this case, there's be little or no mould formation, while again there's be no evaporation from the surface, so case hardening would be virtually impossible too.

My top of the head suggestion would be that case hardening can only occur when the rH is sufficiently low to encourage relatively rapid evaporation from the surface, and the airflow sufficiently high, while the temperature too low to encourage moisture migration from the inside of the sausage to replace that lost from the surface.

[grisell]

Correct, but it is the relative humidity and not the absolute amount of moisture in the air that matters when drying.

But is it the complete answer? We not only want drying, but to avoid bad mould and case hardening etc. Could they be affected by the amount of moisture? There's certainly anecdotal evidence around that this may be so.
[---]
Phil

No, of course not when it comes to microbiology. I was refering to the process of dehydration itself only.

[wheels]

Ian

Thank you, there's a lot of things there to think about.

André

Thanks, I don't think that any of us disagree with you about the drying bit.

Phil

[kimgary]

High Phil

Thanks for the table, "very Interesting", got the grey cells working

I think you have hit the nail on the head "on our enclosed chambers"!

Some figures for you to think about:-

I don't wish to get into point ).00000000etc so will round about figures.
Air at 10°C - 1Kg will have a volume of 1.2m³

Large fridge at 200 litres is 1/6th of 1.2m³ therefore 1/6th of kilogram of air in that space weighs 166grams.

Ok, difference in water weight per kg air from 12°c @75%RH to 15°c @75%RH is 1.4 grams, divide by 6 equals 0.2 grams/cc of water difference within the confines of the chamber, not a great difference.

I tend to think that the air and temp are a sponge in one unit.
If temp is high and RH low then evaporation will be rapid, (the sponge is dry and hot and can soak up water very quickly.
when temp is high and RH is high,the sponge is already very wet this means that it is unable to soak up the water at a high rate and will lead to a slow evaporation.

My head hurts so I shall leave it there at present.

Regards Gazza.