Thursday, January 4, 2018

Insulating pad on window in the door: monofilament holds it in place

We often camp in pretty cold conditions – subfreezing but not sub-zero.  On a recent winter trip, we realized that the frosted window inset into the trailer door was losing a lot of heat.  So when I got home, I rigged up a way to hold a foam insulating pad on the inside of that window.  I think that other folks who camp in cold weather might find this useful.

As I do on our other windows, I used foam flooring material (from Costco), which comes in interlocking panels (about 2 feet square), cut down to fit the window opening.  The foam is about a half inch thick and is fairly stiff.  The window in our trailer's door was about 21 inches by 15 inches.  (Actually, I measured it in millimeters to give myself the illusion of precision.)  I cut the pad to size with a box cutter:

The next issue is how to hold the pad in place?  I thought about Velcro or magnets (my best friends) but decided to string four strands of monofilament fishing line (10 lb. test, I think) across the window to contain the pad.  Here is a shot of the pad on the door:

And this is a close-up of one of the monofilament cross-strings:

In order to create the cross-strings, I first took out a screw (one at a time!) and stuck one end of the string into the hole:

(By the way, please excuse the greasy fingers -- that's what my hands look like on a "garage day.")

I then screwed the screw back into the hole, trapping the end of the monofilament:

After tightening that screw, I removed the screw on the opposite side of the frame.  I cut the monofilament to be about 3/4 of an inch too long and then stuck that end of the line into the screw hole.  As I tightened the screw, it tightened the monofilament, taking up the slack.

In order to install the insulating pad, just slip one end under the lowest string and slide it up into place.  Remove it by sliding it down.  We store it under one of the dinette cushions -- a little extra padding never hurt anything.

Sunday, December 31, 2017

Wooden/Magnetic "Door Halfway-Open" Holder

This is a device to hold the door open halfway in breezy conditions. (It will not work in a high wind, but it works most of the time.)  We need to hold the door open when we are cooking, in order to ventilate the trailer.  We often do not want to open the door all the way -- too cold!  So this does the job without letting in too much cold air.

It is just a curved piece of oak lath (steam bent), with strong magnets at each end.  (I made an earlier version of this out of metal -- it was clunky and hard to deploy.)  It's curved in order to approximate the arc of the door jamb as it swings open -- a straight piece of wood would not do the job. (I tried it already.)

The magnets are on opposite sides of the lath.  One magnet sticks to the strike plate on the door frame, and the other sticks to the metal plate on the door that surrounds the bolt.  The magnets are taken from old computer hard drives.  The mounting plates already had screw holes, but I enlarged them slightly.

Here is the curved lath:

This shows how the magnets are attached to the lath.  The screws are very short -- about a quarter inch.  Note the green tape -- the ends are color-coded for convenience:

This is the inside end, which makes contact at the strike plate on the door frame -- note the red tape:

This is a view of the holder from the inside of the trailer:

And this is a view of the holder from the outside:

When not in use, the holder sticks to a small metal plate on the wall of the trailer, just inside the door.

Sunday, November 20, 2016

A Lighter Dinette Table (Baltic Birch Ply Instead of MDF and Formica)

The stock dinette table weighed a lot -- I think around 30 pounds.  Very awkward to lift when making the bed at night!  So I fabricated a lighter version, using half-inch Baltic Birch plywood.  I finished it with three coats of a wipe-on polyurethane.  We've been using it for over a year, and it has held up well.  It's about half as heavy.

Here is what it looks like when set up for dining:

This is an overview (underview?) of the underside:

And this is a close-up of the socket -- I glued a circular pad of plywood under the table and then screwed the socket into the pad, so that the screws would not come through the tabletop:

Tuesday, November 1, 2016

How to test the working capacity of the batteries

I have two group 31 12V marine deep cycle batteries, which supposedly have a total capacity of 110 amp hours.  In the real world, that means that they have a theoretical working capacity of only 55 amp hours, since I am told that it is not good to draw the battery down below a 50 percent state of charge.  When the measured voltage gets down to 12.1, that's a 50 percent state of charge.

So I wanted to see if I really have 55 amp hours available.  That information is useful for a couple of different reasons: first, if I do have that much available, that tells me that my usual battery maintenance routines are adequate.  If I don't, I have to do something different.  And this reading provides me with a baseline, so that I can tell when the batteries are starting to get old.  Finally, if there is a material difference between my two batteries, that would be interesting, since they were purchased at exactly the same time and have been used in exactly the same way.

(As you'll see below, the results were not what I was expecting.  Hint: this story turns out well.)

