Author Topic: Does yeast in suspension effect hydrometer reading  (Read 1893 times)

Online Richard

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Re: Does yeast in suspension effect hydrometer reading
« Reply #30 on: February 09, 2018, 10:06:53 PM »
Let's go with the poodles thought experiment. The principle of flotation says that an object afloat will displace a volume of liquid with weight equal to the weight of the object. Note that it is only the liquid displaced by the floating object that matters, not the average density of the contents of the container. If you have rocks or flocculated yeast at the bottom of your container they will affect the weight-to-volume ratio of the container as a whole but if the float does not go low enough to touch the rocks or yeast, then only the density of the liquid on top matters.

Now to the poodles. If you have a bunch of normal-sized poodles in a swimming pool and you drop a normal-sized hydrometer float in between them, your reading will be the same as if the poodles aren't there because they aren't being displaced significantly by your float. If you drop your float right on top of a poodle it will not sink at all, and your reading will be grossly affected (giving a very high density reading). Now imagine the poodle getting smaller and smaller until the weight of the float is comparable to the weight of the poodle. Now the extra weight on top of the poodle will begin to push it down in the water and your float will sink a bit, giving intermediate readings. Now imagine that you have billions of microscopic poodles per liter of water and you drop your float on top of them. It will displace many poodles along with the water, and give a reading that reflects the density of (poodle weight in displaced volume + water weight in displaced volume)/displaced volume. Assuming that the poodles are denser than water (my experience is that some poodles are denser than others) then the density reading will go up as more poodles are added per liter. That is the case for yeast. For real microscopic poodles (??), which were assumed to be floating at the start, their density is less than water so a lower reading would be obtained.

Jim's experiment is consistent with this. When he had flour particles in suspension, they were displaced by his float and raised the reading. After the all sank to the bottom they were no longer displaced by his float and his reading was back to that of water.
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Offline Robert

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Re: Does yeast in suspension effect hydrometer reading
« Reply #31 on: February 09, 2018, 10:18:11 PM »
So, two questions:

• A chilled and settled sample will then give a more accurate reading than a sample straight from the fermentor.  But I still want to know, how much suspended yeast (population of poodles) is needed before it causes an error more significant than other factors?

•What does yeast do to a refractometer FG reading, or can we assume (as I do) that this is baked into the cake of refractometer calculators? (OP's OQ before moving the thread was what instrument is most reliable for FG.)
« Last Edit: February 09, 2018, 10:22:03 PM by Robert »
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Offline Frankenbrew

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Re: Does yeast in suspension effect hydrometer reading
« Reply #32 on: February 09, 2018, 10:22:11 PM »
So, two questions:

• A chilled and settled sample will then give a more accurate reading than a sample straight from the fermentor.  But I still want to know, how much suspended yeast is needed before it causes an error more significant than other factors?

•What does yeast do to a refractometer FG reading, or can we assume (as I do) that this is baked into the cake of refractometer calculators? (OP's OQ was what instrument is most reliable for FG.)

I think you have to keep in mind, when wondering how it's baked into the cake, is we usually take our OG readings before pitching the yeast. No?
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Re: Does yeast in suspension effect hydrometer reading
« Reply #33 on: February 09, 2018, 10:26:15 PM »
Let's go with the poodles thought experiment. The principle of flotation says that an object afloat will displace a volume of liquid with weight equal to the weight of the object. Note that it is only the liquid displaced by the floating object that matters, not the average density of the contents of the container. If you have rocks or flocculated yeast at the bottom of your container they will affect the weight-to-volume ratio of the container as a whole but if the float does not go low enough to touch the rocks or yeast, then only the density of the liquid on top matters.

Now to the poodles. If you have a bunch of normal-sized poodles in a swimming pool and you drop a normal-sized hydrometer float in between them, your reading will be the same as if the poodles aren't there because they aren't being displaced significantly by your float. If you drop your float right on top of a poodle it will not sink at all, and your reading will be grossly affected (giving a very high density reading). Now imagine the poodle getting smaller and smaller until the weight of the float is comparable to the weight of the poodle. Now the extra weight on top of the poodle will begin to push it down in the water and your float will sink a bit, giving intermediate readings. Now imagine that you have billions of microscopic poodles per liter of water and you drop your float on top of them. It will displace many poodles along with the water, and give a reading that reflects the density of (poodle weight in displaced volume + water weight in displaced volume)/displaced volume. Assuming that the poodles are denser than water (my experience is that some poodles are denser than others) then the density reading will go up as more poodles are added per liter. That is the case for yeast. For real microscopic poodles (??), which were assumed to be floating at the start, their density is less than water so a lower reading would be obtained.

