As many of you know, my current profession is structural drying. This means I spend long hours of drying out someones basement after their sump pump fails or a pipe breaks. In this business, we use the same principles as growing orchids...only in reverse. We also use a few principles of psychrometry that orchid growers don't often hear about like specific humidity and vapor pressure and I thought I would share them with you guys as I have found them quite helpful.

Specific Humidity:
We orchid growers have nearly always measured humidity in the percentage of Relative Humidity(RH). I think every orchid book I have ever read only mentions this as well. The problem with this is that RH can be quite misleading. Relative Humidity is the percentage of water in the air at a given temperature as compared to the maximum amount that it could hold. Specific Humidity is the actual weight in grains of water in the air instead of a percentage of how much it can hold. Why is this important? If you had a greenhouse that was 60 degrees F and 60% RH, would you consider this a moist atmosphere? You may be surprised to know that this is the same amount of moisture as there is at 80 degrees F and 30% RH, or 46 grains per pound of dry air...and quite dry.
In the drying business, in order to dry out something the humidity in the air must be below 50 grains...so specific humidities in the 40s are great for drying wood furniture and bad for orchids. This is why cold growing species like Pleurothallids and Odontoglossums require such high relative humidites...they need them to get the same moist atmosphere. When the relative humidity is 100%, you get fog, and at 60 degrees F like the above example that's 79 grains. Now you know why cold growing orchids require such foggy cloud forest conditions.
When you consider my growing area usually maintains around 70 degrees F and 80% RH, that's nearly 90 grains and for growing orchids I find above 70 grains is ideal. At work we use slide rule psychrometric calculators, but a basic psychrometric chart can be found here:
http://wwwsam.brooks.af.mil/af/files/fsguide/HTML/Graphics/fig_05-02.gif

I don't think Specific Humidity will ever replace Relative Humidity when it comes to growing orchids, but I do definitely think it should be a consideration when trying to figure out how moist your atmosphere is for different types of species.

Jon

Thanks Jon,

Now, if I keep my day temperatures @ 65 F and nights 50 F; what RH do I need to keep it above 70 grains?

Great info!

Very interesting, Jon. And thanks for the link.

You need humidity to grow orchids????? DOH!!!!!!



Thanks Jon! Very informative.

Thanks Jon

I also lock my RH values in to >70% to fudge for temp effects, and it seems like the orchids seem to appreciate that number too.

Are there cheap meters that opperate in SH instead of relative H?

Obviously water cannot hold 70 gpp at 50 degrees F, and I should have wrote that at least 70 gpp is my preference in warmer temps. At any warmer temp, always try to avoid going under 50 grains as this causes plants to transpire faster than they can uptake, thus dehydrating the plant.

I don't know of an inexpensive source for a SH meter, as ours are over $500.
We use Delmhorst meters and Vaisala thermohygrometers. An inexpensive meter that gives temp and RH will suffice if you use the chart or a slide calculator.

Jon

Very interesting, thanks for sharing, Join.

<<<Chun