The PMO’s Corner

This issues topic is Stevenson Screens/Instrument shelters and wet and dry bulb thermometers.

Figure 1. A hygrometer set and shelter needing some maintenance

Figure 1. A hygrometer set and shelter needing some maintenance

Figure 2. A well maintained hygrometer and shelter

Figure 2. A well maintained hygrometer and shelter

When was the last time you took a good hard look at the outdoor thermometers and the Stevenson Screen protective shelter on your ship? Does it look like Figure 1 or Figure 2? If it looks like Figure 1, then we have some work to do. Unlike the barometer or barograph, this weather observing gear is the least maintained. Because this instrument is exposed to the harshest elements, it is susceptible to wild variations because of poor maintenance. Here are 4 parts that make up shelter and thermometers and how you can take simple steps to ensure your ship is reporting the most accurate data possible. Figure 1 is a picture taken of a Stevenson Screen and thermometers that need maintenance actions. Figure 2 is how it should look.

  1. Shelter (Stevenson Screen).
    Make sure that the shelter (Stevenson Screen) is painted flat white. I have seen these boxes painted all sorts of colors, so as to match the ship’s paint scheme or varnished a dark wood grain. All of these efforts to make the box look “better” defeat the purpose of the screen. Darker colors will cause the thermometers to read higher than the ambient air temperature, as much as 10 degrees higher. This not only affects the dry bulb, but also the wet bulb temperatures. The screen should be free of salt spray and stack soot. Make this unit part of any regular fresh water wash-down maintenance routine.
  2. The wet bulb reservoir.
    The water reservoir should be free from contaminates like mildew and salt residue. These contaminates will cause the wet bulb temperatures to be cooler than what the actual readings should be. To keep this container free of these contaminates, just rinse the reservoir out with clean water and use a clean cloth or paper towel to completely dry it out. Let sit for a few minutes to ensure it is completely dry, then refill with clean water. If the reservoir has become encrusted with contaminates, clean it out as much as possible and fill with cool water with a drop of bleach. Let it sit for a few minutes to kill the mildew. Then, using a small nylon brush or clean cloth, rinse it out for several minutes, to remove any of the bleach and contaminate residue. Repeat as necessary to return the tub back to its like new condition (Figure 2). If you are unable to get it clean, contact your local PMO for a replacement.
  3. Wick
    The wick is the most neglected item. As seen in the Figure 1, this is what a neglected wick looks like; covered in soot, salt spray and mildew contaminates. In the case of this unit, contaminates had hardened and would not allow the water to travel up the wick to the bulb of the thermometer, resulting in a false wet bulb temperature. To correct this, just remove the wick and rinse every time you add water to the reservoir. If you are unable to get the wick back to a clean and white color, or it will no longer wick the water up to the thermometer bulb, replace with a new piece of wick. For times when you are out of wick or you are waiting for a resupply from your local PMO, any 100% cotton material like new white shoe laces or a piece of a white tee shirt, held completely around the thermometer with a small piece of string or rubber band, will work until you can get the proper wick.
    Figure 3. The fluid show in this thermometer has separated and the unit needs to be replaced

    Figure 3. The fluid show in this thermometer has separated and the unit needs to be replaced

  4. Thermometers
    Lastly, take a look at the thermometers. As seen in Figure 1, if corrosion or build-up of residue has encrusted the bulbs, ask for a replacement unit from your local PMO. This corrosion can be avoided by giving the unit a good fresh water wash-down on a regular basis. Just remove the thermometer set from the Stevenson Screen and run the entire unit under a stream of fresh, cool water for several minutes. This will help dissolve the salt spray and keep corrosion under control. Dry the unit off with a clean cloth and replace back in the screen. Also, look closely at the thermometers. If any bubbles develop in the alcohol fluid (Figure 3), request an immediate replacement of the unit. These bubbles develop over time and will result in the thermometer giving a false temperature reading.

So, by taking a few minutes a week and performing some very basic maintenance steps, you can ensure your ship is reporting the most accurate temperature data possible. If you need replacement parts, please don’t hesitate to contact your local PMO for assistance.

PMO Dave Dellinger

Dave Dellinger has been PMO in Miami since November of 2008

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