Using weight instead of volume for calibrating sight glass

So I'm a little anal when it come to calibrating my new sight glass, and I want it to be as close to perfect as is possible. I have a high accuracy scale and my plan is to weight out 1/2 gallon at a time as opposed to measuring it by volume. I have an 8 cup Pyrex measuring cup so I can measure the full amount but I feel it wouldn't be as accurate as doing it by weight. My research turned up a value of 8.345 lbs per gallon, but everything I could find said this varied due to pressure and temperature. What I couldn't find out was by how much. If it's a matter of 0.005 lbs every 50 degrees or so, then no big deal but it's 0.05lbs every degree that could make a substantial difference. Any chemistry majors out there know the answer? Anyone else use this method for calibrating a sight glass?

Again, I know, overkill but that's how I roll!

A little more research turned up this which appears to be fairly accurate although different from my previous results:

http://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html

Does this look right? My current tap water is about 72° so I'm thinking somewhere around 8.32 lbs would be accurate?

Well there is quite a bit of difference between the density of water at boiling vs room temp... but what you really need to decide is at what temperature are you going to take your measurements. Everything I have seen says to take your measurements on the cold side before any heat is applied, and I believe most software takes this into account as well. I would make your measurements at room temp (77F) and use 997g/L as your baseline. I did that years ago and it has worked well for me.

Well, you got me interested, and I was able to find this link. Looks like at room temp it will be a little less than that (it looks like the standard is at 4C). I'd be surprised if the pressure of your place is going to differ much from standard, so I wouldn't worry about that. Of course, this is also pure water, where your water is likely to have some dissolved salts and what-not, adding to the weight. It also depends on how accurate your scale is (and how recently its been calibrated).

I'm confused. You're going to measure a weight of water, pour it into your kettle, and mark the sight glass according to the level?
So, the sight glass is measuring a volume, ultimately. Say water's density changed dramatically between 60 and 80 degrees. You measure 1 gallon of 60 degree water, mark it on your sight glass, and then tomorrow you use the sight glass to move 1 volumetric gallon of water at 80 degrees. What have you accomplished by having a sight glass that's calibrated by weight, if you subsequently use it by volume?

Cromwell said:

I'm confused. You're going to measure a weight of water, pour it into your kettle, and mark the sight glass according to the level?
So, the sight glass is measuring a volume, ultimately. Say water's density changed dramatically between 60 and 80 degrees. You measure 1 gallon of 60 degree water, mark it on your sight glass, and then tomorrow you use the sight glass to move 1 volumetric gallon of water at 80 degrees. What have you accomplished by having a sight glass that's calibrated by weight, if you subsequently use it by volume?

Click to expand...

If you measure water volume at any temperature you are still going to have the same issue when you change the temperature. Your sight glass measurement is only accurate at the temperature of the water you calibrated it with.

Cromwell said:

I'm confused. You're going to measure a weight of water, pour it into your kettle, and mark the sight glass according to the level?
So, the sight glass is measuring a volume, ultimately. Say water's density changed dramatically between 60 and 80 degrees. You measure 1 gallon of 60 degree water, mark it on your sight glass, and then tomorrow you use the sight glass to move 1 volumetric gallon of water at 80 degrees. What have you accomplished by having a sight glass that's calibrated by weight, if you subsequently use it by volume?

Click to expand...

Water volume will vary based on temperature if I calibrate with volume or weight it doesn't matter, 1 gallon will be different volumes at either temperature no matter what.

RiverCityBrewer said:

Well there is quite a bit of difference between the density of water at boiling vs room temp... but what you really need to decide is at what temperature are you going to take your measurements. Everything I have seen says to take your measurements on the cold side before any heat is applied, and I believe most software takes this into account as well. I would make your measurements at room temp (77F) and use 997g/L as your baseline. I did that years ago and it has worked well for me.

Click to expand...

Interesting, I had assumed you'd want to mark for cold temps, but if I really wanted to be anal I could calculate equivalent weight of one gallon at 212 for BK and ~150 for HLT. That way I'd know true values of volume for each vessel at typical operating temps!

Told you I was anal

I know Beersmith factors in a "cooling factor" for volume. I've always assumed it was for strike water and MT run-offs so that the final volume is correct after cooling.

Don't forget to factor in latitude, altitude, local topology and geology, air density, and the position of the Sun and Moon when weighing your water!

Umm to be honest with you guys you might be over thinking this. RDWHAHB.

I went with 8.33 lbs/gal after doing some google searches.
I had no way to check the calibration of my scale. That could easily
be the largest source of error.

The brew calculators I have looked at assume you are measuring cold water.
And they they typically include fudge factors (eg: boil kettle loss, mash tun loss)
so it is easy to dial in workable values once you have some experience with your system.

Thanks guys, I ended up using the 8.32 value, and calibrated the whole keg. I know it might sound overly complicated and unnecessary, but I think true volumes are important and measuring this sightglass correctly the first time was important to me. (I was honestly joking about calibrating each vessel differently) I'm comfortable now knowing that my markings are as accurate as possible with the tools I have on hand.

Pure water weighs 8.33 lb/g, salt water around 8.5. With all the crap in that Ohio water, I imagine yours is somewhere in between.