How on earth did this project cost $100 when cast acrylic costs $$$$?
April 19, 2013 3:56 AM   Subscribe

Quick question. Say you were building this algae reactor, which the guy claims he built for 100 bucks. But you're pricing out the same cast acrylic tubing and it costs more than $85 a foot, putting the price of the tubes at hundreds of dollars. Is there a secret source of 3' long, 3.75 OD cast acrylic tubing? (By the way, it has to be cast acrylic. Not extruded.)
posted by sunnichka to Science & Nature (10 answers total) 1 user marked this as a favorite
Why don't you ask the author? Also, I doubt that cast/extruded makes so much a difference as wall thickness and lack of imperfections. McMaster Carr is a little better on the price, with cast 3.75" acrylic tubing at $36.86 per foot for their thickest walled (1/8"). Looks like the expensive tubing in your link has 3/8" walls and figuring (OD/ID)/2, the project uses 1/4" walled.
posted by exogenous at 4:13 AM on April 19, 2013 [1 favorite]

The price quote you linked is for 0.375" wall thickness tubing, but the project text specifies 0.250" tubing. But based on the project photo, the tubing he actually used is 0.125" tubing.

Specifically, this photo does not look like 0.250" thick tubing. It looks about half that thickness. (I used an image editing program to count the pixels and do some math.)

I suggest you contact the project author, verify the tubing dimensions, and ask him where he bought it. I imagine small quantity tubing prices vary quite a bit.
posted by ryanrs at 4:15 AM on April 19, 2013 [2 favorites]

By the way, cast acrylic is stronger than extruded, so don't be too quick to write off the difference. The plastic dip stuff might be a factor, since extruded acrylic doesn't like solvents.

I'm not saying extruded definitely won't work, but the project author's preference is plausible.
posted by ryanrs at 4:26 AM on April 19, 2013 [1 favorite]

Try Tap plastics, they also have polycarbonate (lexan) tubes, which can be much stronger (in some applications). You might also look into clear PVC pipe. Chemistry labs and other industrial applications use it for drain pipes so that waste water can be monitored. It's nice because you can use standard pvc pipe fittings and glue, and it has a pressure rating. For situations with acid waste, glass is used, so that may be another material.
posted by 445supermag at 6:10 AM on April 19, 2013

For growing green algae, you will want something that passes a lot of blue light at 400-450 nm. I haven't looked into it too much, but this link suggests that polycarbonate might be worse than acrylic.
posted by exogenous at 6:30 AM on April 19, 2013

Could be he bought used/surplus tubes.
posted by cardboard at 7:14 AM on April 19, 2013 [1 favorite]

Seconding surplus. The local surplus store has lexan/acrylic way cheaper than it is "new"
posted by thylacine at 7:40 AM on April 19, 2013

It should be easy enough to get the pressure rating for any piece of tubing, and not terribly hard to calculate the pressure it's going to be under (most of it is going to have to do with the height of the water column unless you're being super aggressive about aeration).

I'd check his assertion that extruded won't do with some math. It's possible he got a crappy length of tubing.
posted by Kid Charlemagne at 10:35 AM on April 19, 2013

I don't think pressure is the real issue - in my experience with the stuff, the extruded variety just doesn't last very long when exposed to sunlight. I suspect that the extrusion process leaves it with stresses and micro-cracks/crazes that quickly get worse when subject to daily heating/cooling (& temperature gradients across/around the tube wall) & possibly UV exposure.

(Though, a quick bit of math in my head suggests that a 2' tube would be subject to ~1.25lb/f2, which is just under the ~1.5lb/f2 typically used as a rule-of-thumb for 1/4" thick extruded acrylic. So maybe he's right - it'd certainly be worth double-checking my math & typical acrylic specs, to be sure…)
posted by Pinback at 3:03 PM on April 19, 2013

Looking at this as an engineer, I think the must-be-over-2'-long versus must-be-strong-enough criteria have another solution.

Build the tubes horizontally, with a slight cant to one side. Gas will still collect at one point; you'll have the length desired (which is really an area issue, not a length one), and the water pressure won't be nearly as high. You may need to support the tubes midway, but that's pretty simple.

With a lowered need for wall strength, you can get thinner (cheaper) tubing.
posted by IAmBroom at 5:41 PM on April 20, 2013

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