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Thread: Effect of relative humidity on roasting

  1. #1
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    Effect of relative humidity on roasting

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    I know that it has already been discussed here; http://coffeesnobs.com.au/YaBB.pl?num=1169177791/40#40

    But how does humidity effect the home roaster? If at all?

    I can see that a higher relative humidity means less available oxygen in the air for gas powered pro drum roasters. In http://coffeesnobs.com.au/YaBB.pl?num=1239023412/50#50 Andy also mentioned “humidity makes a big difference in drum roasting to both the roasting (air flow) and cooling (more air flow)”.

    Would humidity really effect an electric heat source (heatgun) that much?
    I have heard from a pro roaster that he doesnt roast at high humidity but have read in a thread that others using a popper do.

    I roast in the garage with a Coretto. I pre heat to about 80C. Have found that ambient temp has little effect if any on the roast. I would have it as a guess that humidity would be the same.

    The Coretto also has a fast response compared to a big drum roaster. Could this also be another reason why I dont see much effect of temp and maybe humidity?

    Does anyone out there measure and record relative humidity before home roasting? Does it make a difference? By that I mean objectively, not it "feels like 40%".
    Is it worth it to go out and buy a Hygrometer?

  2. #2
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    Re: Effect of relative humidity on roasting

    No need in most areas to buy your own Hygrometer as the BOM does it for you (assuming you are roasting outside) http://www.bom.gov.au/products/IDV60...01.94830.shtml is my local 500m from home :).

    Generally being coastal we run 30-60% all the time and cool when I roast at between 15-23 degrees.

    Extreme humidity like in the tropics will cause a problem and I have seen some of the North Americans have problems with below zero roasting ::)

    Pleased I live in a mild climate :)

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    Re: Effect of relative humidity on roasting

    Quote Originally Posted by 282F2B242C263323242D4A0 link=1239671550/1#1 date=1239674849
    No need in most areas to buy your own Hygrometer as the BOM does it for you (assuming you are roasting outside) http://www.bom.gov.au/products/IDV60...01.94830.shtml is my local 500m from home :).
    You lucky man!

    The closest weather station to me is 35km away!
    The next closest is 40km away.
    My house is 200m above sea level which makes it different anyway. At the bottom of my driveway is a creek. The temp there can be upto 10C different!
    The figures from the BOM are meanless to me :(
    And I roast in the garage.

  4. #4
    Mal Dimal's Avatar
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    Re: Effect of relative humidity on roasting

    Gday "bassway,

    I stand to be corrected but it seems to me that extremes of relative humidity will influence not only the average moisture content of the beans prior to roasting, but also the time (and energy) it would take to reduce the water content of the beans down to a level that will allow roasting of the beans to commence. Low relative humidity would equate to less energy and time and vice versa for high relative humidity conditions.

    Cheers :)
    Mal.

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    Re: Effect of relative humidity on roasting

    Quote Originally Posted by 123F3B373A560 link=1239671550/3#3 date=1239693391
    Gday "bassway,

    I stand to be corrected but it seems to me that extremes of relative humidity will influence not only the average moisture content of the beans prior to roasting, but also the time (and energy) it would take to reduce the water content of the beans down to a level that will allow roasting of the beans to commence. Low relative humidity would equate to less energy and time and vice versa for high relative humidity conditions.

    Cheers :)
    Mal.
    Yep, understand and agree.

    What I was refering to was to the relative humidity of the climate that a roaster is in, not the moisture content of the coffee bean.

  6. #6
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    Re: Effect of relative humidity on roasting

    Thats what I was referring to mate... As well as the situation with the beans.

    If where the roaster is located, the RH is High on any particular day, more energy will be required to remove the moisture from the beans than on a Low RH day. This is because energy is required first, to reduce the moisture load in the air so that the moisture within the beans can evaporate and be carried away. It probably results in a double whammy situation because the moisture content in the beans will be higher than usual as well.... Particularly if the beans have been in storage on the premises during a high RH period,

    Mal.

