David Elfstrom Profile picture
Nov 7, 2021 10 tweets 4 min read Read on X
#corsirosenthalbox flow tests for Lasko 3733 and 3M Filtrete 1900 20x20x1 filters.
Shroud optimization: 20 cm radius (15.75" diameter) /1
Effect of shroud size on outlet flow. Having a shroud is important, but don't stress over it. Without it, air flows backwards at the corners instead of through the filters. /2
The supply voltage makes a difference! Fan speed changes linearly with voltage. Here's the flow range for the accepted 110-125V variation of the nominal 120 V North American distribution supply: /3
Like with flow, the variation in voltage supply directly affects the fan power consumption in watts. /4
But here's what we really want to know: How can we compare a homemade air cleaner made with quality furnace filters to a commercial HEPA air cleaner? Here's the estimated comparable ratings for this Lasko 3733 with 3M Filtrete 1900:
252 CADR, room size 390 square feet. /5
With these estimations in mind, variations of these air cleaners can be tested for performance in accordance with AHAM AC-1-2020 standard. NOTE that in more recent versions it specifies tests at 115 V. Earlier versions specified 120 V. /6
So if we were to do an independent comparison today, we would be testing at 115 V even though supply voltage to homes is 120 V or higher in practice. Here's the estimations at 115 V: /7
Smoke CADR is intended for removing cigarette smoke. What about for viruses? Can we use a larger target particle size? Thread asking for discussion: /8
Back to the shroud, I tested it without one at all, and the flow is lower than if there's the optimal size shroud running on speed 2 instead of speed 3. So adding a shroud is like getting an extra speed setting! /9
This is a 23% improvement over no shroud. When I did it before, I got 36% improvement. Why? The greater the resistance on the inlet, the more likely air will be drawn in at the front near the tips. Earlier test was done with only one filter.

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More from @DavidElfstrom

May 31
Vancouver Coastal Health has released an updated Ventilation and Indoor Air Quality resource for Schools and Childcare Facilities () but their CO2 page needs some edits. vch.ca/en/document-li…
States high levels of CO2 and other contaminants ... can contribute to [list of five health effects, such as headaches, fatigue, difficultly concentrating], says CO2 should be below 1000 ppm, then says (incorrectly) that health effects from CO2 occur at levels above 5000 ppm. No, those effects can occur below 5000 ppm.  It also states the CO2 concentrations do not indicate a risk of infectious disease transmission in a space. No, ASHRAE's position statement on CO2 states that elevated CO2 concentrations CAN reflect higher risk. Obviously, because it indicates poor ventilation, which IS an i...
Vancouver Coastal Health "CO2 concentrations do not indicate a risk of infectious disease transmission in a space". No.
ASHRAE's position document on indoor CO2 says "higher CO2 conc correspond to lower ventilation and potentially increased risk of airborne transmission"
Vancouver Coastal Health "Note that health effects from CO2 occur at levels above 5000 ppm". Did WorkSafeBC interfere? Because that's contradicted by your Health Canada reference in the sentence immediately before it.
Read 10 tweets
Mar 28
Three years ago on this day ASHRAE stated that Airborne transmission of SARS-CoV-2 is significant.

Today, WHO finally released a model () of #CovidIsAirborne risk by experts in the field. /1 iris.who.int/handle/10665/3…

Landing page for the WHO document titled: Indoor airborne risk assessment in the context of SARS-CoV-2: description of airborne transmission mechanism and method to develop a new standardized model for risk assessment
Four years ago on this day WHO said : "FACT: #COVID is NOT airborne."