My overall plan was to hook up a lamp and then to see how long it took to draw the battery down to roughly 12.1.

I started with a fully charged battery  (which reads 12.9 V when it comes off the charger) and then let it rest for a day, so that the initial reading was 12.7 volts. 

I then hooked up a 60 watt incandescent bulb, plugged into a small inverter, which was plugged into a "cigarette lighter socket" adapter, which has alligator clips that go to the battery terminals.  (If you don't have one of those adapters, they are really handy when you want to hook a 12 V appliance directly to a battery.)  I then used my multimeter to find out how much current the bulb and the inverter were drawing, which was 6.1 amps.

(If you already know how to measure the amount of current that a device is using, skip this paragraph.)  Put the red (positive) multimeter plug into the "10 amp" socket on the front of the multimeter.  Turn the multimeter on to the 10 amp setting. It should read "zero."  With the lamp still plugged into the adapter, unhook the cigarette lighter adapter's black alligator clip from the negative terminal of the battery.  Touch the multimeter's black lead to the negative terminal.  Touch the multimeter's red lead to the unhooked black alligator clip on the cigarette lighter adapter.  The meter will display the amperage.

I left the light on for two hours, thus consuming 12.2 amp hours. I unplugged the light and let the battery rest before taking a reading.  I was told that it had to rest for two hours to settle down.  But I discovered that after a half hour of rest, the voltage had plateaued and did not change.  So for the rest of the experiment, I let the light run for two hours, followed by a half hour of rest, at which point I measured the voltage and then plugged the light in again.
Here are the results in a crude graphical format:













Time            2hr      4hr          6hr          8hr       10hr      

So this means that after ten hours of actual run-time (consuming 61 amp hours), the battery got down to 12.2 volts, i.e., with 60 percent of capacity still remaining.  By extrapolating from this straight-line graph, it looks like I could have gone two more hours, for a total working capacity of 67 amp hours.

That is a lot better than the 55 amp hours of working capacity than I was expecting!

I did this experiment twice, once with each battery, and got exactly the same results.  This tells me that this wasn't a fluke.

I am not sure how it is possible that my batteries are outperforming their rated capacity, but I'm not complaining.  This won't change my consumption patterns when we are camping – we are very careful about electricity.  But this is encouraging news, and it gives me a baseline for subsequent comparisons.


Tuesday, July 12, 2016

Magnetic "Door Halfway Open" Holder

Whenever we cook (which is twice a day), we have to open the windows and the door in order to get enough air – otherwise, the propane/carbon monoxide alarm goes off.  In warm, dry weather, that's no problem – we just open the front door all the way, we step outside, and we latch it open.  But in cold or wet weather, I would prefer not to have to go outside, and we often don't want the door open all the way.  But there is no latch for holding the door open halfway.

So I came up with a rather simple "Mickey Mouse" solution.  I mounted very strong magnets on either side of a short strip of flexible steel.  One magnet attaches to the bolt assembly on the door, and the other magnet attaches to the strike plate.  Here's what it looks like when deployed:

The magnets are neodymium or rare earth magnets, salvaged from old computer hard drives.  (I have a friend in the computer business, but I would bet that you can get these things from any local computer repair shop.)  The magnets have holes in the mounting brackets; using a drill press and some care, it is possible to enlarge the holes to accommodate very small machine screws.

I used a mallet to flatten out a 1 foot strip of galvanized roof flashing.  I then drilled holes for the magnets and put them on either side of the steel strip:

Note that on one side, I mounted a small piece of wood, just to keep the strip of steel from flexing too much.

I then added an extra piece of perforated steel to the inside of the door frame, above the existing bolt assembly -- this is to provide more surface area for the magnet to grab onto: 

It was necessary to use a flexible steel strip, rather than something more substantial, because the bolt assembly on the doorframe and the strike plate are not exactly in the same plane.  So each end of the steel strip flexes slightly, to accommodate this minor disparity:

The whole thing can be deployed from inside the trailer:  I open the little hatch in the screen door next to the latch.  I stick the far end of the holder out onto the bolt assembly.  I then attach the near end of the holder onto the strike plate.  Here is what it looks like from the inside:

These killer magnets seem to be strong enough to hold the door in place, even in a fairly good wind.  But I haven't field-tested this yet, so we will see how it does in the typical strong afternoon Sierra breeze.

Finally, for storage, I screwed a small piece of perforated steel on the inside wall of the trailer, next to the front door frame.  The magnet sticks firmly onto that little piece of steel.