CHECKING YOUR HYDROMETER

Rev. H

Recently I was plagued by a persistent problem with specific gravity of beers and wines coming out too low. I suspected my hydrometer, but I had checked it several times against plain water, it always read 1.000. So my attention shifted to other areas, to no avail. Finally I decided to check the hydrometer some more.

What I use is the ordinary Crosby & Baker "beer and wine" hydrometer that you can buy lots of places for six bucks or so. It is a long glass bulb with what looks like a blob of lead in the bottom and a paper scale curled in the skinny top of the tube.

I made up a 22% by weight sucrose and water solution and checked two hydrometers. This solution should read 1.092 at 20C. Hydrometer #1 (the one I had been using) read 1.084. #2 (which had been hiding in the back of a drawer for 7-8 years) read 1.091.

Then I started over, made up another 22% solution, making up 50% more solution and using a different graduated cylinder to measure the water, and repeated the readings. #1 = 1.085, #2 = 1.092. These readings were all at 60F which is the reference temperature for my hydrometer.

So I began to believe those numbers. But I couldn't understand how the #1 hydrometer could be off like that, since I had checked it in plain water and it looked OK. So I decided to check both of them in plain water again. Results: #1 = 1.000, #2 = 1.000.

It becomes clear that a hydrometer cannot believed unless it is checked at two points. Just checking it in plain water is necessary but not sufficient.

After publishing my results onto beer and wine discussion groups, I was requested to provide a procedure that could be used to check a hydrometer. I had done all my tests using carefully weighed amounts of sugar on a triple-beam balance with 0.1 gram precision, and graduated cylinders for volumetric measurement. Not everyone has access to this kind of equipment. What is needed is a procedure that gives reasonably accurate results but can be simply performed with ordinary household or kitchen equipment.

Ordinary kitchen measuring cups, measuring spoons, and scales are notoriously inaccurate. So how can we get accurate results? After scratching my head over this for a while, I finally realized that we can let the physical properties of the materials themselves provide the accuracy. Looking around in my reference books, I determined that we can make up a saturated solution of table salt (sodium chloride, or NaCl) that has well-defined properties. Then we can easily and accurately dilute this solution to a usable concentration and check the hydrometer with it.

So here is the procedure:

1. Determine the reference temperature for your hydrometer. It should be marked on the instrument or given in the instruction sheet that came with it. There should also be a table for correction of readings taken at other temperatures. Try to get your solutions as close to the reference temperature as you can so you don't have to rely on "corrections" of unknown quality. Within 5 degrees F should be good enough.

2. Using plain tap water, take a hydrometer reading. Take care to avoid air bubbles sticking to the hydrometer. Bob it up and down, spin it. Record this reading. It should be 1.000, but write down whatever it is. Make sure you know the temperature of the water being measured.

3. Make up a reference salt solution (procedure given below) having a temperature close to the hydrometer reference temperature. Rinse your hydrometer flask with the salt solution, then fill with enough for a reading. Take a hydrometer reading on this reference solution. Write it down along with the temperature. It should be 1.106.

I will leave it to you to interpret your results. I performed the procedure given above using ordinary kitchen stuff and got 1.000 and 1.103 with my hydrometer #2 (1.094 with #1, the bad one). I consider this adequate. What is good enough? I dunno, it depends on what you want. If you want to just be able to read it and use the answers directly, then you ought to not accept poor results. On the other hand, if you are willing to take what it says and apply a correction, it really won't matter how bad it is, but it will be more trouble to use. My experience is limited to two of them, and both read very well in plain water. This is an important region for wines, because you generally want the S.G. to get below 1.000. I would say it ought to be within one or two gravity points with plain water. I would say it ought to be within 3-5 at 1.100. Remember that these are cheap hydrometers and you can't expect each one of them to exhibit high accuracy. You might have to buy two or three to find a good one, and that might be cheaper than buying a more expensive one. If your hydrometer is close at the low end, but off by say 6 points at the high end, then it will probably be off only about 3 points at 1.050, so you will probably be OK if you aren't brewing barley wines or mixing wine musts at 25 Brix. You will have to form your own opinion about what is good enough for you.

The main thing is to know what you have and not to assume it is good, even if it reads OK in plain water. If you only use the hydrometer infrequently, you might choose to apply an offset and scale factor correction based on the outcome of these tests.

If you get a bad result on the salt solution, before giving up on your hydrometer, please start over and repeat the procedure to reduce the chance of just making a procedural error. You can't believe the data unless they are repeatable.

