Measuring SG with a scale

[Calypso]

I just read about a method for measuring SG that uses a jeweler's scale (which I have). I'm pretty sure this method could be used with hot wort.

http://www.forrestwhitesides.com/node/49

Does anyone know of or see a problem with this approach?

 

[Qhrumphf]

It will only work for hot wort if you're accounting for the heat in the volume measurement. Water (and wort) expand with increase in temperature, and as it expands, the density goes down, and the gravity goes down accordingly.

 

[Calypso]

It will only work for hot wort if you're accounting for the heat in the volume measurement. Water (and wort) expand with increase in temperature, and as it expands, the density goes down, and the gravity goes down accordingly.

Good point. So I would need to calculate the mass of an equal volume of water at an equal temperature.

So if we say the wort is 150F, we'd want to divide the mass of 40 mL of wort by the mass of 40 mL of water at 150F. Which is... 39.21. Did I get that right? (I just used this website to calculate the density of water at a particular temperature: http://antoine.frostburg.edu/chem/senese/javascript/water-density.html)

 

[Qhrumphf]

As specific gravity is a ratio, my understanding was that if you had the water at the same temperature and same volume, just weigh the wort then weigh the water and divide.

 

[Qhrumphf]

I guess the point here is that it's a lot of work to do it this way. While physically possible in a jam (say, as in your first link, your hydrometer breaks at an inopportune time), it's a lot less work to just chill a sample and read with a hydrometer.

 

[Calypso]

I guess the point here is that it's a lot of work to do it this way. While physically possible in a jam (say, as in your first link, your hydrometer breaks at an inopportune time), it's a lot less work to just chill a sample and read with a hydrometer.

I respectfully disagree, for the following reasons:

1) Chilling ~75 mL of wort to 60F from 150F takes me almost 20 minutes.
2) It's really hard to get a good reading from my hydrometer. Either the cylinder is fogged up or dirty or the hydrometer is facing the wrong way or I can't tell what line it's at.
3) It requires a lot of wort. If I can pull 40 mL rather than 75 mL, that's a win.

 

[Psylocide]

Hydrometer definitely sounds easier.

You're describing it like the black and white parts of infomercials.

 

[Calypso]

Hydrometer definitely sounds easier.

You're describing it like the black and white parts of infomercials.

Order now and get not one, not two, but THREE graduated cylinders for the low low price of $19.95 plus shipping and handling! But WAIT, there's MORE!

I guess we all have different tolerances for different kinds of hassles. For me, weighing a sample and doing some simple math beats the pants off of dealing with a hydrometer.

 

[Psylocide]

Yeah... I've just never experienced any of those problems. It's so easy, I don't even have to think about it.

Chill to calibration temp... spin drop. If it's facing the wrong way, look on the other side. As for chilling taking 20 min... If you miss preboil what are you going to do about it? Add DME? Also, you can use a temp correction calc to get a decent idea of what it will be right away.

 

[1977Brewer]

I'm perfectly happy taking hydrometer readings. Especially post fermentation. Flat warm beer that I made with my hands beats...nothing really. It just helps.

 

[Calypso]

Yeah... I've just never experienced any of those problems. It's so easy, I don't even have to think about it.

Chill to calibration temp... spin drop. If it's facing the wrong way, look on the other side.

You make it sound so easy. I should take a video of me trying to get a reading. It's like a comedy of errors.

 

[Psylocide]

Reread that post too. It can't be as hard as you're making it out to be.

 

[doug293cz]

To do this right you need something called a pycnometer, which is a very precise volumetric flask. A graduated cylinder is nowhere near precise enough. Having done some procedures with a pycnometer back in school, there is no way I would choose that over a hydrometer.

If you need something to satisfy your inner geek, then go for it. But, do yourself a favor: do a sensitivity analysis to determine how much the probable errors in volume and weight measurements will affect the results.

Brew on

 

[1977Brewer]

You make it sound so easy. I should take a video of me trying to get a reading. It's like a comedy of errors.

Like a WaxVac infomercial, or more like a ShamWow infomercial?

 

[Calypso]

Chill to calibration temp... spin drop. If it's facing the wrong way, look on the other side. As for chilling taking 20 min... If you miss preboil what are you going to do about it? Add DME? Also, you can use a temp correction calc to get a decent idea of what it will be right away.

Well, part of this stems from the fact that it was recommended in another thread that, in order to diagnose a problem, I take SG readings during mashing every 15 minutes. But yes, if I miss my target SG by enough, I likely would add DME to hit my target.

I thought that hydrometer scales become increasingly inaccurate the farther you get from the calibration point, even after you correct?

 

[Qhrumphf]

Or, refractometer. A few drops is all you need, and by running the bulb of a pipette under cold tap water you can cool your sample in literally seconds.

 

[jtratcliff]

The more things you have to measure, the more error creeps into the final value. With the scale, you need to accurately measure the temperature, volume, and mass of the wort... with measurement errors associated with each measurement.

With the hydrometer, you are directly measuring density. 1 measurement.

Which is what specific gravity is... the ratio of the density of your sample to the density of water

 

[Calypso]

The more things you have to measure, the more error creeps into the final value. With the scale, you need to accurately measure the temperature, volume, and mass of the wort... with measurement errors associated with each measurement.

