Hydrometer? Refractometer? Or something better?

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Hydrometer? Refractometer? Or something better?

Post by PeeBee » Tue May 05, 2020 1:07 pm

I've been promising this writeup for sometime. Now I've got it altogether (...ish) it's time to bug you all with it.

I've always voiced a preference for refractometers over hydrometers because they are not so easy to break (either by dropping or having too hot a sample). Err, I should say here that you can break refractometers! With refractometers there's no "meniscus" on the sample's surface to bother about distorting the result. But most of all, refractometers require a few drops as a sample, and a hydrometer might require 250mls with the added concern "should I put the sample back and risk cross-infection, or should I ditch it?".

But some people just can't get on with refractometers because the reading must be converted if fermenting to allow for alcohol (and because you are measuring predominantly a maltose solution whereas a refractometer's native units are "BRIX", a scale designed for sucrose). Both devices utilise scales of tight packed fine lines. So what else is there?

The scales of fine lines (and meniscus) are not good if your sight isn't so good. Me? My sight isn't so good as sharpness at close range deteriorates with with age, quite common, so reading glasses required? Add large parts of my field of view are subject to double vision, and add to that "saccadic intrusions" or something you don't want to know about! So what I'm about to write may not be for everyone, or may answer a problem you have with hydrometers and/or refractometers. But before anyone tells me a hydrometer or refractometer is better … well you don't know what you are talking about!

And the device I'm promoting? (I'll start writing about them in a separate post as this "introduction" is quite big enough).

A Pycnometer.

NOTE: Often pronounced "pick-nom-eater", but as it's derived from the Greek word for "dense" ("πυκνός") I'd prefer "puk-nom-eater". Well, this is the most controversial thing I might say in this thread!
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Pycnometer

Post by PeeBee » Tue May 05, 2020 2:47 pm

Fancy name, but a pycnometer is just a bottle with a hole in the stopper. There are fancier versions with built in thermometers, even very expensive digital ones, but the simple bottle at room temperature is good enough.
20200504_130825_clip.jpg
20200504_130825_clip.jpg (45.27 KiB) Viewed 913 times
Pictured is one that is tiny at 25ml, about as small as I'd dare go given the accuracy of weighing scales I have. Which I must point out; you will also need a good set of weighing scales. The cheap (£10-15) Chinese scales often used for weighing brewing water treatment salts that weigh grams to two decimal places are not reliable enough, but the larger scales used by the eliquid (vaping) crowd (£25-30) are up to the job and will have a capacity of 500gms. Such scales will also weigh water treatment salts (and hops).

You can get pycnometers calibrated or uncalibrated, but I'm not sure how well they are calibrated so I went for uncalibrated ones. The 25ml ones can be about £4-5 from China and I'd recommend two (they are glass and you can accidentally break them). Get 50ml or 100ml bottles if your weigh scales are not very reliable. If calibrated bottles are accurate a 100ml one has the advantage of displaying the SG on the scales (÷10), but an uncalibrated 25ml one hardly involves complicated maths.

To calibrate: Make sure everything has been in the room (at near 20°C) long enough for temperatures to stabilise. Weigh the dry bottle (with stopper) and record. Fill bottle with distilled (or deionised) water, careful not to create air bubbles. Replace stopper and allow fluid to be pushed out of the stopper's hole. Dab any excess fluid from the bottle and around stopper (not directly over hole in stopper or water is drawn out of the capillary hole). Note no lining up with meniscus. Weigh the bottle and subtract the bottle's dry weight. You have the weight of water at 20°C (record), which you can divide by 0.9982 (the density of water at 20°C) and record as the bottle's volume (in mls). Record this volume as a weight (in grams) at 4°C. We'll come to why two weight measures later.

Calibration done. You only do this once.


There's nothing new about any of this. But it is only very recently that weighing scales have become so cheap for such fine accuracy. Years ago, before there was hydrometers, brewers would weigh enormous volumes (in barrels) and talk of "Pounds" of extract (sugar) in their worts.

EDIT: I knew I was getting this wrong, but couldn't put my finger on what I was doing wrong: Messing up weight and volume, that's what! The error will be insignificant, but I'll start correcting it in this post, and the next if necessary, before anyone else notices. Corrections in-line, in red where I can (i.e. not deletions).
Last edited by PeeBee on Sat May 09, 2020 5:52 pm, edited 1 time in total.
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"Specific Gravity" and reference samples

Post by PeeBee » Tue May 05, 2020 4:56 pm

I've put this in a separate post 'cos it's so kooky you might want to skip it! I'm not even confident I'll get this right!

