Would Hop Oils effect Hydrometer Readings?

[Brewmegoodbeer]

I recently read that Sierra Nevada begins dry hopping when a batch is within 1 degree plato of target FG. This allows any oxygen that is within the hops to be pushed out by the co2 from the active yeast. My question is, would the addition of hop oils from dry hopping give you an inaccurate hydrometer reading? Hydrometers measure density. Oil does not mix with water, therefore these micro droplets that are hop oils are not homogeneous with the rest of the beer. Would this oil cause your hydrometer to float incorrectly and give you a false reading?

 

[JimRausch]

Someone correct me if I'm wrong, but I believe they measure Dissolved solutes in solution. So hop oil floating on the surface would not affect SG.

 

[GrogNerd]

Someone correct me if I'm wrong, but I believe they measure Dissolved solutes in solution. So hop oil floating on the surface would not affect SG.

I believe you are correct, including your use of "affect" :rockin:

bits of gunk floating around also does not affect gravity

 

[Brewmegoodbeer]

Someone correct me if I'm wrong, but I believe they measure Dissolved solutes in solution. So hop oil floating on the surface would not affect SG.

I know Wikipedia is not the most credible source to gather information, but since one of my professors in pharmacy school actually uses it to source some of his info in his lectures, I will give it a go: "A hydrometer or areometer is an instrument that measures the specific gravity (relative density) of liquids—the ratio of the density of the liquid to the density of water."

Do droplets of oil within water affect density?? This may have to be a test ill have to do myself. water vs. 1 tablespoon of vegetable oil within water and hydrometer readings of both. Going to have to do this.

 

[Brewmegoodbeer]

I believe you are correct, including your use of "affect" :rockin:

bits of gunk floating around also does not affect gravity

I realized I used the wrong "affect". I could not edit this after I submitted the post. I am not dumb I swear, just dumbfounded.

 

[Brewmegoodbeer]

Alright, so I just did the quick experiment. I added 1 tablespoon (15 mL) of vegatable oil to my hydrometer in water and I got a way off reading. I then added 1/2 teaspoon (2.5 mL) and I got a reading accurate with water. I did not want to find the threshold because I was sick of washing oil out of my tube. Lol so oil can have an affect on a hydrometer reading. It just depends on how much oil we are extracting from these hops. Sorry for the sideways pics.

 

[Brewmegoodbeer]

Since 2.5 mL of oil did not have an "affect", I would suspect that the amount of oil that is drawn into our tube to float the hydrometer in is negligible and will not "affect" the hydrometer reading.

 

[Gavin C]

Since 2.5 mL of oil did not have an "affect", I would suspect that the amount of oil that is drawn into our tube to float the hydrometer in is negligible and will not "affect" the hydrometer reading.

WRT to your experimental model. Oils will have an effect on the reading.

Just do a little thought experiment and let's assume a calibration temperature of x for the hydrometer.

Place a 1ft long hydrometer of suitably wide range (e.g. 0.9-1.1) in a half filled swimming pool of distilled water at x°F 1ft deep. It reads 1.000 as it floats on the surface

Now add 1 ft worth of vegetable oil (assume oil SG 0.92) to the pool

Allow time for everything to stratify

What does the hydrometer measure as it sits on the surface of 1ft of oil at x°F atop 1 ft of water?

0.92 is the obvious answer

If a smaller amount of oil is added (lets say 2cm deep) The hydrometer will sit in mostly water but also some oil.

It will read SG <1.000

As more oil is added to the pool a greater volume of the hydrometer is immersed in oil and less volume in water.

The upthrust force on the hydrometer must remain constant (it is always equal to the weight of the hydrometer) and as oil density is lower than water the hydrometer sinks lower and lower till the extreme example as outlined above is met.

Archimedes Principal is the simple law in play here.

The upthrust on a body partially or completely immersed in a liquid is equal to the weight of fluid displaced by that object.

With hop oils dispersed in the beer (i.e. not floating on the surface but dispersed throughout) the SG will change.

The oils are soluble in alcohol and have a lower density than beer. Adding hop oils will lower the density (SG reduction).

The effect is negligible however given the volume of beer v volume of hop oils.

