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Offlineteladi
FUNKSOULBROTHER


Registered: 06/27/17
Posts: 1,189
Loc: South Africa Flag
Last seen: 1 year, 5 months
PCing at altitude * 6
    #25530374 - 10/11/18 04:37 PM (6 years, 3 months ago)

I thought I'd share some math with everyone. I live at an altitude of 6000 ft, and for some time have been concerned that I haven't been sterilizing correctly. I've been using a Presto 23 Qt PC for more than a year now, operating it at 15 PSI and increasing my sterilization times by 20% to account for the difference in air pressure. Boy was I wrong! I needed to sterilize for 210% longer than the suggested times.

...

At altitude, you need to adjust your sterilization pressure or duration. I hope you remember that the boiling point of water decreases with altitude, so much so that water boils at 71C/160F at the peak of Mount Everest. This is due to a reduction in air pressure, since pressure dictates water's boiling point.

Your PC raises the boiling point of water, but it'll only raise it relative to the air pressure outside of it. The gauge and pressure regulator on your PC measures this relative pressure, so the interior temperature will be lower the higher you are, resulting in less than ideal sterilization. There are two ways to sort this out: Increase pressure, or increase time.

Method 1: Increase interior pressure

The target absolute air pressure for the interior of your PC should be 29.7 PSI, to yield an interior temperature of 121C/250F. What should your pressure gauge read to ensure this? For that you need to work out what your atmospheric air pressure is, and subtract from 29.7 PSI. At sea level, this is 14.7 PSI, your gauge should read 15 PSI. At 8000 ft, your gauge should read 18.8 PSI.

Formula: 0

Use your favourite method to increase the sterilisation pressure of your PC. For me it's large washers on the weight of my Presto 23 Qt. There are plenty of teks which'll teach you how.

Method 2: Increase sterilization time

In the field of moist heat sterilization is a concept known as F0. This is the equivalent exposure time at a temperature different to the reference sterilization temperature, and is calculated using the following formula:

Formula: 1

Where T is the temperature your PC operates at and F0 is a multiplier to adjust sterilization time. This formula is slightly different to what is presented in scientific literature, and is changed to provide the adjustment required. This assumes a z-value of 10C, which is good enough for most PC calculations. The exponential function means changes to sterilzation temperatures translates to large changes in sterilization times.

Assuming your pressure gauge is set to 15 PSI, you need to adjust your sterilization times as follows:

Formula: 2

To use, look up your altitude, and multiply your sterilization time by the time multiplier to get your equivalent sterilization time. At an altitude of 3000 ft, you should be sterilizing for 48% longer than what you would at sea level. This translates to 28 extra minutes for a 60 minute sterilization. At 7000 ft for Mexico City, you should be sterilizing for more than double the suggested time. It's ok to be imprecise. If you're at 1000 feet and need to sterilize agar for 20 minutes, you don't need to adjust your sterilization time to exactly 23 minutes. 20 minutes is fine at 1000 ft or even 2000 ft.

We generally err on the side of caution for sterilization, especially given that many of the substrates we use are excellent heat insulators. Sterilization of wood based substrates is significantly longer than sterilization of metal tools.  The provided formula indicates that steam sterilization at 100C requires sterilizing for 125 times longer than at 121C. Bulk substrates are often sterilized for a few hours in a PC, which would translate to an equivelant in a bulk steam vessel. Much of the sterilization time using a PC is spent bringing the entirety of the substrate (grains, wood chips, etc) up to temperature. Finding reasonable sterilization times at temperatures much lower than 115C requires experimentation, but 5-8 hours is generally quoted.

...

I have adjusted my PC to operate at just over 18 PSI, a touch more than the suggested 17.9 PSI. I should hopefully experience fewer losses to contamination, which I now suspect may be due to me sterilizing my grains for an equivalent of 56 minutes. Too low. If you prefer going for much longer at pressures and temperatures much higher than what is suggested, then you go mad, as long as you don't burn your substrates. See here for a list of what plastics you can and can't sterilize using the various methods.

...

Much more information presented in a much better way than I could is available at https://web.archive.org/web/20180219141204/https://www.fedegari.com/wp-content/uploads/2016/07/ST19_EBook_F0-What-it-means-How-to-calculate-it-How-to-use-it.pdf.


--------------------

Edited by teladi (10/11/18 05:00 PM)

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Offlineverum subsequentis
seeker of truth
I'm a teapot User Gallery


Registered: 03/22/16
Posts: 8,732
Last seen: 2 years, 7 months
Trusted Cultivator
Re: PCing at altitude [Re: teladi]
    #25530512 - 10/11/18 05:47 PM (6 years, 3 months ago)

nice man. I'll admit, i didn't read it all but i scanned it. I'll come back later but now it's time to crack a beer and watch the US men's national futbal team tangle with Columbia!

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Offlinemy0mycelium
Stranger
Female
Registered: 01/06/22
Posts: 16
Loc: Utah
Last seen: 2 years, 1 month
Re: PCing at altitude [Re: teladi]
    #27795853 - 05/27/22 10:03 PM (2 years, 7 months ago)

A million thanks!!

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