Jim's experiment is consistent with this. When he had flour particles in suspension, they were displaced by his float and raised the reading. After the all sank to the bottom they were no longer displaced by his float and his reading was back to that of water.
We need a volunteer. Who has a poodle they can set their hydrometer on and see what the reading is?

Offline Robert

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Re: Does yeast in suspension effect hydrometer reading
« Reply #34 on: February 09, 2018, 10:26:45 PM »
So, two questions:

• A chilled and settled sample will then give a more accurate reading than a sample straight from the fermentor.  But I still want to know, how much suspended yeast is needed before it causes an error more significant than other factors?

•What does yeast do to a refractometer FG reading, or can we assume (as I do) that this is baked into the cake of refractometer calculators? (OP's OQ was what instrument is most reliable for FG.)

I think you have to keep in mind, when wondering how it's baked into the cake, is we usually take our OG readings before pitching the yeast. No?

This question originated on another thread, where the question was about whether yeast really causes huge errors, as claimed by Palmer,  and whether hydrometer or refractometer would be more accurate for FG.  This question interests me most, because I think we understand pretty well how to choose a measurement method for wort.
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Offline Frankenbrew

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Re: Does yeast in suspension effect hydrometer reading
« Reply #35 on: February 09, 2018, 10:31:33 PM »
Gotcha. But if we're measuring attentuation, don't we need to level the field between OG and FG?
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Re: Does yeast in suspension effect hydrometer reading
« Reply #36 on: February 09, 2018, 10:34:57 PM »
So, two questions:

• A chilled and settled sample will then give a more accurate reading than a sample straight from the fermentor.  But I still want to know, how much suspended yeast (population of poodles) is needed before it causes an error more significant than other factors?

•What does yeast do to a refractometer FG reading, or can we assume (as I do) that this is baked into the cake of refractometer calculators? (OP's OQ before moving the thread was what instrument is most reliable for FG.)
I'm assuming suspended solids will diffuse light, not refract it. Meaning solids do change the degree of but at some point would diffuse it to the point that it's difficult to tell exactly where the line is.

A 2ml sample should be easier to settle than a 200ml sample, and reheat to proper measuring temp.

But the outcome of this thread is up to the reader. I was just curious if it was true that suspended solids effect hydrometers. And it's quite obvious to me that they do.

Offline Robert

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Re: Does yeast in suspension effect hydrometer reading
« Reply #37 on: February 09, 2018, 10:37:51 PM »
Gotcha. But if we're measuring attentuation, don't we need to level the field between OG and FG?
I think you're inching toward another question I raised:   IS anybody actually measuring attenuation?  Or do we just look for an APPARENT attenuation that cues us to move to the next stage of fermentation or cellaring?  In which case, consistency matters, accuracy doesn't. But that is really separate from OP Jim's query.
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Offline charles1968

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Re: Does yeast in suspension effect hydrometer reading
« Reply #38 on: February 09, 2018, 10:38:06 PM »
You could filter the yeast suspension through a coffee filter. Dissolved carbohydrates and salts will pass through, suspended particles won't. Then take gravity of the filtrate. I'm 99% sure it will be higher than distilled water.
Go for it. But also measure it as is. So just water, water with yeast, then that water minus yeast. Probably don't even need to filter, just let it settle to the bottom. Will there be a difference between in suspension and settled out? I suspect yes.

I don't have a hydrometer any more - gave it to my sister-in-law's nephew. Only a refractometer.

I don't think the suspended matter would affect a refractometer in the same way. The refractometer is measuring how much light is refracted when it enters a transparent medium that's more dense than air. Suspended matter makes the liquid non-transparent and screws up the reading. You can only get a valid reading by removing the gunk.

Online Richard

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Re: Does yeast in suspension effect hydrometer reading
« Reply #39 on: February 09, 2018, 10:40:40 PM »
Refractometers are calibrated to read correctly for water with a simple content of sugar. We need wort correction factors because wort has many sugars and other things that the refractometer was not designed to measure. Same for alcohol, so we need to correct refractometer readings after fermentation has started. If you add yeast to the solution you will need to correct for it because it isn't the sugar the refractometer was calibrated with. I don't know if the typical quantity of yeast per volume is enough to change the reading significantly, but I do know that cloudy liquids scatter the light in a refractometer and give very fuzzy lines which make it hard to get a decent reading.
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Online klickitat jim

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Re: Does yeast in suspension effect hydrometer reading
« Reply #40 on: February 09, 2018, 10:41:06 PM »
You could filter the yeast suspension through a coffee filter. Dissolved carbohydrates and salts will pass through, suspended particles won't. Then take gravity of the filtrate. I'm 99% sure it will be higher than distilled water.
Go for it. But also measure it as is. So just water, water with yeast, then that water minus yeast. Probably don't even need to filter, just let it settle to the bottom. Will there be a difference between in suspension and settled out? I suspect yes.

