What is your relevant humidity?

It is March of 2002. The place is a dock in New Delhi, India, where a container of high-end, Canadian-manufactured kitchen cabinets has been waiting for a month to clear customs and be delivered. The ambient temperature in the sun approaches 50 C. The relative humidity is over 90 percent. Inside the steel boxes … it’s anybody’s guess.

There is no need to guess, however, at the state of the product on delivery. The joints were swelled, the finish was cracked, the boxes were twisted and the shipment was rejected. In addition, the reputation of Canada in India for quality goods was damaged.

Humidity only affects everything...

Humidity only affects everything…

Relative humidity is one of the most-discussed and least understood aspects of the production system, and it is one of the most important, affecting employee health, wood moisture content, tool life, product quality and safety. Not a bad list for something that is all relative. Taking just moisture content, that factor alone is credited with causing the majority of quality issues in finished wood products.

Just to get started, we need to set some terms of discussion, those being absolute humidity, relative humidity and equilibrium moisture content (EMC).

Absolute humidity is what we illustrated on the dock in New Delhi. It is the weight of water in saturated air, and can run from near zero to about 75 grams per cubic meter at 50 degrees C at 90 percent relative humidity, as on the dock. For perspective, if the cabinets were fabricated and stored in Canada at -10 C at 20 percent relative humidity, the weight of the water in the air would be ½ of one gram.

Relative humidity — the term we all deal with at home and on TV — is the water content of the air expressed as a percentage of absolute humidity relative to the maximum for that temperature. In the example of the dock, the maximum absolute humidity (100 percent relative humidity) would be 83 grams. The 90 percent figure we provided is simply 83 X .9 (74.7 grams, to be exact).

In general terms, the secondary wood industry tries to machine wood at about 8 percent moisture content. This is viewed as a middle ground where it will not shrink too much in the drier air of Canadian winters or expand too much in the moist air of the summers. However, “too much” is a big variable. The actual moisture content of the wood will vary depending on whether it’s one species or another, and whether it is solid wood or a composite. The speed with which it will vary also changes, depending on the specific density of the material, the absolute humidity in the environment, the temperature and any number of other variables. To complicate things, the relative humidity and the absolute humidity will vary widely depending on your product’s destination. You do not need to go to India to find an environment hostile to products created in Canada. Arizona is hot and dry, Florida is hot and wet, Inuvik is cold and dry and Toronto sucks.

In addition, the humidity in the environment affects the moisture content of wood much more slowly than it does the air. This makes it important to manage the environment and the wood as you prepare your products for delivery.

“Too much” humidity is a  big variable

“Too much” humidity is a
big variable

Management of moisture content is not as simple as adding or subtracting water to the air. Because the absolute humidity of air in Saskatchewan in December at 70 percent is lower than the absolute humidity of air in Tampa in December at 70 percent, products for a building project in Tampa should be conditioned for their intended moisture content in their intended environment, not the factory. This is achieved, in part, at the drying kiln, but can also be continued with shop humidification.

So far, the discussion has focused on quality issues. However, one of the primary areas of focus by North American regulators is becoming the issue of dust explosions. In its Hazard Communication Guidance for Combustible Dusts, the U.S. Department of Labor cites moisture content and ambient humidity as the top two variables in the ease of ignition and severity of explosion of combustible dust, following particle size. (See Wood ­Industry, January/Feburary 2013)

Given the current legal focus on due diligence following accidents and the current state of insurance, it would be worth checking with your own insurance agent to see whether jobsite humidification would affect premiums or reduce legal exposure.

Another area of long-standing concern regarding wood moisture content and relative humidity in production is the area of tooling. Wood, being a cellulose fibre, both absorbs moisture and reacts in different mechanical ways to its dryness. Dry fibres, for example, can shatter or split more easily than wetter, more supple fibres.

Similarly with tooling; chip removal, sharpness, speed and virtually every other function of a cutting tool is affected by moisture content and humidity, and the ability to maintain a stable humidity level is also the ability to measure and manage cutting-tool performance.

Finally, the Canadian Centre for Occupational Health and Safety has released a range of publications regarding human health and the work environment as they relate to relative humidity. For example, Cold Environments – Working in the Cold says of humidity, “water conducts heat away from the body 25 times faster than dry air.”

The publication Inspection Checklists – Sample Checklist for Manufacturing Facilities specifically asks whether ventilation equipment, as well as fume hoods and dust collection are “working effectively.” In addition, the publication, Humidex Rating and Work explores the relationship of humidity to comfort and current efforts to set standards.

Currently, there are several types and many brands of shop humidification systems, ranging from small, one- to three-man shops up to large factories, and running the gamut of steam, low-pressure, high-pressure and ultrasonic modes. Among those modes, it is possible with digital monitoring to increase humidification output as indoor relative humidity changes due to changes in weather, ventilation, etc. This makes it possible to maintain a relative humidity of 50 percent, for example, even though the outside environment would otherwise cause heaters to kick in and relative humidity would drop to 10 percent – not uncommon in Canadian factories in winter.

It may not be possible to overcome the difference between Canadian manufacturing processes in late winter and a shipping dock in New Delhi. However, new advances in humidification technology and new interest by workers’ groups and regulators strongly suggest a review is in order and change is on the way. Now might be the time to, at minimum, weigh costs and options between now and the next round of -30 C high-pressure ridges out of the arctic.

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