Testing a cheaper laminar flow hood | Hacker News Reader

Testing a cheaper laminar flow hood

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64 pointssurprisetalk17 comments

This doesn’t get around the bulk, but at least in the metro Boston area, there are loads of auctions from closing labs or failed biotechs where you can get all sorts of lab equipment for pennies/nickels on the dollar.

If you have space for a commercial lab hood, the cost for used/auction is low, if you’re slightly patient. (This is partly because of the bulk, of course. Lots of people might bid on the small, benchtop equipment, but the giant equipment often goes for the minimum bid.)

I wonder who snatched up all the set equipment from Breaking Bad...

I have a legit laminar flow hood (airclean 600, 32 inch) in my home for plant tissue culture. I gotta say, I thought it was going to be easy street to sterile culture- it was not. Sterile protocol matters so much that I think it would have been just as easy to do what I needed to do in a glove box with all the extra precautions I had to take. It's honestly hard to beat an enclosed container with zero airflow.

> I left them exposed for 30 minutes. This is a ridiculous amount of time to leave agar plates open for, since usually you’d only open them for a few seconds.

It's actually really common to let them exposed that long. Not when using them, but to make them: once the agar medium is melted and poured in the petri dishes, closing the lid during cooling causes condensation. Having the petri dish full of water when using it is more difficult and annoying, so it's better to let them cool down with the lid open (a space-efficient way to do so is to have a pyramid of plates where each lid rests on two plates and can support one)

Also, two contaminations out of 4 plates sounds really really bad, but then the blog doesn't say how the agar plates were prepared (how many plates without exposures were contaminated?) and how long they were incubated (sometimes something starts growing after a week or two, if you're culturing a fast-growing bacteria then it's mostly irrelevant).

As someone who has spent an unreasonable amount of time mucking around in this exact swamp, here are some things worth knowing:

* HEPA is the extreme edge of a much larger scale called MERV, which rates filtration performance.

* You'd be surprised how few contamination threat models _mandate_ HEPA. If you're doing petri dish work, contaminants you care about are probably in the 1 to 3 micron range or larger.

* This means you can get adequate results from lesser filters, which has massive implications for cost (because filters are cheaper) and ergonomics (because the same dollar buys a bigger filter, which is less cramped to work in).

* Laminar filters are only laminar under specific circumstances, which include both pressure and distance. Too much or too little force and the output will be turbulent (non-laminar). Fans need be adjusted to get this pressure right, across the entire face of the filter.

* As you get farther away from the filter, air falls back into too-low speeds and becomes turbulent again. The lids in the "clean" test for this post are farther away than I'd put them.

* As the filter traps more particles, it becomes harder to push air through, so optimal fan pressure varies for every filter across time.

* The pressures we're talking about here are very small. It's normal for correctly pressurized airflow to be undetectable to a bare hand. Easy to overshoot.

* In theory, you can test this with an anemometer. In practice, I've not found a cheap one that will so much as _turn the prop_ under such a light load.

* A cheap but inexact test of laminar flow is a vertically held lighter's flame being pushed to a flickerless ~45deg angle.

* When working in front of a hood, movement technique matters. If you put a dirty hand in between your hood and your dish, you're still blowing contaminants onto the workpiece, you're just doing it in a straight line.

* Or a turbulent line: obstacles like the end of the acrylic sheet can cause air to whip around the edge and become turbulent.

* Gordotek has a video that's probably of interest here[0], where he shows a cheap filter blowing zero PPM, as measured by a swanky lab particle meter that costs more than most people's rent. He received an enormous amount of heat from keyboard warriors that said this is BS and doesn't work. Notably, the number of haters who came with particle sensor readings of their own is also zero PPM.

* A laser PM2.5 sensor with 99%+ accuracy in the ranges we care about (1+ micron) can be had for under $50. This moves your measurement -> result interval from days to seconds, making it feasible to test more parts of the filter.

* Owning such a sensor, I've seen a duct fan taped to a trash bag taped to an unfinned MERV16 blow every bit as clean as a real-deal university lab HEPA filter in the 1+ micron range.

[0] https://www.youtube.com/watch?v=lInfdAVvBts

> * In theory, you can test this with an anemometer. In practice, I've not found a cheap one that will so much as _turn the prop_ under such a light load.

Hot wire anoemometer ? Given it's rapid response you could probably even measure the turbulence in the flow. Probably possible to DIY too

The fires in Northern California the past few years have been a little better, but at their worst e.g. 2020, everyone had these box fans with filters taped on. I always wondered how effective those were at filtering contaminants ( I know they were effective at removing pollutants because I tested: https://igor.moomers.org/posts/minimal-viable-air-quality)

I'm sure it should help bring down the background levels of airborne pollutants, which should help as kind of a layered defence. But I expect it would be unlikely you could get it low enough to be in the same ballpark as a laminar hood, just because of the huge difference in air volume (the room is huge in comparison to the space the hood covers, so you need to filter way more, unless it was extremely well sealed in which case you'd just have to filter for long enough)

Ah, I've actually built one myself recently and it works great for plant tissue culture. Been using it for years now. Just used a commercial HEPA filter, a centrifugal fan, and a wooden box to equalize the pressure. Then it's a matter of regulating the voltage to the fan until you get the right amount of pressure to get laminar flow.

Poor man’s method is working right next to an open Bunsen burner. It actually works remarkably well.

For laminar flow, or for filtering of particles?

> I honestly don’t know why these things are so expensive. I’m guessing it’s a premium from the brand name, as well as quality assurance.

Being used mostly by medical (a small market with relatively rich companies) is the reason. It's a lot of testing to make proper one for not too massive market so the price is "eh, they can't be arsed to try to DIY that" basically

lol I wanted hood of a car flow. This is cool, but not that cool

I always thought these hoods were for sucking away germs or fumes so they didn’t get out into the room. Nope. That’s backwards. Trying to keep the sample clean here.

Yeah intern contaminated by samples is easier to replace than the samples ;)

Grad students are more plentiful, and come prepaid and pretrained.

There's extraction hoods that are aiming to do the opposite, it depends on what you're more concerned about. (doing both, is of course, much more annoying)

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