I don't have a hydrometer any more - gave it to my sister-in-law's nephew. Only a refractometer.

I don't think the suspended matter would affect a refractometer in the same way. The refractometer is measuring how much light is refracted when it enters a transparent medium that's more dense than air. Suspended matter makes the liquid non-transparent and screws up the reading. You can only get a valid reading by removing the gunk.
Correct. Disolved sugar refracts, solids diffuse

Offline charles1968

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Re: Does yeast in suspension effect hydrometer reading
« Reply #41 on: February 09, 2018, 10:41:54 PM »
Let's go with the poodles thought experiment. The principle of flotation says that an object afloat will displace a volume of liquid with weight equal to the weight of the object. Note that it is only the liquid displaced by the floating object that matters, not the average density of the contents of the container. If you have rocks or flocculated yeast at the bottom of your container they will affect the weight-to-volume ratio of the container as a whole but if the float does not go low enough to touch the rocks or yeast, then only the density of the liquid on top matters.

Now to the poodles. If you have a bunch of normal-sized poodles in a swimming pool and you drop a normal-sized hydrometer float in between them, your reading will be the same as if the poodles aren't there because they aren't being displaced significantly by your float. If you drop your float right on top of a poodle it will not sink at all, and your reading will be grossly affected (giving a very high density reading). Now imagine the poodle getting smaller and smaller until the weight of the float is comparable to the weight of the poodle. Now the extra weight on top of the poodle will begin to push it down in the water and your float will sink a bit, giving intermediate readings. Now imagine that you have billions of microscopic poodles per liter of water and you drop your float on top of them. It will displace many poodles along with the water, and give a reading that reflects the density of (poodle weight in displaced volume + water weight in displaced volume)/displaced volume. Assuming that the poodles are denser than water (my experience is that some poodles are denser than others) then the density reading will go up as more poodles are added per liter. That is the case for yeast. For real microscopic poodles (??), which were assumed to be floating at the start, their density is less than water so a lower reading would be obtained.

Jim's experiment is consistent with this. When he had flour particles in suspension, they were displaced by his float and raised the reading. After the all sank to the bottom they were no longer displaced by his float and his reading was back to that of water.

Seems logical to me. Another way to think of it is imagine taking the gravity of thick porridge, runnier porridge, even runnier...

So suspended yeast might inflate gravity readings, but in a beer that's attenuated the yeast will drop out as sugar disappears, so it shouldn't be a big problem.

Offline Phil_M

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Re: Does yeast in suspension effect hydrometer reading
« Reply #42 on: February 10, 2018, 01:23:22 AM »
Adding anything to water will change the density. Think about quicksand. The sand isn't dissolved in water, but the density is such that you can "float" on top of it easily.

Or antifreeze in your car. Or nitrogen bubbles in a freshly poured Guinness. (I've been recently schooled by the latter recently)

Back to the antifreeze, that brought to mind a specific point: Gravity is NOT a measure of sugar content, it's a measure of density. As in any Engineering problem, you have to keep the units straight...
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Offline Robert

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Re: Does yeast in suspension effect hydrometer reading
« Reply #43 on: February 10, 2018, 01:32:22 AM »
Gravity is density, not sugar content, BUT:  The Plato scale is an expression of density based on percent sugar in solution, and refractometers are calibrated to measure an analogous scale, calibrated to refractivity of a pure sugar solution.  So in actual brewing practice,  you have to account for the fact that you are not in fact dealing with a pure sugar solution, even though that's what all your measuring systems assume.  (Assume. You know, as in "Assume a spherical cow....")
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Offline narvin

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Re: Does yeast in suspension effect hydrometer reading
« Reply #44 on: February 10, 2018, 03:26:09 PM »
Flour settled to bottom, new reading

How long did that take?  There's a lot of flour at the bottom, so another hypothesis is that enough flour temporarily provides nucleation points for rising dissolved gasses and due to surface tension lifts the refractometer.  Any ideas how to test this?

Also, ascorbic acid which is in dry yeast as an antioxidant is soluble in water and is a derivative of glucose.  How much is in there, I don't know. 

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Ascorbic acid (vitamin C, C6H8O6)is a water-soluble vitamin. A solution containing 81.0g of ascorbicacid dissolved in 230g of water has a density of 1.22g/mL at55oC.
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