### Author Topic: HDPE fermenters, oxygen and positive pressure  (Read 863 times)

#### saaz amore

• Cellarman
• Posts: 53
• When your beer has a smell like a Pilsner Urquell
##### HDPE fermenters, oxygen and positive pressure
« on: September 17, 2022, 01:21:48 pm »
I was talking with my brother the other day. He's been homebrewing much, much longer than me. We got to the topic of HDPE fermenters, and I mentioned that HDPE is much more oxygen-permeable than PET or stainless steel (he's been fermenting in his Speidel for years), and that I recently started fermenting in cornies.

I told him I was worried about oxygen ingress in HDPE fermenters, especially with lagers, and not so much with ales. He said as long as there's positive pressure (i.e. outward airlock activity), there shouldn't be any oxygen coming in through the walls of the fermenter.

It made sense to me, but I have no idea what the actual science is on this. Any ideas?

#### Richard

• Senior Brewmaster
• Posts: 1018
##### Re: HDPE fermenters, oxygen and positive pressure
« Reply #1 on: September 17, 2022, 02:18:36 pm »
He is wrong. Gas diffusion is driven by partial pressures, not total pressures. The pressure inside the fermenter is almost completely CO2, not oxygen. That pressure will keep CO2 from diffusing in, but oxygen will continue to diffuse through the permeable walls until the partial pressure of oxygen on the inside is equal to the partial pressure on the outside.

https://en.wikipedia.org/wiki/Partial_pressure
Original Gravity - that would be Newton's

#### Fire Rooster

• Guest
##### Re: HDPE fermenters, oxygen and positive pressure
« Reply #2 on: September 18, 2022, 03:17:31 am »
oxygen will continue to diffuse through the permeable walls until the partial pressure of oxygen on the inside is equal to the partial pressure on the outside.

https://en.wikipedia.org/wiki/Partial_pressure

Using your guesstimations, what would the pressure difference be ?,
and how long for pressure to equalize in mentioned fermenter ?

Thanks
« Last Edit: September 18, 2022, 06:16:26 am by Fire Rooster »

#### Richard

• Senior Brewmaster
• Posts: 1018
##### Re: HDPE fermenters, oxygen and positive pressure
« Reply #3 on: September 18, 2022, 09:31:16 am »
oxygen will continue to diffuse through the permeable walls until the partial pressure of oxygen on the inside is equal to the partial pressure on the outside.

https://en.wikipedia.org/wiki/Partial_pressure

Using your guesstimations, what would the pressure difference be ?,
and how long for pressure to equalize in mentioned fermenter ?

Thanks
The partial pressure of oxygen inside is essentially zero, while the partial pressure outside is roughly 0.2 bar. The rate of diffusion depends on the material, the thickness and the temperature. I don't have the dimensions available, but making some rough guesses I would say that oxygen would diffuse into the head space at a rate of about 100 ppm/day. At that rate it would take a VERY long time for the pressure to equalize completely. The real question, though, is what the impact on the beer would be to have a few hundred ppm of oxygen in the head space. The oxygen there would tend to be blown out the airlock along with the CO2 during active fermentation, so it wouldn't build up linearly with time until fermentation was finished. Even then, what matters is the dissolved oxygen in the liquid, and I don't know how 100 ppm of oxygen in the headspace translates to ppm of dissolved oxygen in the beer.
Original Gravity - that would be Newton's

#### Fire Rooster

• Guest
##### Re: HDPE fermenters, oxygen and positive pressure
« Reply #4 on: September 18, 2022, 11:27:01 am »
oxygen will continue to diffuse through the permeable walls until the partial pressure of oxygen on the inside is equal to the partial pressure on the outside.

https://en.wikipedia.org/wiki/Partial_pressure

Using your guesstimations, what would the pressure difference be ?,
and how long for pressure to equalize in mentioned fermenter ?

Thanks
The partial pressure of oxygen inside is essentially zero, while the partial pressure outside is roughly 0.2 bar. The rate of diffusion depends on the material, the thickness and the temperature. I don't have the dimensions available, but making some rough guesses I would say that oxygen would diffuse into the head space at a rate of about 100 ppm/day. At that rate it would take a VERY long time for the pressure to equalize completely. The real question, though, is what the impact on the beer would be to have a few hundred ppm of oxygen in the head space. The oxygen there would tend to be blown out the airlock along with the CO2 during active fermentation, so it wouldn't build up linearly with time until fermentation was finished. Even then, what matters is the dissolved oxygen in the liquid, and I don't know how 100 ppm of oxygen in the headspace translates to ppm of dissolved oxygen in the beer.

Thanks

#### saaz amore

• Cellarman
• Posts: 53
• When your beer has a smell like a Pilsner Urquell
##### Re: HDPE fermenters, oxygen and positive pressure
« Reply #5 on: September 21, 2022, 11:55:36 pm »
Thanks for the information. I had never heard of partial pressure. I think I'll stick with cornies for the time being.