 

[Brewmegoodbeer]

WRT to your experimental model. Oils will have an effect on the reading.

Just do a little thought experiment and let's assume a calibration temperature of x for the hydrometer.

Place a 1ft long hydrometer of suitably wide range (e.g. 0.9-1.1) in a half filled swimming pool of distilled water at x°F 1ft deep. It reads 1.000 as it floats on the surface

Now add 1 ft worth of vegetable oil (assume oil SG 0.92) to the pool

Allow time for everything to stratify

What does the hydrometer measure as it sits on the surface of 1ft of oil at x°F atop 1 ft of water?

0.92 is the obvious answer

If a smaller amount of oil is added (lets say 2cm deep) The hydrometer will sit in mostly water but also some oil.

It will read SG <1.000

As more oil is added to the pool a greater volume of the hydrometer is immersed in oil and less volume in water.

The upthrust force on the hydrometer must remain constant (it is always equal to the weight of the hydrometer) and as oil density is lower than water the hydrometer sinks lower and lower till the extreme example as outlined above is met.

Archimedes Principal is the simple law in play here.

The upthrust on a body partially or completely immersed in a liquid is equal to the weight of fluid displaced by that object.

With hop oils dispersed in the beer (i.e. not floating on the surface but dispersed throughout) the SG will change.

The oils are soluble in alcohol and have a lower density than beer. Adding hop oils will lower the density (SG reduction).

The effect is negligible however given the volume of beer v volume of hop oils.

Thank you kind sir!!!

 

[ajdelange]

A hydrometer is effected by the following forces:
1) Its weight - down
2) The weight of the liquid it displaces (immersed part of stem and bulb) - up
3) The weight of the air displaced by the stem - up
4) Surface tension at the liquid air interface

The hydrometer settles into the solution to the point at which the sum of those 4 forces is 0. So whether adding something to the liquid produces an effect or not depends on whether it effects those 4 forces. Immiscible things added to the liquid if denser than it sink to the bottom in which case they have no effect on a hydrometer reading. A bowling ball dropped into a swimming pool will not effect a hydrometer floating at the surface. If the immiscible stuff is less dense than the liquid it will float to the surface and, if the stem or bulb are in the surface layer, forces 2, 3 and 4 will be reduced and the hydrometer will sink. If the immiscible stuff is the same density as the liquid it will stay in suspension and no force changes, except perhaps surface tension, will be observed.

Now if something is put into the liquid which is lighter or heavier than the liquid but is kept in suspension as in a stable emulsion the particles of this material do get displaced by bulb and stem, along with the liquid, and do have an effect on the measured density.

The answer, as is often the case, is 'it depends'.

I'll also note that as I have never seem a 'hop oil slick' on top of a beer or wort I suppose that these have become emulsified and would effect hydrometer readings were it not for the fact that the concentrations are so low.

 

[Gavin C]

Thank you kind sir!!!

No worries mate.

AJ's outline of the forces on the hydrometer are more complete of course.

I hadn't mentioned surface tension or upthrust from displaced air. I did not think of those. Thanks AJ.

It's fun to construct these mental free body diagrams.

Surface tension upward force

 

[dyqik]

There's also an effect of surface tension on the shape of the meniscus around the hydrometer stem. A change in the meniscus shape may affect the way you judge where the reading should be on the scale. I think on the wide-range hydrometer shown above, you are supposed to read the bottom of the surface, not the top of the meniscus. So you can't actually see the reading in any of those photos.

That might give a 1 gravity point error on a typical wide-range hydrometer like the one pictured, but it won't give much error on a narrow-range FG hydrometer.

Generally you may not want to completely fill a hydrometer tube either, as was done in the pictures above, because the meniscus shape at the top will be different to when it's only partially full, and that could affect the reading a tiny bit.

 

[ajdelange]

You should read WRT the mesniscus as directed by the manufacturer. The good, narrow range brewing hydrometers are calibrated with wort - not water with some salt dissolved in it. I have found, by comparison with a digital densitometer, that these can, with a little care, be read to an accuracy of better than 0.1 °P (0.0004 SG), perhaps half that.