Density is very specifically mass (weight) divided by volume (we'll use g/cm³ here, or g/ml, but it could be converted to kg/litre or whatever units are convenient). Specific Gravity (SG) is a ratio of the sample's density to a reference's density. There are times when the two (density and SG) look the same, but they are not the same! SG is a ratio, it has no units as it's just a number.

So, that "reference's density"? Water is often used at it's most dense, which for water is 4°C (actually 3.98°C is closer, but 4°C is close enough). The reference is 1g/ml at this temperature so this makes any maths easy. The sample can be a different temperature, and 16°C and 20°C is often used. This is handy, because SG and density is the same thing at a given temperature. But, when calibrating a brewing hydrometer we can't be bothered with this nonsense and both sample and reference is deemed to be 20°C.

Still with me? Am I doing your head in yet? 'Cos all this is doing my head in!

So, let's put this in action (there's the calibration weights and a 50p for scale included in the shot):
20200504_130825_WEB.jpg
My pycnometer bottle weighed 18.28g. I filled it with my sample (piccie) so my sampled liquid weighed (39.01-18.28) 20.73g. Now I'm going to compare the results to a table produced using a scientific instrument so we must use that 4°C reference worked out in the previous post. So, 20.73/26.36 (the sample liquid's weight divided by the 4°C reference weight) is SG 0.786. Obviously isn't beer at that gravity! Look it up on my tables and … this is really jammy, I wouldn't have expected such a result because the scales are not that accurate, but are fairly decent … the SG for 100% isopropyl alcohol is 0.786. Sceptic as I am I was convinced this 99.9% isopropyl alcohol bought through eBay was a scam, but seems I was wrong!

(EDIT: Okay, this is supposed to be a mathematical proof and I'm taking liberties without explanation! SG is a ratio of densities, and I'm showing a ratio of masses. But as density is mass/volume and volume is the same, or near enough the same, in both cases I've ignored volume. Well I'm a homebrewer, not a flippin' mathematician!). (EDIT-2: I'd copied in 30.01 into above maths. What a clown I am, the piccie clearly shows 39.01.).

If the SG was to be what a "Noddy" brewing hydrometer would have shown, it'll have been 20.73/26.31 or SG(20°C/20°C) 0.785.

I need a drink. Ah, here's one :beer: . Blah, that was the isopropyl alcohol!

And why am I fiddling with such tiny values? Well I didn't decide we have to measure SG. And SG is described to three decimal places. So messing around with SG is going to mean messing with measurements to three decimal places! Bummer that.

Right, let's see it in action doing some proper work …
Last edited by PeeBee on Tue May 05, 2020 7:53 pm, edited 1 time in total.
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Real life usage

Post by PeeBee » Tue May 05, 2020 7:34 pm

This is for real! No mathematical proofs, just no fannying about, let's just get on with it …
20200505_184658_WEB.jpg
We must have those numbers: The weight of the bottle (18.28g) and the weight of water (26.31g at 20°C) it holds.

44.84-18.28 = 26.56g (weight of beer sample)
26.56/26.31 = SG(20°C/20°C) 1.0095(FG of this beer)

This is the same gravity taken yesterday with a refractometer, so the beer really has finished. The refractometer reading just now suggests the same gravity, but is actually difficult to read because of the blurred "line" due to the beer being so murky at this stage. One up for the pycnometer?
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More proof?

Post by PeeBee » Thu May 07, 2020 1:37 pm

VERY IMPORTANT! Don't let all this prattle put you off trying a pycnometer. I'm only trying to prove that the SG they return is true, not illustrate what you need to do to use one.

Well, I don't seem to have caused controversy yet (give it time!) but I'm slightly worried that I've been taking liberties with the maths and are perhaps hiding minor blunders and "liberties" from you (and myself!). (EDIT: Yeah, I found them! All corrected now. I think?)

You don't need to do this in-depth math to use a pycnometer, but knowing it has been done should give confidence that what they tell you should be correct. And I'm not doing this to "show off". I'm just providing anyone who is cleverer than me at this caper (i.e. a good chunk of the population) where I'm coming from should they wish to correct me on a point of maths.

So going back to basics: Specific Gravity (or, more descriptively, Relative Density) is equal to the density of a substance divided by the density of a reference substance:

SG = ρ-sample÷ρ-reference

where ρ (Greek letter "rho") represents density.

ρ = Mass÷Volume

We've got "Mass" (weight) of the pycnometer. To be subtracted from the total mass of the sample plus bottle being used (to give the mass of only the sample). I do not use the "tare" function because that would mean you'd have rinse and dry out the pycnometer completely before every use.