Procedure for Making Up a Reference Salt Solution

Measure one cup of HOT tap water into a bowl or other container. Use your microwave to get it a little hotter if your tap water is not very hot. Boiling temperature is OK. Add about one-third cup of salt to the water and stir. Use canning or pickling salt, which is about all I can find that doesn't have additives and is pure NaCl. Rock salt, ice cream salt or unrefined sea salt can have as much as 5% impurities (something other than NaCl) according to the Morton Salt Company. Table salt can have potassium iodide up to 100 ppm, dextrose to stabilize the iodide, prussiate of soda, and calcium silicate. Kosher salt also has additives.

You may need more solution than this, it depends on how large your hydrometer flask is and what you will use to measure the equal volumes of saturated solution and water later in the procedure. It is better to make more than less, because the errors in measurement will tend to have less effect. However much solution you make up, use the same ratio, one cup water to one-third cup salt, as a starting point. Salt is cheap, so make up plenty solution.

The objective is to dissolve as much salt into the water as it will take. So stir, stir, stir. It will turn white. If all the salt dissolves, then add more. You need to end up with extra undissolved salt in the water. Using a blender with hot water is probably the best way (thanks to Rick Wiles for this idea). When you are sure that no more will dissolve, you want to cool it to room temperature, preferably close to your hydrometer reference temperature. Put the container of water and salt into a bowl and add some ice and water to the bowl. Don't get any in your solution. Let it sit for a while to clear and cool. Then carefully decant the liquid off the solid, pouring it into some other clean, dry container. Do not pour any of the salt crystals into the new container. If you do, don't worry, just let it settle and decant it again. You can leave a little liquid in the first container with the salt, since we won't need all of it, but just leave enough to avoid pouring out any salt crystals or any visible sediment.

This solution should be 26.38% NaCl by weight (at 20C/68F). Specific gravity is 1.202, which is way off your hydrometer scale. So we need to dilute it. Diluting one part of this solution with one part water will give a concentration of 14.40% with a specific gravity of 1.106, which should be OK for the ordinary wide-range hydrometers.

At this point, let's discuss sources of error so you can avoid them. We don't have to measure any amount of anything with absolute accuracy, but we do have to be able to get the same amount of liquid twice. You can use an ordinary measuring cup if you want to, but you will want to measure exactly the same way twice. During both measurements, use the same cup sitting on the same surface oriented the same way and measure to the same line. Be careful whether the liquid level comes to the bottom of the line or to the top of the line so you can do it the same way both times. You will get best results with a container having a high ratio of height to diameter, in other words, a "tall" shape. You can use a drinking glass with a piece of tape put on it as a reference mark. A salad dressing bottle with a skinny neck or a beer or wine bottle (preferably clear) is good (make sure the liquid level is into the neck). What chemists use to get an accurate volume is a thing called a "volumetric flask", which is a flask with a big round bottom part and a little skinny neck with a line on it. These will yield a couple tenths of a milliliter accuracy in a liter. So anything tall and skinny, even a flower vase, is good to use to get a repeatable volume. I have demonstrated repeatability of better than 1% measuring about 325 ml in a 12 oz. beer "pilsner" glass, using a mark on a piece of Scotch tape on the glass to mark the level of the liquid.

You can use more water and salt than I specified above if needed to fill your container, just make sure you have enough salt so that it won't all dissolve in the hot water. You also need to make sure the container is dry each time you start to make a measurement in it so as not to dilute your solution (don't be overly concerned about it, just wipe it out). Keep your spoon handy and stir the solution occasionally, wiping your spoon dry before each stirring.

Now pour the saturated solution of salt into your measuring container. Bring the liquid level exactly to your reference line (or mark your reference line at the level of the liquid). The absolute amount here is not important. What is important is that you can measure exactly this amount again. Now it's time to get really careful.

Pour this amount of solution into another clean dry container, trying to get it all. Shake the drops out into the second container. Now put in tap water at close to the same temperature (within 15-20 degrees F) and adjust the amount to be exactly the same amount you had before when you had the salt solution in the measuring container. Use a small spoon if you need to dribble small amounts of water into the container to make it come to the right place on your reference line. Pour this measured water into the equal amount of salt solution you already measured. Stir, stir, stir, to thoroughly mix. Pouring between two containers is also good for mixing. When this is well mixed, it is ready to pour into your hydrometer and find out the good or bad news.

Be sure to rinse your hydrometer and flask with the salt solution before filling it and taking a reading.

Unused solution can be stored in an evaporation-proof container (glass) for later use.

If you want to check at another point, add two more measures of water (for a 1:3 dilution). This will give a 7.55% by weight solution having specific gravity of 1.055 at 20C.

Stanley E. Prevost
Huntsville, Alabama
sprevost@knology.net
1/13/99

Rev. G 4/12/1999
Rev. H 1/2/2002

© Copyright 1999, 2002 Stanley E. Prevost All Rights Reserved. This document may be copied and distributed for noncommercial use provided that this copyright notice is maintained with the document.