Category: Analysis

Anything that I have tested empirically, or explain with detailed calculations, or just generally bother to get all the facts straight.

Post 1798: Forest fire smoke and easy air cleaning.

 

With smoke from the Canadian forest fires continuing to generate air pollution alerts in the U.S., my wife suggested that I re-up my articles on using a box fan as an air cleaner.

This is a re-telling of Post #1792 and Post #1794.  Refer to those posts if you want more background information.

Three simple points

Point 1:  A standard 20″ box fan and a high-end 3M Filtrete HVAC filter together make a simple and effective air cleaner.  Get a 3M 1900 filter (rated MERV 13), place it on the back of the fan, and turn the fan on.

The key here is that the 3M electrostatic filters produce little “back pressure” or resistance to air flow.  That’s why you can have the low-powered fan draw air through that filter and still have significant air flow.

You can do the same thing with standard high-resistance MERV 13 filters, but you would need to construct a “Corsi Box” to provide enough surface area.  That is, tape four together into a hollow box, to provide enough surface area to allow for adequate air flow.

The 3M filters are expensive, but in my experience they last for months.  Arguably, this being almost July, you’d only need one for the entire summer.

 

Point 2:  This is more effective than a typical room-sized HEPA filter.  The reason is that with heavily-polluted outdoor air, filtering a lot of air reasonably well (fan + filter) beats filtering a small amount of air extremely well (HEPA unit).

Above is the labeling on that Filtrete (r) 1900 filter. In a single pass through the filter, it removes

  • 62% of the tiniest particles (0.3 to 1.0 mircons)
  • 87% of the mid-sized particles (1.0 to 3.0 microns)
  • 95% of the larger particles (3.0 to 10 microns).

That’s nowhere near as good as a HEPA filter, which removes on-order-of 99.97% of all such particles in a single pass.

So why does the fan + filter win?

First, outdoor air infiltrates into indoor spaces at a fairly rapid rate.  Typical tight older construction has one air exchange per hour.  That is, every hour, enough outdoor air enters the building to replace the entire volume of indoor air.

In the current situation, that means smoky outdoor air is more-or-less pouring into your living space, continuously.  Even with the windows and doors shut.

Second, a box fan moves a lot more air per minute than a typical room-sized HEPA unit.  A box fan on high can move about 2000 cubic feet of air per minute.  Depending on the fan, a box fan on low can move on order of 1000 cubic feet per minute.  A typical room-sized HEPA unit might move just over 100 cubic feet per minute.

The end result is that the slower HEPA filter can’t keep up with the steady inflow of dirty air.  Or, more properly, can’t keep up as well as the fan-and-filter combination.

On the left, you see the results of a numerical simulation of the two types of filtration.  Left is the box-and-filter, right is a typical HEPA unit.  Horizontal axis is time, vertical axis is the density of particulates in the air.  (See prior post for full details of simulation).

The equilibrium level of particulates in the room is vastly lower with the high-volume, lower-efficiency filter (left graph above).  Why?  Because the slow pace of the HEPA filter (right graph) can’t keep up with the level of outside-air infiltration that is typical in older construction.

Point 3: Availability.  As we learned during the pandemic, if there’s a sudden surge in demand (e.g., for N95 respirators), the shelves are soon stripped bare.  So if everybody goes out looking for an air cleaning device, those will soon become unobtainable.

As of today, my local Home Depot has well over 100 20″ box fans in stock, on the floor, ready to be purchased.  By contrast, they have just five room-sized HEPA units in stock. 

Which makes sense.  Those fans are commodity items costing about $25 each.  The Honeywell HEPA unit, by contrast, goes for just about $300.    Home Depot couldn’t afford to keep 100 of those in stock, on the off chance that there might be a run on air cleaners.

Summary

Sometimes, simple and cheap is what you want.  In this case, a box fan and a 3M 1900 air filter together cost much less than a room-sized HEPA filter.  And in this situation — where you are trying to filter pollution arriving from outdoor air — the much higher air flow of the fan-and-filter combination actually works better than a typical HEPA air cleaner.

Nothing prevents you from dealing with this problem by wearing an N95 respirator inside.  But note from the simulation above, the fan-and-filter combination provides air that is almost as clean as you would get, breathing through an N95 respirator.  So you get almost the full benefit of that, without the hassle of wearing a mask 24/7.

As a bonus, while the mask protects your lungs, the fan-and-filter combination protects both your lungs and your eyes.  If eye irritation is an issue for you, filtering the indoor air is the only way to go.

