# Hydrometers: Plato vs SG

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#### ljm

##### Member
As part of the process of tweaking my brewing game I am about to abandon my old and mostly trustworthy lhbs 0.999-1.17 hydrometer in favor of a precision hydrometer or two. To go along with this I have also nearly completed a new spreadsheet that works with either Plato or specific gravity measurements.

My question: Are there any sound reasons for choosing between Plato and SG hydrometers? As I understand, all the work is based on Plato's tables with which I only have a second-hand familiarity. Is there something in them or the way they were constructed that would favor one hydrometer over the other? In any case, with calculations there is always a need to find one quantity from the other.

The most precise hydrometers I have seen online (and can afford) are single scale and have graduations of 0.0005 SG or 0.1 Plato/Brix. Both seem fine and the precision difference between them isn't significant.

Thanks in advance for any and all input.

What the Plato comission did was measure the specific gravity of solutions they had made up to known °P (% w/w) strength so there is a 1 to 1 correspondence between the two. It's like Faherenheit and Centigrade in the sense that if you know one you know the other. The only advantage to the Plato scale hydrometers that I can think of is that if you have the Plato value dividing it by 100 and multiplying that by the weight of the wort gives you the mass of extract in it. This is very handy if you have load cells under your kettle but few do. In that case you need the specific gravity of the wort which you get by using the tables backwards. You then convert that to density and multiply by the volume to get the weight of wort. I suppose one advantage of the specific gravity scale is that you can plug it into a formula which gives you °P. To go in the other direction you have to invert than formula using something like root bisection or the Excel Solver which home brewers seem to be afraid of.

Whichever you decide on you will be converting to the other.

Plato is more 'professional' if that matters to you.

You have illuminated a solution to the major frustration with my current setup - the %\$!* dipstick for volume "measurement". Measurement when boiling, constant corrections for temperature and, well, seemingly "needless" approximation that does not feed my need for better precision and accuracy. The chain of going from grain + water to fermenter is only as good as the weakest link - volume measurement in my case.

Although I expect load cells are beyond my \$\$ range, I can certainly see the advantage of good mass measurement of water and wort. With a fairly good mass (or less likely, volume) determination, the chain looks solid with extra sig figs at the end. As others including myself have pointed out, only the quality of the final product is really important. But for better and sometimes worse I am a quantitative guy.

You must have a very nice setup...

Although I expect load cells are beyond my \$\$ range...
They ain't cheap.

...and they drift.

You have illuminated a solution to the major frustration with my current setup - the %\$!* dipstick for volume "measurement". Measurement when boiling, constant corrections for temperature and, well, seemingly "needless" approximation that does not feed my need for better precision and accuracy. The chain of going from grain + water to fermenter is only as good as the weakest link - volume measurement in my case.

Although I expect load cells are beyond my \$\$ range, I can certainly see the advantage of good mass measurement of water and wort. With a fairly good mass (or less likely, volume) determination, the chain looks solid with extra sig figs at the end. As others including myself have pointed out, only the quality of the final product is really important. But for better and sometimes worse I am a quantitative guy.

You must have a very nice setup...

Why don’t use the perfect cylinder approximation for volume?

...and they drift.
I haven't found that to be a problem. I put up to a hundred pounds of grain and about 250 lbs of water in at mash in and find that they return to within a pound or so of tare at the end of the day.

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Why don’t use the perfect cylinder approximation for volume?
I have a tuna can (short, squat) style BK with a rounded edge at the bottom, throwing off the perfect cylinder model (I have tried it). My *&%! dipstick is a little more accurate because I measured the water for each mark. And even that is only marginally OK when the brew is boiling. (What is the water level?) And we still have a volume correction for temperature.

I like the idea of taking a sample, getting Plato, SG and wort mass (mw) and then calculating the 20C volume ( V = mw/(dens(H20) * SG) ). How cool would that be? But alas it is only a fond dream unless I can find some cheap, decently accurate industrial scales to set under mash tun and BK and my wife agrees to clutter up the garage even more.

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As I would be certain to be unable to find a dipstick when I needed it I took the approach of adding known volumes of water to the tuns and measuring the distance from the rim to the surface of the water. This allows for irregularity in kettle cross section (caused in my case more by the presence of steam coils than shape). The numbers into a table which is on my computers with a laminated copy that stays in the same drawer. Then all I need to be able to find is a tape measure and I've got 3 or 4 of those around so I can almost always find one.

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