With the hydrometer, you are directly measuring density. 1 measurement.

Which is what specific gravity is... the ratio of the density of your sample to the density of water

Well, you have to measure the temperature as well. But point taken.

 

[jtratcliff]

true... you need the temperature of your sample. So perhaps that error is a wash in your error budget. So you just need to figure out the error of your volume measurement and the error in your mass measurement and see if that better or worse than the hydrometer error

 

[Calypso]

My plan at this point (I'm happy to post a follow up on how it goes if people like) is to buy a 20 mL syringe and 100g x 0.01g scale. I'll put the empty syringe on there and tare, draw 20 mL, then weigh again. Then I'll divide by the weight of 20 mL of water at whatever temperature.

The scale I looked at has an accuracy of +/- 0.02g, which at 20 mL gives me an error of +/- 1.001 SG. Which I can live with.

 

[Calypso]

So, I thought I'd come back and report on how it worked and give you all a chance to get your "toldyasos" in. It turns out the problem is not the accuracy of the scale. The problem is the difficulty in getting an accurate volume. Eyeballing 20 mL means that being off by 1% (which is trivial) is the difference between 20.8/20 = 1.040 and 21.1/20 = 1.060. Whoops! In order to have an acceptable margin of error I'd have to be measuring out something on the order of a liter of liquid.

Learning is fun!

 

[doug293cz]

I would like to know which scale you used. Most of the low range scales I have seen only read to 0.1 gram, but I'd like to get one that goes to 0.02.

Brew on

 

[Qhrumphf]

So, I thought I'd come back and report on how it worked and give you all a chance to get your "toldyasos" in. It turns out the problem is not the accuracy of the scale. The problem is the difficulty in getting an accurate volume. Eyeballing 20 mL means that being off by 1% (which is trivial) is the difference between 20.8/20 = 1.040 and 21.1/20 = 1.060. Whoops! In order to have an acceptable margin of error I'd have to be measuring out something on the order of a liter of liquid.

Learning is fun!

Yep. No one denied that you could do it. Just said it wasn't practical.

Toldyaso :fro:

In all seriousness, brewing is much like everything else in the world, just because you can doesn't mean you should.

Now, that does provide an interesting idea in my book.

If you know the temperature and gravity of the wort, and have an accurate enough scale (with a high enough weight tolerance), you can determine the volume, probably with more accuracy than the "quart by quart" self measurement, because we all know how accurate fermenter volume marks (and kettle/sight glass volume marks) are.

 

[Calypso]

I would like to know which scale you used. Most of the low range scales I have seen only read to 0.1 gram, but I'd like to get one that goes to 0.02.

Brew on

I picked up this scale: http://www.amazon.com/gp/product/B0012LOQUQ/?tag=skimlinks_replacement-20

The precision is 0.01g and the tolerance is +/- 0.02g. Max is 100g.

 

[Gavin C]

You make it sound so easy. I should take a video of me trying to get a reading. It's like a comedy of errors.

I want to see that. Maybe add some Benny Hill music to it.

Regarding the time to chill. You can put the sample in a metal/glass bowl that you had placed in the freezer. That will chill it very quickly. Put it in a ziplock bag and place it in cold water. That will work fast too. There are lots of ways to chill it fast.

I'm in no rush. I take my pre-boil gravity (170F)and put it in the freezer for a few minutes. It's down to 60F very quickly but it's not data I need fast anyway.

After the boil, I chill and take my sample from the kettle (under 70F). Prep it the same way, this time even faster. But again no rush.

If you are having trouble reading it, take a picture and make it big or buy narrow range hydrometers. I got a set of three. Not really crucial but I am a geek so I just couldn't help myself.

Pictures are easy to read. Just an idea.

I think your solution lends itself to more problems than it solves through compounding measurement errors.

Calibration temperature of my hydrometer (old one) is 68F

Pre-boil of a recent batch

Final gravity of the same batch after fermentation is complete

 

[dunbruha]

Pre-boil of a recent batchView attachment 281629

Final gravity of the same batch after fermentation is complete[/SIZE][/B]View attachment 281630

Just out of curiosity, what gravity values did you get for these two pictures? I'm still having trouble deciding where to read it.

 

[Gavin C]

Just out of curiosity, what gravity values did you get for these two pictures? I'm still having trouble deciding where to read it.

Just checked my Beersmith data

Recorded as
1.039 preboil gravity and 1.010 FG

 

[dunbruha]

Just checked my Beersmith data

Recorded as
1.039 preboil gravity and 1.010 FG

Thanks. I'm still not sure how to read the hydrometer. I thought you were supposed to look at the bottom of the meniscus to get the reading. Based on your preboil picture, I would read 1.042, and 1.012 for the FG. So maybe I'm reading it wrong, or maybe the pictures don't give a good representation of the reading?

 

[Gavin C]

I think the images are accurate are but I am biased I suppose.

I would not aggree with your interpretation although I could concede the preboil being interpreted as 1.040 based on the image.

 

[ncbrewer]

Another aid in reading the hydrometer is a magnifying glass. My vision isn't very good, and this helps me a lot, and doesn't take much extra time.

If you find the scale is facing away from you, you can turn the cylinder clockwise or counter clockwise a few times. The hydrometer will follow slowly. This will get it facing you in less than 10 seconds usually.