Today, WHO finally released a model () of #CovidIsAirborne risk by experts in the field that warned of airborne even earlier, but were shut out. /2
iris.who.int/handle/10665/3…
WHO Tweet March 28, 2020 FACT: #COVID19 is NOT airborne.   The #coronavirus is mainly transmitted through droplets generated when an infected person coughs, sneezes or speaks.  To protect yourself: -keep 1m distance from others -disinfect surfaces frequently -wash/rub your hands -avoid touching your eyes / nose / mouth  Cover page of Indoor airborne risk assessment in the context of SARS-CoV-2
This document has been a long time coming. As described by @jljcolorado, Lidia Morawska, co-chair of the group that published the new WHO airborne model, was previously cut off by John Conly when making the case that #COVIDisAirborne to WHO. /3
Acknowledgements: WHO ARIA Technical Advisory Group:  First name: Lidia Morawska (Co-Chair) (Queensland University of Technology, Australia) (many other names) John Conly (University of Calgary, Canada)
Read 4 tweets
Feb 23
Air purifier manufacturers say HEPA should always be the filter of choice, and their product's proprietary filter delivers. Which HEPA? ISO 35H at 99.95% or ISO 40H at 99.99%? Why not ISO 50U? That's 10x better at 99.999%. Why stop there? Go for ISO 70U at 99.99999%! /1
The answer is, single-pass filtration efficiency DOESN'T MATTER except in specific cases like Powered Air Purifying Respirators (PAPR), clean rooms, operating theaters, or nuclear laboratory exhaust—HEPA's original purpose. /2
For portable/in-room air cleaners, all that matters is the Clean Air Delivery Rate (CADR) for a target particle size and type, within acceptable for sound power and frequency characteristics for the people in the room. /3
Read 13 tweets
Jan 3
Four years into this and we can't keep duct-taping in-room filter solutions for clean air. It's just filter(s) and a fan. We need open-source, optimized design, certifiable product, efficient, repairable using commodity filters and commodity components. /1
We need air cleaners assembled and distributed by not-for-profit community-based social enterprise. No more lock-in to proprietary filters. Verified replacement commodity filtration performance for safety. /2
Low income with donated CR boxes will pay over time in electrical costs for the duct-taped solution for clean air.
Power utilization for Smoke CADR, same filters:
Conventional CR Box: 4 CADR/W. (77 W)
PC fan array air cleaner: 24 CADR/W. (8 W)
/3
Read 4 tweets
Aug 28, 2023
1/ Levoit Core 400S versus Austin Air HM400 in a challenge to see which portable air cleaner removes submicron salt particle aerosols the fastest! Which do you think will win, and by how much? Poll in next tweet below... A large indoor tent with taped seams on a concrete floor is shown. On the left is a bench with instruments. A webcam is mounted on the top of a chair facing the instruments. In the middle are two air purifiers: The cylindrical Levoit Core 400S and the larger boxy Austin Air HM400. In the back right corner is a circular floor fan pointed diagonally upward into the middle. At the front right is a chair holding what looks like a tiny pitcher with a spout. A long thin clear plastic tube is connected to the side of this device, and exits the frame. Power cords are on the floor.
2/ Which has a higher CADR (Clean Air Delivery Rate):
Levoit Core 400S, or Austin Air HM400?
See if you can find the manufacturer's claims for both, and then come back and vote:
[sarcasm] Not only is the Austin Air bigger and far heavier, it also draws way more power, is much louder, and more expensive. It couldn't possibly be *worse* than the Levoit, right? Right?
Read 13 tweets
Jun 12, 2023
When to change the filter on DIY #CorsiRosenthalBox? Holder et al (onlinelibrary.wiley.com/doi/10.1111/in…) showed that performance degradation depends on contaminant: Dust reduces airflow. Smoke kills electrostatic properties. @JimRosenthal4 @joeyfox85 @CorsIAQ Table for MERV 13 single-fi...
@JimRosenthal4 @joeyfox85 @CorsIAQ Note the test substance for the CADR PM2.5 performance measurement was simulated wildfire smoke.
The same would likely apply to a commercial HEPA air cleaner. Monitor for clearing time for a fixed release of contaminant with a simple PM meter.
Read 6 tweets

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