Post G23-031: The iron law of back-yard gardening.

 

Nature doesn’t merely abhor a vacuum.  Nature is a vacuum.

You name the delicious food plant that you’d like to grow, and I’ll find a pest that will hoover it up before you can.

Thus, the Iron Law of Backyard Gardening:

Anything that can be eaten, will be eaten.

Plan your garden accordingly.

Machine-gun emplacements optional

I had a friend over yesterday, during which time I showed him the various deer defenses for my garden area.  These are pretty much in line with my original garden plan for the year (Post G23-009), and include:

  • A square enclosure,
  • fully surrounded by a 6′ trellis,
  • which is in turn surrounded by an electric fence,
  • with motion-activated water squirters inside,
  • plus a handful of Wireless Deer Fence units,
  • a motion-activated light.
  • And some chemical deer repellent, held in reserve for the fall, when the deer get really aggressive.

I can see where some might think this is a bit extreme.  But, so far, this seems to be keeping the deer out of my garden. I’ll settle for that.

Outside the defensive perimeter, the only survivors are plants that deer won’t (typically) eat.  This year, that’s mustard, various “deer-proof” flower mixes, marigolds, and zinnias.

I don’t plant that because I’m particularly fond of it.  I plant it because the deer aren’t.

A multiple-choice question for testing your gardening maturity.

When you consider planting sweet corn in your garden, the first thing that comes to mind is:

  1. How great fresh corn will taste.
  2. Where’s the best spot to put it.
  3. How to keep the @#$)$(# squirrels (raccoons, deer) from eating it.

Score your gardening maturity level as follows:

  1. Novice
  2. Intermediate
  3. Veteran

Why?  Because if you don’t have a good solution to c) above, a) and b) just don’t matter.

This is no mere academic exercise.  I’ve been thinking about planting a little Silver Queen sweet corn in that garden.  It ticks all the boxes for something I’d like to grow.  My whole family likes it, and we can’t buy it around here, not even at the local farmers’ markets.

The old me would have simply cleared some space, planted some seeds, watered and fertilized according to directions … and hoped.  But in the squirrel-infested suburbs, I suspect that all I’d be doing is buying myself a ton of heartache down the road.

Why?  See the Iron Law of Backyard Gardening above.

Probably the existing electric fence etc. will keep the deer out.  Maybe the squirrels won’t recognize it as food.  Maybe the local crows won’t discover my tiny batch of corn.  Maybe my cucumber beetles (which double as the Southern Corn Rootworm) won’t find it.

But when I add all those up, the likelihood that I’m going to get to eat that sweet corn is pretty slim.  Absent some fairly strong and pro-active defensive measures.

Before I put a seed in the ground, I have to work out how am I going to keep the squirrels off my corn.

All else is folly.

I dis the sisters

The first thing I came across is the traditional “Three Sisters” planting method, above.  It’s actually pretty sophisticated, in that you don’t just randomly inter-plant corn, beans, and squash.  Instead, the squash is planted to form a defensive ring around the corn/beans plot.  With the idea being that (e.g.) raccoons and squirrels don’t like pushing through the prickly squash leaves, and so will leave your corn alone.

Funny thing about it, though.  As with so many things for the home garden, a lot of people repeat that story.  Almost nobody tests it.  And almost nobody reports the results of that test.  I found exactly one individual who tried it, and said it was a miserable failure for keeping squirrels out of the corn.

And I believe that, because my squirrels had no problem at all waltzing through my cucurbits in order to gnaw on my winter squash and pumpkins.  What finally put a stop to that was wrapping the pumpkins in floating row cover, which, apparently, led the squirrels to forget that there were pumpkins there.  Or something.

(Upon reflection, it’s entirely possible that modern squash varieties are a lot “tamer” than what Native Americans would have grown.  A thornier squash might in fact make a pretty good barrier against squirrels and raccoons.)

So the idea that a row of squash plants is going to deter them from going after a prize like sweet corn, that’s just fantasy.  Might work somewhere, but I’m not staking my corn crop on it.

And then for every other “home remedy” approach to keeping squirrels off corn, you can find plenty of people who tried it and had it fail. Hot pepper powder.  Peppermint oil.  Chemical repellents.  And so on.

Near as I can tell, my options are to grow my corn in a squirrel-proof cage (including six inches under the soil, and a fence roof on top), or produce enough of an electric fence/net that the squirrels can’t get over it to get to the corn.  Or, alternatively, exterminate my local squirrels, or use a squirrel-hating dog as a garden watch dog.