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    Re: Effect of relative humidity on roasting

    Quote Originally Posted by 735E5A565B370 link=1239671550/5#5 date=1239697159
    If where the roaster is located, the RH is High on any particular day, more energy will be required to remove the moisture from the beans than on a Low RH day. This is because energy is required first, to reduce the moisture load in the air so that the moisture within the beans can evaporate and be carried away. It probably results in a double whammy situation because the moisture content in the beans will be higher than usual as well.... Particularly if the beans have been in storage on the premises during a high RH period,

    Mal.
    Mmmm....

    According to http://www.engineeringtoolbox.com/heating-humid-air-d_693.html
    you only need another 0.2% energy to heat air 10C that is 50% humidity. It seems that ambient temperature is far more important.

    If the high RH was effecting the moisture content in the beans;

    500g of beans gaining 2% moisture would mean another 10g of water to heat.
    Assume 20C starting temp. 10g/1000 x 80C = 0.8 calories.
    Assuming that enthalpy is a little over 5 times the energy you would get a total extra energy required of about 5 calories.
    A 1500Watt heatgun puts out about 4000 btu/hour.

    I dont really think that it would effect the roast that much?
    Above seems like the worst case.

  8. #8
    Mal Dimal's Avatar
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    Re: Effect of relative humidity on roasting

    Well have to wait for Andy or one of the other pro roasters to chip in then, to explain what happens I guess :-? ...

    I take it that your calcs take into account the latent heat of evaporation required to facilitate an increase in RH differential so that the moisture in the beans can reduce sufficiently? I know when designing air conditioning systems years ago, the RH of the air being cooled/heated was a significant load factor to be accounted for within the system. Failure to adequately account for this always resulted in a system that operated at significantly less than optimal efficacy....

    Mal.

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    Re: Effect of relative humidity on roasting

    I think so Mal,

    Thats the enthalpy bit. The energy required to push 100C water to steam.

    But I was just surfing the net to try and find answers. I could be wrong.
    Im not in it for an argument I am just curious. I want to know!
    As an engineer you would know far more about this my poor little technician education/experience.

    Dont heat pumps de-humidify? This would require a large amount of energy. The heatgun isnt really de-humidifying. Just heating the air.

    I did do a little experiment tonight.
    I drilled a hole in the side of one of the adapters that attach to the end of my heat gun. I attached a K type thermocouple to it and measured the output. I also grabbed a cheapy hygrometer today. It was only $13 but claims +- 3% humidity accuracy.
    Once the heatgun output stabilized I sprayed water in the air to increase the humidity. The heatgun output went up!
    I only got the humidity up from 37% to 43%. The heatgun output went up from 390 to 396. This was done in my garage. Not really a result.

    So I went into the laundry. Starting ambient temp was 14.second crack, RH was 52%.
    Turned on the heatgun, it stabilized at 400C. Switched on the the steam iron to generate humidity.
    Came back, the gun had dropped to 390!?!?
    I started to steam up the room.
    In the end I got the room up to 72% RH. The heatgun output was 393C. Gone up? Not really. The ambient temp also went up to 17.4C. So there is the 3C increase. The humidity didnt seem to make any effect.

    Where did the first reading of 400C come from?
    When I switched off the steam iron the heatgun temp went up. Switched on the iron, went down. Line voltage made the biggest difference to the heat output of the heatgun!


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    Re: Effect of relative humidity on roasting

    Behmor Coffee Roaster

    Interesting conversation.

    Last year I considered buying a moisture meter as I wanted to monitor the moisture content of my raw beans. Once I discovered that any decent piece of equipment was going to cost upward of $1k I quickly reverted to the feel and smell method of assessment.

    Im sorry, but I wouldnt think a $13 hygrometer is worth the batteries, as far as reliability goes. This goes for cheap digital scales too. ;)

    I need to make significant adjustments for consecutive roasts - even with the same beans. I also consider time of day, temperature, and general weather apart from a whole lot of other stuff. Once the drum is charged at a set temperature, the time and temperature of the turn are and should be predictable if youve done it all before.

    In most cases I find this true within a tenth or two of a degree. But there are times when this isnt quite the case, and I only have a hunch that its because of the weather, humidity included, and perhaps even how much gas is left in the bottle.

    Does it affect the roast? I dont think so, but could easily be wrong. I guess thats why roasting coffee is, and hopefully will always remain, a combination of science and art.

    Cheers!




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