## Saturday, December 30, 2023

### Another day in paradise

I gained weight

The bad news is that my weight ballooned two pounds over the holidays.

Back on-track with weight lifting

The good news is that I was finally able to execute four sets of ten-repetitions of 190 pounds. I used the straps. Experience taught me that I need a five minute of recovery between each set. My plan is to repeat this performance two more times with an appropriate number of recovery days between sessions. Then to bump up the weight another five or ten pounds.

Physically, the lifts were not overly challenging. I was able to get it off the ground without straining excessively. I had enough extra strength to play-around with the repetitions. It went more smoothly, for instance when I used delts and lats add speed to the bar just before getting to knee level.

In general, I am not very self-conscious but I had to wonder what the athletes using the other equipment thought of the Shrek-like shape deadlifting.

Drywall

Mrs ERJ remembered that I have very modest skills at repairing drywall. In this case, "very modest" means that more than 50% of the time the repair looks better than the original damage.

So, I now have some assignments around the house.

Convection Cells

One of the quirks of using a dead-air space as an insulator is something called Convection Cells.

Picture in your head our hero sleeping on a cot suspended 8" above the ground. Consider that the ground and the ambient air is 40F, our hero is 95F. A one-dimensional model would assume a linear top-to-bottom gradient from 95F at the cot to 40F at the floor. With me so far?

What happens when you introduce the second dimension? Let's assume the temperature gradient at the center-line is the same as the one-dimensional model. The first-order approximation of the temperature immediately adjacent to the skirting that separates the dead-air space from the ambient air will be the significantly colder than the 1-D model. In fact, if there are drafts in the ambient space, it will be closer to the ambient temperature than to the 1-D model.

Cold air sinks. If the cot is standing in free-space, then two convection cells will arise where air rises in the center of the dead-air space beneath the cot and air cools-and-sinks at the skirts. Instead of a static, gentle temperature gradient, a convection model where the warmest air next to the bottom of the sling is scrubbed away by moving air comes into play.

That is why you want to fill that "dead air space" with something...waste bubble wrapping, popcorn, crumpled newspapers, beverage containers... almost anything.

#### 5 comments:

1. Is this not why beds have bed skirts? I keep trying to convince my bride to bring back Victorian bed curtains but she is not having it.

2. You assume then, an infinite cold-sink under the sleeping person. It isn't that simple. Once you warm the floor a degree or two, then the model changes drastically. As stated above, a simple bed-skirt will make a big difference. Yes, insulating the ground will also make a difference, as will preventing the circulation. Even better is a second layer between you and the ground, acting as a thermal barrier. THis way, the ground sees the barrier and the barrier see you, a much less transmissive system.
As you say, it really matters little what you fill that space with. almost anything will help a bit.

3. LOL, I read that and thought about the 'rainfall' in the blimp hangars...

1. Or the clouds and rain that commonly happened in the 500+ foot rocket building at Kennedy Space Center.

4. Many years ago someone with expertise and experience in the matter explained that "insulation is the encapsulation of a dead air space but to have dead air you have to kill it first."
Stop the infiltration and convection movements or your insulation will not accomplish what you want it to.

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