Given that I already have an electric fence driver, I believe that if I go through with growing Silver Queen, I’m going to surround it with a mesh-type electric fence.  If I can find or make one cheaply enough.

This is my gardening life

In hindsight, this really and truly reflects how I garden now, after just four years of intensive back-yard gardening.  Each year, what I grow is a smaller and smaller subset of what I’d like to grow.

My vegetable garden increasingly consists of plants that can fend for themselves, and those that I can feasibly protect.  Poisonous leaves (potato, tomato)?  Perfect.  Painful thorns (cane fruits)?  Ideal.  Or plants that are mostly left alone, as long as I can keep the deer off them (peas, beans, sweet potato, cucurbits).  That’ll do.

But sweet corn?  No natural defenses.  Attractive to everything that flies or walks through my yard.  Tall enough that fencing it in is a chore.  Plus, plentiful insect pests.

With sweet corn, it’s not a question of whether something will eat it.  Just when, and how much.

If I can figure out some cheap and feasible squirrel defense, I’ll give it a try.  But if that’s a crash-and-burn, I’ll buy my corn at the store just like everybody else.  And move on.

Post #1797: Rethinking mail-in voting in Virginia

 

I’m not one to bash the USPS.  For two decades, my business-related financial transactions went through the mail.  The only time a check ever got “lost in the mail” is when a client sent it to my prior address.  To within rounding error, in all that time, the USPS had more-or-less a 100% success rate.

But for this post, I’m going to make an exception. Continue reading Post #1797: Rethinking mail-in voting in Virginia

G23-030: Shocking mustard. Maybe not the best idea I’ve ever had.

 

Ever get partway through a task and thought, hmm, maybe this wasn’t such a great idea?

Such was today’s task, making mustard shocks.  That is, bundling mustard stems together so that the mustard plants would stand upright to dry, rather than lying on the ground. Continue reading G23-030: Shocking mustard. Maybe not the best idea I’ve ever had.

G23-026 Winnowing, or, Rube Goldberg does agriculture.

 

On the plus side, I bet you didn’t expect a blog post about winnowing.

On the down side, this entire blog post is about winnowing.

Image above: Winnowing Grain, Eastman Johnson, 1879, via https://www.wikiart.org/

Continue reading G23-026 Winnowing, or, Rube Goldberg does agriculture.

G23-025: Springtime drought in the DC area. What can you do?

 

Here in the Washington DC area, we’ve been flirting with drought conditions all spring.  By mid-April, we were at the center of a little isolated area of drought.  Now, we sit at the southern edge of an area of moderate drought extending from Canada southward.   This, per the National Drought Monitor:

Continue reading G23-025: Springtime drought in the DC area. What can you do?

G23-022: Ladies and gentlemen … the beetles.

I hung a couple of bag-a-bug (r) Japanese beetle traps yesterday.  The scholarly literature suggests that these do more harm than good.  I believe the opposite.  So, every year, I hang two traps near my garden.  I think they keep the Japanese beetle population down, if used correctly.  Follow the instructions, hang them well away from and downwind of the space you are trying to protect.  The idea being that as beetles fly upwind, lured by the scent of your delicious landscaping and garden plants, they will be diverted by the lures in these traps and DIE DIE DIE.

But this post isn’t about Japanese beetles per se.  It’s about growing degree-days.

Growing degree-days and my pest calendar

Source:  NC State University growing degree days explorer.

I used to think that various insect pests arrived on or about some fixed calendar date every year.

That’s not exactly correct.  As it turns out, various species emerge, pretty much like clockwork, after a given amount of springtime warmth has occurred.  That warmth is typically measured by growing degree-days with a 50 degree F reference point.  In effect, it’s an estimate of the cumulative time and extent to which the air temperatures in an area exceed 50F.

Both the Japanese beetle and the squash vine borer show up right around the 1000 growing degree-days.  Once you’re aware of that fact, you can pretty much set your calendar by their arrival.  Last year, they were right on time (Post g22-023, Post g22-024).

Last year, my first Japanese beetle occurred on June 18.  But this year is running a bit cooler than last.  Which means a bit later than last year.  Based on growing degree days, we’re about 100 degree-days behind where we were last year.   Which, at current temperatures, should be about four days.  That means I ought to see my first Japanese beetle on or about June 22 this year.  And my first squash vine borer not long after that.

So I have my Japanese beetle traps up now.  I can forget about them until it’s time to take them down and dispose of them.