[OP]
Contributor
I was wondering at what humidity level everyone else's shops are at? I have a 3 gauge with temp humidity and atm pressure. My shop, which is in my basement, started the day at 50% (normal according to the gauge). I then unplugged my dehumid and it rose to ~53%. I am trying to determine if I want to run it constantly like I was, or just intermittantly during the summer. Any thoughts?
Member
What did the temperature do during that time. You do know that they are related right? And maybe it goes without asking - I'm sure you measured 'Relative Humidity". The dehumidifyer likely increases the temp in the room.
[OP]
Contributor
The temp had climbed slightly...so I was not surprised the humidity went up, plus I was sweating which only added to it. The dehumidifier was not plugged in at the time...
Contributor
Jay,
The answer is what ever the humidity will be in the room the furniture you make will be in. Climate controlled house is easy especially if your shop shares the same climate control. However in an unconditioned house the humidity will fluctuate wildly in some parts of the country and in the same climate your basement will be more humid because of the temperature drop. Same air that has been chilled = higher humidity. That is why it is a mistake to ventilate basements and crawl spaces. You just keep bringing in more and more moisture from the warmer outside air. I personally keep my dehumidifier on 50% in my basement and try to keep my house around the same year round but here in CT that is not easy if you like getting fresh air into the house.
James
Guest
I'm running two whole house dehumidifiers in my shop / garage / basement.
Member
I took over my 3 car garage as my workshop. Milled some kiln-dried ash for a workbench. Measured with moisture meter in 3 places on each board at that time the moisture reading and marked on wood. Was planning on gluing up the top and finishing the bench throughout the winter. Now each time I check the same wood it is gaining moisture to meet the condition of the workshop. My question...since the workbench will be in my shop, do I wait till it quits gaining and stabilizes or run out and buy a dehumidifier to keep the shop humidity down.
Member
James:Originally Posted by James White
What you are saying is referance to relative humidity.
However relative humidity is NOT a measure of how much moisture, (water vapour) is in the air. Relative humidity over simplified is: The percentage of moisture in the air in comparison to the total moisture content possible at a given temperature. 100% RH is the saturation point at any temperature.
RH could be used as an indication of how comfortable you may feel.
The moisture content of air (Specific Humidity) is measured in Grains Per Pound (GPP). The picture below is of a Psychrometric Calculator. The outside ring is Relative Humidity (RH) the inner ring is Temperature (red numbers) in degrees f.
As you can see, I have set the RH to 50% and the temperature to 60 degrees. The GPP is 39.
At 50 degrees, the RH is 72% the GPP is still 39.
At 70 degrees the RH is 35% the GPP is 39.
What wood reacts to is Spesific Humidity not Relative Humidity.
In the above examples, the RH measurement is meaningless since the moisture content of the air is the same.
Making decisions on what is happening to the RH can be misleading.
Greg
Last edited by Greg Sznajdruk; 10-25-2009 at 9:51 AM.
Member
50% RH is really low compared to the Gulf Coast area. Our typical RH in summer is around 80 to 90%. We are always at our limits as to RH factor when to finish or not to finish.
Retired, living and cruising full-time on my boat.
Currently on the Little Tennessee River near Knoxville
Contributor
Hi Greg,
Please correct me if I am wrong. Doesn't EMC (equalibrium moisture content) depend on RH (relative humidity). Most of the kiln schedules I have seen require higher and higher RH as temperature goes up in order to control the amount of MC loss per day.
In regards to basements its the cold concrete that condenses and adsorbs water and therefore increases RH and specific humidity. Again I am no expert but this is how I understood it. I am always looking to learn.
Member
I'm not so sure that your last statement is relevent. If all else remains the same, Specific Humidity only changes with atmospheric pressure which generally does not vary much throughout the day. On the other hand Relative Humidity can change drastically throughout the day. When using finishing products, RH trend throughout the day is easily monitored and forecasted. Apparently SH stays relatively the same. When it comes to paint and finish products , the Product Data Sheets make reference to RH and not Specific Humidity.
Also the two have different ways of measurements. The SH is based on the actual content of moisture and is measured by weight whether you want to use grams or pounds. RH is a percentage of the same moisture content as related to the same volume of air but at different temperatures. If my RH is too high, one way to drop the RH is to turn on the heat because the warmer air can hold more moisture without sweating - the dew point thing. To drop the SH, you would have to remove the moisture. If you remove the moisture, (which is another way to drop the RH) you have also dropped the RH. So, the two are not really inseparable but the numbers are used for purposes.
