blueprince PC tek
Posted 29 April 2012 - 12:10 AM
How to use a 6" in diameter, 304 grade, 18/8 gauge stainless steel stock pot to make a +9.3psi PC safely.
I've scoured the interwebs for a way to make a PC because of its prohibitive cost in my country. I can't even find 1 for sale at retail that goes to 15psi. The ones that are for sale have no indication of psi which i take it to mean they're in the 7-9psi range. Even these cost just too much for me so i took it upon myself to make a *safe* and cheaper alternative. Unfortunately i have yet to get my DIY PC up to 15psi, but 9.3psi isn't too bad; just have to sterilize a bit longer is all.
(I've since gotten a 18lt china-made model that works great)
Disclaimer: If anyone attempts this, you *must* do the math & the testing. The strength testing of the clips you use is crucial to accurately gauge the pressure range.
What you need:
- 6" Stock pot, 304 18/8 stainless steel
- Measuring tape / ruler
- Calculator or you can go here: http://www.csgnetwor...calculator.html
- Some brain power
So, how do we go about this? Simple, we need to know how much pressure is being applied on the lid of the pot at 15psi assuming it has been sealed and there is a safe failsoft system to regulate excess pressure. Here's how:
1. Measure the diameter of the pot lid from the edges where its exposed to the inner pot. For this exercise we'll assume 6". We'll use this measurement in the steps below.
2. Calculate the surface area: area = PI * (diameter / 2) * (diameter / 2) or
22/7 x 3 x 3 = 28.3 square inches (rounded up)
3. Calculate the pressure at 15 psi: 28.3 x 15 = 424.5lbs of pressure on the lid at 15psi
or 193 kg (rounded up)
4. Calculate the circumference: PI * diameter
22/7 x 6 = 18.85" (Rounded down)
Now you have 3 very important pieces of information required to convert the stock pot into a PC but you're still missing 3 more pieces. Before we go into that though, i want to bring up the fact that while there *is* 193kg of pressure on the lid at 15psi its *still* just 15psi and that is well within the parameters of 304 18/8 stainless steel. So no, the pot will *not* explode.
The next things you'll need are:
- High temperature silicone sealant. I use the type rated for automotive repair which resists heat up to 650f.
- Lubricant of choice. I use regular vegetable cooking oil.
- 2-3" length of wire, does not matter what kind. I use a straightened paper clip.
- Precise ruler, those small metal ones are great for this.
- Sharp blade
1. Turn the lid upside down and apply a thick layer of silicon to the lip.
2. Coat the lip of the pot with lubricant all over where the silicone will meet it
3. Leaving the lid upside down, turn the pot over and put it on the lid. No need to press down, gravity will do the job for you. Just make sure its level. Leave to dry.
4. Once dry, run the blade carefully around the edge to remove excess silicone
5. Separate pot and lid. Check to make sure there are no rough spots in the silicone which could indicate a gap. It should be uniformly smooth all the way around. Little air bubbles are acceptable.
6. Put the pot right side up and put the lid on. Using the wire, bend it to measure the thicknesses of the pot lip + silicone + pot lid. For the purpose of this guide, lets say 2.5mm.
Now you have the 4th piece of information; the thickness. Just 2 more to go. The next thing you need are strong clips. I use this kind, black binder clips:
- Strong clips of choice
- Measuring scale
- Weights, i use a combination of old weight lifting weights and stuff lying around the house.
- Precise ruler
1. Tie of 1 end of the clip to something sturdy and tie off a string to the other end of the clip.
2. Hang a weight off the free hanging string and and measure how much the clip has opened. You want to get a precise weight to make the clip open exactly to the combined thickness of the pot lip, silicone and lid combined. Use the measuring scale to determine exact weight. For this guide we'll assume it to be 10kg.
3. Measure the width of the clip. For our purpose, we'll assume 1.5".
And that's it, you have all the information you need. Now to use it:
- Pressure @ 15psi = 193kg
- Strength of clip = 10kg
- Circumference of lid = 18.85"
- Width of clip = 1.5"
1. Divide circumference by width of clip: 18.85 / 1.5 = 12 (Rounded down)
This is the number of clips you can fit around the pot.
2. Multiply by strength of clip: 12 x 10 = 120kg
This is the max pressure the clips will withstand before steam begins to escape.
3. Calculate the percentage of 15 psi this represents: 120 / 193 * 100 = 62.17% (Rounded down)
4. Calculate actual psi: 62.17 / 100 * 15 = 9.3255 psi
With all the rounding up and down we've done + the fact we didn't take the weight of the lid into account, *should* account for the increase in metal flexibility at higher temperatures.
To finish it, we need to put in a pressure release system so the pressure can be released after use so as not to cause a vacuum. For this, i'm using the same coupling system as on my pot still except instead of a copper pipe leading out to a condenser I've got a 1.5" length of 1/4" copper crimped and then brazed closed.
After you're done sterilizing, wear an oven mitt and use a wrench to turn the fitting until the pressure is released.
When its in use, you'll notice that when the pressure builds too high for the clips, 1 edge of the lid will gently open enough for steam to come hissing out. If you always put the lid on the pot in the same position, the steam will always escape from the same spot. You can attach a deflector on the lip of the lid for the escaping steam to condense on. This could be as simple as a piece of aluminum foil and duct tape or go as far as a metal bracket bolted to the lid with a welded on sheet metal deflector. I'd go with duct tape and aluminum foil myself.
Great place you all have here; I look forward to being a part of the community.
Posted 30 April 2012 - 02:29 PM
i cant endorse this one..............
Posted 30 April 2012 - 02:46 PM
Posted 30 April 2012 - 06:14 PM
As mentioned, i'm now the proud owner of a 18lt gasket PC which is fantastic and cost about USD100 (Stuff is expensive where i am; think other side of the globe). The original design used a pot still i had lying around to make essences so all i really had to do was crimp and braze a 1/4" copper pipe shut; doesn't get more cost effective than free now, does it?
Eh, didn't really think anyone would *endorse* this; its a tek of last resort for when the great PC/canner extinction event happens :) But it worked for me when i needed it, so i thought i'd share.
Posted 30 April 2012 - 06:30 PM
Eh, when the great PC/canner extinction event happens
this exactly why i keep my engineering team on the speed dial.