We've got "Volume" as the volume of the pycnometer. It changes with temperature, but not enough to worry about (it was measured at it's "working temperature" of 20°C, and the only other temperature it will be, optionally, referenced at is 4°C, and pycnometers are often made of borosilicate glass which has a very low coefficient of thermal expansion).

So: ρ-reference (H2O@20°C) is 0.9982g/cm³

and: ρ-reference (H2O@4°C) is 1.000g/cm³ (as near to as makes no difference)

We don't need to get these figures with the pycnometer, because tables with density of water at varying temperatures are all over the Internet.


My pycnometer filled with distilled water weighs 44.60g (at 20°C). Less the weight of the bottle (18.28g) is 26.32g of water. To be perfectly correct this would be 26.32÷0.9982g, or 26.37g @ 4°C, and as the density of water at 4°C is 1g/cm³ the volume of the pycnometer is 26.37cm³ (or ml). The accuracy is all well and good, but the instruments and conditions we use them under can't really justify such accuracy, but it'll be more than close enough for our usage.

Don't forget, all the numbers arrived at so far are done only once (per pycnometer). So now we get to actually using the device (I'll use the example in the previous post).

Weight of sample wort (at 20°C) is 44.84-18.28 or 26.56g. Density of sample at 20°C is 26.56÷26.37 which is 1.0072g/cm³. Remember, this number is the density not the SG! So going back to the beginning of this post …

SG = ρ-sample (at 20°C)÷ρ-reference (at 20°C) = 1.0072÷0.9982 = 1.009

… or what I had before but without the hidden tricks (with perhaps some irrelevant little differences because the equipment isn't "laboratory" standard, nor does it need to be thankfully, plus I might of pulled barely relevant dirty tricks in my previous posts).
Last edited by PeeBee on Sat May 09, 2020 6:06 pm, edited 1 time in total.
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Re: Hydrometer? Refractometer? Or something better?

Post by richard080561 » Thu May 07, 2020 6:01 pm

"should I put the sample back and risk cross-infection, or should I ditch it?".
The answer is to take a sip, for QA purposes, then pour it back.
A Pycnometer.

NOTE: Often pronounced "pick-nom-eater", but as it's derived from the Greek word for "dense" ("πυκνός") I'd prefer "puk-nom-eater".
Shouldn't it be "pick-no-meter"?

I use a refractometer before fermentation and a hydrometer once fermentation is underway
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Re: Hydrometer? Refractometer? Or something better?

Post by PeeBee » Thu May 07, 2020 9:50 pm

richard080561 wrote:
Thu May 07, 2020 6:01 pm
… I use a refractometer before fermentation and a hydrometer once fermentation is underway
Double the trouble! I've described why I don't want to use (can't use?) either of those instruments, and suggested what I might think of anyone suggesting those instruments are the ones to use. But I suppose you're not really waving them about as superior to what I suggest, so I'll let you off. :)

Cheers!
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Re: Hydrometer? Refractometer? Or something better?

Post by richard080561 » Thu May 07, 2020 9:57 pm

PeeBee wrote:
Thu May 07, 2020 9:50 pm
richard080561 wrote:
Thu May 07, 2020 6:01 pm
… I use a refractometer before fermentation and a hydrometer once fermentation is underway
Double the trouble! I've described why I don't want to use (can't use?) either of those instruments, and suggested what I might think of anyone suggesting those instruments are the ones to use. But I suppose you're not really waving them about as superior to what I suggest, so I'll let you off. :)

Cheers!
How gracious of you.
I find the instruments I use simple and efficient. I didn't need to read four long posts to understand how to use them. I wouldn't be so rude as to tell people what they should use or what I think of them if they don't like instruments that I use.
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Re: Hydrometer? Refractometer? Or something better?

Post by guypettigrew » Fri May 08, 2020 8:16 am

My instrument of choice is a digital refractometer. Drop the liquid in, leave it a while to let the temperature stabilise and get an easy to read Brix value. You can tell when the temperature has settled because the reading doesn't alter when repeated. I find it much easier to use than either a hydrometer or an optical refractometer.

Then I use the Northern Brewer calculator to tell me what the gravity is.

It may not be 100% accurate, but it's certainly good enough for home brewing. And it's another toy to play with!

Guy

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Re: Hydrometer? Refractometer? Or something better?

