Dissolved Oxygen Meter Conversion Math

[micraftbeer]

I have a Milwaukee MW600 Dissolved Oxygen meter I am playing around with to assess the impact of different aspects of my brewing and fermentation process. Most of the brewing literature references Oxygen in units of ppm, but the meter reads out in mg/L. It appears in a liquid solution, mg/L is interchangeable with ppm. But I'm not sure if it should be ratio'd by the density of the fluid (wort)?

Out of the brew kettle yesterday I had 3.0 mg/L, and after 2 minutes of oxygen fed from a tank through a stone, I had 8.1 mg/L. I am trying to understand if in my 1.044 SG wort, that is 8.1 ppm of Oxygen, or 11.7 ppm.

Any experts out there on this topic?

 

[Vale71]

Your DO meter doesn't have the faintest idea what your wort density is. If it did you'd have gotten yourself a free densitometer but that's not the case, unfortunately. It just assumes that you're measuring water at a standard density of 1.000 grams per liter. The only thing it corrects for is temperature.

 

[micraftbeer]

I assumed it didn't know my gravity, that's why I was wondering if I needed to do the math myself based on me knowing the density.

Like if it thinks based on the oxygen content it measured, that would be 8.1 mg per Liter of water. So on a 1.044 SG wort that would be 8.1 x 1.044? Or 8.1 / 1.044? Or it doesn't matter, and it's just 8.1.

 

[Vale71]

It really doesn't matter as the total amount of oxygen won't change. The yeast only cares about how much oxygen it can absorb, not some arbitrary weight/weight ratio.

 

[micraftbeer]

In this particular case, I'm trying to figure out how to get the recommended PPM of Oxygen for the yeast. I'm using Imperial A38 Juice, and their website notes " This strain has demonstrated the need for higher wort dissolved oxygen (DO) levels than most ale strains. Target 20-25 ppm DO or set the oxygen regulator flow to 50% higher than normal. "

So in this case, I am interested in the actual value. And of course I have the meter to measure actual values, and would like to be accurate in sharing results from experiments where I measure dissolved oxygen whether it's in water, wort, or beer. In some cases, water will be much easier to experiment a bunch of different process steps, but I will compare end results at some point to an actual batch of beer.

I reached out to Milwaukee to get their feedback. I'll share once I get it.

 

[Vale71]

The manufacturer is using the same kind of meter your are (maybe not the same brand and model but identical as far as reported values) and they're surely not correcting for actual gravity so their values are directly comparable to yours.
By the way, 20-25 ppm O2 is really a lot. Expect a lot of off flavors but since this will be a NEIPA then that's probably expected of it...

 

[micraftbeer]

Just closing the loop here, as I just talked to the tech lead at Imperial Yeast. He confirmed as @Vale71 was saying, that the meter just reports oxygen per unit of volume, it is not standardized to the density of water or anything. mg/L is the same units as ppm, so my reading of 8.1 mg/L was a good bit shy of the 20-25 ppm they recommended with this yeast.

As a bonus tip, he noted that rather than the method I used of just putting the oxygen wand stone at the bottom of the tank and letting it sit there the whole time, he recommended I swirl the wort around (or the oxygen stone) to get the oxygen well dispersed throughout the wort.

 

[day_trippr]

Yeah, that swirling part is pretty much required to have anything close to "efficient" absorption (which is to say "not so gawdawful").
That - and the "smaller pores the better" thing - is why I went with the Williams SS O2 wand with the .5 micron stone...

Cheers!

 

[Vale71]

You probably already know this but just to be sure... Since your meter has a polarographic sensor to ensure correct measurement you must:

1 - wait for the sensor to be polarized
2 - keep moving the sensor in the liquid during measurement

 

[micraftbeer]

@Vale71 I've been reading about the polarization, and unfortunately, all I can find in Milwaukee's literature is to "be sure it is polarized", but no explanation of how long that is, or how to know when it is ready to go. Do you have experience with this meter, or is there a general rule of thumb for polarographic sensors?

While doing my measurements, I am using swirling motion with the probe in my small glass of wort/beer sample.

 

[micraftbeer]

Actually, I was able to hook up with Milwaukee Instruments tech support via their online chat this morning. They said for this meter, the polarization happens pretty quickly. Turn it on, wait for the display reading to go to max (which takes a few seconds), and then I'm good to go.