In lumber drying, one not only raises the temperature to increase the drying capability by reducing the RH but also circulates the air and removes the moisture. By removing the moisture both the RH and SH are changes. For most non-scientific applications such as woodworking, painting etc., RH is the standard.
Last edited by Tony Bilello; 10-25-2009 at 10:32 AM.
Retired, living and cruising full-time on my boat.
Currently on the Little Tennessee River near Knoxville
Member
Greg - Your explanation is correct except for this last part. Wood in equilibrium with the air will have a EMC that is specifically related to the relative humidity, not the absolute humidity. This is why EMC tables for different species of wood are written out in terms of RH.
The reson has to do with the driving force for mass transfer (mass in this case being water vapor). If the RH is very low, even if the absolute humidity is relatively high, there is a high driving force for the wood in contact with this air to give up moisture, as the air "wants" more water vapor. The opposite is true as well.
Anyway, to the OP - one of the reasons to run a dehumidifier in your shop doesn't have anything to do with the wood - it's the best way to prevent rust on your tools. While one can always wax the table saw table, it's not easy to coat internal parts with a rust-preventing lubricant.
Generally speaking, steel/iron surfaces will not appreciably corrode if you keep the relative humidity below about 55%.
Member
The equilibrium moisture content (EMC) occurs when the wood has reached a water content equilibrium with its environment and is no longer gaining or losing moisture. In Kiln drying wood this is a science in order to prevent checking and case hardening the kiln is controlled to specific levels over the period of drying.
In the past we did not instruments that could read Specific Humidity and SH was calculated using temperature and RH. In the last 5 years, there are now instruments that can read SH directly.
Concrete generally is cooler that the ambient air (room temperature) as far as it’s effect on the RH it would depend on what the Dew Point Temperature is at any given temperature and RH. The bottom window of the calculator show in the picture is the Dew Point. Dew Point is a consideration when you have cast iron in your shop. If the cast iron is at or below the dew point water vapour will condense.
Greg
Last edited by Greg Sznajdruk; 10-25-2009 at 11:32 AM. Reason: Spelling
Member
Until recently the only meters we had did not, have the capability to read Specific Humidity so RH and temperature were used to calculate SH. Yes there is a correlation between RH and SH but RH is a percentage of what moisture could be held by the air
Increased temperature allows the air to hold more water vapour. More water vapour in the air allows a more effective way to remove this moisture.
I have obviously taken this discussion off on a tangent, not my intent. The original discussion was in regards to dehumidification. The point I should have made is that residential dehumidifiers are not capable of removing water vapour below 55 grains. This is where GPP comes into play if the GPP is below 55 grains your dehumidifier is simply generating heat. You can find the rating of your dehumidifier at the AHAM (American Home Appliance Manufactures) site.
Greg
Guest
Well there are a couple ways of looking at it.
The old school guys didn't have dehumidifiers and had to build things that would tolerate seasonal change. One suspects that the old school furniture users were already well accustomed to the effects of changing humidity and just accepted it as a matter of course. Inter Alia, they learned to make their baked goods taking into account the ambient humidity. What choice did they have?
Now people have different expectations and an awful lot of those expectations are entirely dependent on the things no one really thinks about much like their house hold HVAC being the reason their furniture doesn't change shape and the floors are not cupping why they can follow a recipe when making bread and cake and it comes out the same every time instead of them having to adjust for the season. These things all fall into a back ground noise that most folks never give much thought to.
So if you are making stuff for people with these sorts of expectations well, maybe you have these expectations too.
So maybe getting a dehumidifier might not be such a bad idea.
My humidity levels were so bad for much of the year that I had to oil or paint any exposed steel. My home is on a hill and in that hill are a thousand springs. The water is great but it's everywhere. And the foundation is hand laid field stone three feet thick. It's pretty good but the humidity still come s through.
I remember when I first moved in, going into my shop and finding an indoor cloud fogging my whole shop. It was most disconcerting.
Member
Wow
I’m a two-finger typist and I’m developing calluses.
This discussion is leading to the Second Law of Thermodynamics and Thrust calculations. Wet will go to dry if can. RH is the percentage of moisture that the air could hold. SH is the actual amount of water in the air by weight.
Rusting of steel is determined by Dew Point. At 55 % RH and temperature of 65 degrees f, the dew point is 48 degrees.
At 55 % RH and temperature is 55 f the dew point is 34 degrees f.
Therefore, as a rule of thumb it would appear to be valid.
Greg
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