Post by PeeBee » Fri May 08, 2020 12:52 pm

guypettigrew wrote:
Fri May 08, 2020 8:16 am
My instrument of choice is a digital refractometer. …
They do look the bee's knees! I'll have to get one of them … %*!$#$! (flippin' rich kids!). :wink:
richard080561 wrote:
Thu May 07, 2020 9:57 pm
… I find the instruments I use simple and efficient. I didn't need to read four long posts to understand how to use them. …
I find it fascinating that while my eyes dance about trying to fixate on something (such as fine parallel lines), what I see is generally rock solid, no dancing about at all, but no sharp detail either. It's all in me head so there aren't any glasses that can fix it. That's why I think hydrometers and refractometers are cr&p.


So I resurrect a little used instrument (in brewing circles) to fix that issue for me, but others might chose pycnometers for other reasons. But the trouble is how to describe how they work in a few words, so I have to resort to "long posts".

But look what I'm describing in those posts: How a hydrometer works, so I can copy it with a pycnometer. And most brewers haven't the first idea what a hydrometer is telling them. So I'm disadvantaged from the start. First off I've got to educate folk what a hydrometer tells them and then describe how to get that out of a pycnometer. If I don't and provide a simplistic view of pycnometers, someone in the know will tear a strip of me for describing the use of pycnometers all wrong.


Let me give an example. The specific gravity of water. Which everyone knows is 1.000. Now to get that from a pycnometer, we fill it with water (@20°C) weigh it and work out it's density (which using the pycnometer example above):

26.32g ÷ 26.37ml = 0.998g/cm³

A bit weird? Except hydrometers don't read density, they read relative density (specific gravity). No problem, the density of the reference material is also water (at 20°C) so just divide the density of water by itself:

0.998g/cm³ ÷ 0.998g/cm³ = 1.000

This is what a "Noddy" brewers' hydrometer is calibrated to. What if we used a scientific precision hydrometer calibrated to a 4°C reference water sample?

The density of water at its densest (3.98°C, or 4°C) is near enough 1.000g/cm³. Water's density at 20°C is 0.9982g/cm³. So a scientific hydrometer is calibrated so that when in water at 20°C it is indicating 0.998 (0.998 ÷ 1.000: Damn, I've been writing it wrong on a couple of preceding posts as 1.002!). Many brewers who had a hydrometer that recorded 0.998 in water would chuck it in the bin! So home-brewing hydrometers are "calibrated" (often roughly) to a 20°C reference sample.

EDIT: Minor alterations 'cos I was mixing up weights, volumes and temperature (or … I'm just plain mixed up anyway).
Last edited by PeeBee on Sat May 09, 2020 7:25 pm, edited 1 time in total.
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Re: Hydrometer? Refractometer? Or something better?

Post by richard080561 » Fri May 08, 2020 1:09 pm

Tl,dr
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Re: Hydrometer? Refractometer? Or something better?

Post by guypettigrew » Fri May 08, 2020 1:11 pm

PeeBee wrote:
Fri May 08, 2020 12:52 pm
guypettigrew wrote:
Fri May 08, 2020 8:16 am
My instrument of choice is a digital refractometer. …
They do look the bee's knees! I'll have to get one of them … %*!$#$! (flippin' rich kids!). :wink:
Not really, it's just what you choose to spend cash on. My eyes aren't all that great, and a nice big digital read out is really helpful!

Guy

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Re: Hydrometer? Refractometer? Or something better?

Post by Silver_Is_Money » Sat May 09, 2020 10:10 am

This one makes life simple, albeit that it breaks the bank:

https://www.coleparmer.com/i/mettler-to ... er/2575330

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Re: Hydrometer? Refractometer? Or something better?

Post by PeeBee » Sat May 09, 2020 11:11 am

guypettigrew wrote:
Fri May 08, 2020 1:11 pm
PeeBee wrote:
Fri May 08, 2020 12:52 pm
guypettigrew wrote:
Fri May 08, 2020 8:16 am
My instrument of choice is a digital refractometer. …
They do look the bee's knees! I'll have to get one of them … %*!$#$! (flippin' rich kids!). :wink:
Not really, it's just what you choose to spend cash on. My eyes aren't all that great, and a nice big digital read out is really helpful!

Guy
You know I was kidding 'cos you know I have one of those even more expensive "Tilt" thingies that have the extra feature of doubtful accuracy too! Anyway, the Rich Kids have got one of those toys Silver_is_Money posted!
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Re: Hydrometer? Refractometer? Or something better?

Post by guypettigrew » Sat May 09, 2020 12:29 pm

Yup, knew you were kidding! That Silver_is_money thing is amazing. Just as well it can't be shipped over here to the UK or we'd all be ordering one--not!

Guy

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