Best Humidity setting for a Dry Cabinet?

Hi, I just got a FORSPARK 50L dry cabinet to store my Hasselblad 503CW and four lenses, and I'd like to hear about your suggestions. I would think something within 35% and 45% should be good, but I don't want to make a costly mistake. Depending on your geographic location, feel free to mock me, reminding me that Zeiss glass should be used, not stored... In the Republic of Panama, where I live, humidity rarely falls under 80% (and the annual average is 86%). My Nikon lenses once started to grow fungus, I sent them to Nikon USA, but two years later, the issue recurred.

"Ideal" storage conditions for perishables like pharmaceuticals is usually given at 50% RH, 74 deg F. Without chilling (or desiccation) and reheat, you can't achieve both, but you can achieve the humidity goal by raising the cabinet temperature. In the old days, I'd consult enthalpy tables. Now I would read the cabinet instructions.

 

In tropical climates, make sure your equipment is at ambient temperature before exposing it to the air. Condensation, especially internal condensation, will practically guarantee fungus growth. I spent a lot of time in Puerto Rico (dewpoint 88-92 deg F), where I kept the equipment bagged indoors, waiting at least an hour before unbagging once outdoors.

Edited April 1, 2019 by Ed_Ingold

Thank you Ed, the cabinet instructions indicate 40%-50% for optical equipment. Extreme desiccation might be a problem for the camera itself which must stay correctly lubricated. As for the temperature, this cabinet does not control it, so it's rather uniform albeit rather high (around 30 C = 86 F). You're right, keeping it refrigerated could very likely be counterproductive. Right now, I set the cabinet at 45% for the time being.

The dew point is proportional to the absolute moisture content of the air. Relative humidity is the ratio between the vapor pressure of water at the dew point and at the actual temperature. Assuming the dewpoint remains constant, you can reduce the relative humidity by raising the temperature, which is precisely how a dry cabinet works. In all likelihood the cabinet is located in an air conditioned space, which keeps the absolute humidity low, requiring less heat for control.

 

If you wish to control both the humidity and air temperature simultaneously, you must inject moisture (steam) or reduce the dew point by chilling the air, then heat or cool the air to the desired temperature.That is well above the pay grade of a dry cabinet, as well as most HVAC systems designed for comfort only. It is routine, however, for process air handling and manufacturing environments for sensitive products.

 

Relative humidity can be measured reasonably well using a hygroscopic membrane, which shrinks or contracts with relative humidity. You then heat the air in the cabinet until the relative humidity equals the set point. Process systems rely on quantitative dew point measurements, using a thermoelectric or conductometric device.

Edited April 1, 2019 by Ed_Ingold

Many years ago, I lived in Hilo, on the island of Hawaii.

I ran a dehumidifier in my closet (where I had my camera gear and leather shoes) and it pulled out about a half gallon a week.

The closet was not sealed, just a standard sliding door bedroom closet, but still that was a lot of moisture getting into the closet.

The dehumidifier did not have a calibrated control, only relative, so I had no idea what it kept the humidity level at.

I don't know if there are specific RH guidelines for cameras, but in the world of photofinishing the standard recs are somewhere around 45-55% RH. This is seen as a balance between 1) low enough to prevent rusting of equipment, and 2) high enough that dust problems in printing (largely due to static electricity) are minimized.

 

I don't know of any problems with camera gear going lower on the humidity. As a point of reference, homes in cold climates frequently have low relative humidity; below 20%, and even 10% RH is not uncommon. And I'm not aware of any particular problems with cameras in such places. So my guess is that you'll be fine with your initial thoughts about running at 35-45% RH.

 

As a note, I looked up your unit on Amazon. It refers to "thermoelectric cooling," which I presume is for the purpose of removing water vapor. (One side of the thermoelectric element cools enough that water condenses out, just like on the outside of a cold drink.) So some water is removed, and thus the RH is lowered, while the cabinet remains more or less at ambient temperature; no need to heat the cabinet.

Thank you again Ed for clarifying the difference between Dew Point and Relative Humidity, thank you Gary for sharing your personal experience, and thank you Bill for taking the trouble of checking out the cabinet specs. I won't worry about the temperature (life in the Tropics) and focus on the ideal relative humidity setting. I suspect that by maintaining the Zeiss lenses below 50% I should be able to prevent fungus. Since I also keep my 503 CW Hasselblad body in the cabinet, I wonder if there's a risk of going too low, thus affecting lubrication for example.

If you cool with condensation without reheating, the relative humidity will be maintained close to 100%. If the surrounding room temperature is high enough, you may not need additional heating. However if you actually get condensation in a cooling element, ambient heat leakage is probably not enough. You get about 20 deg F cooling from a simple thermopile in dry air. If there is condensation, the temperature drop will be much less, due to the heat of condensation.

 

I don't think anything in the equipment would be affected by low humidity, but high temperature will affect the life of lubricants and possibly the anti-reflective coating inside lenses and the Hasselblad.

If you cool with condensation without reheating, the relative humidity will be maintained close to 100%. If the surrounding room temperature is high enough, you may not need additional heating. However if you actually get condensation in a cooling element, ambient heat leakage is probably not enough. You get about 20 deg F cooling from a simple thermopile in dry air. If there is condensation, the temperature drop will be much less, due to the heat of condensation.

 

I don't think anything in the equipment would be affected by low humidity, but high temperature will affect the life of lubricants and possibly the anti-reflective coating inside lenses and the Hasselblad.

Thank you again, Ed. I'm afraid I'm stuck at ambient temperature (30 C) since my cabinet only works controlling humidity. I am checking with an independent sensor and it's doing a good job. I've set the humidity at 45%.