Specific Gravity Standards

Joe_BJoe_B Posts: 318Solar Expert ✭✭✭✭✭✭
Today I was equalizing my batteries and after measuring the SG and correcting for temperature, all of my cells read 1.244 and all of them were within .005 of each other. These batteries are pretty new and perform to specs. So it occured to me that since these are all healthy, the low SG is probably due to the accuracy of my bulb type hydrometer. So I was wondering if they made a standard test solution with a known SG that I could use to determine a correction factor for my readings. I was thinking the plain H2O had a SG of 1.000 but my hydrometer starts at 1.100 so I need something denser. What hydrometers do you guys use and how do you calibrate them?

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Specific Gravity Standards

    Right you are on the SG of pure water being 1.000 and most hydrometers start at 1.100.
    Things that affect an SG reading other than the actual liquid include temperature and elevation. At 3,000 feet my SG readings are low by 0.1 consistently.
    The trick is to take initial readings when you first get the batteries to establish a baseline to compare future readings to. Keep that written down somewhere safe, like burned with a soldering iron into the wall near the batteries, and then you're good. Until you have to get a new hydrometer because the old one broke. It happens.

    I don't know of any "calibration liquid" available, but that doesn't mean there isn't one. All readings are relative anyway, even from a "known" source.
  • Joe_BJoe_B Posts: 318Solar Expert ✭✭✭✭✭✭
    Re: Specific Gravity Standards

    Well, I did a bit of googling and came up with this, I am a little hesitant to use NaCl because of what happens when it gets mixed with H2SO4. (yeah I watched a lot of submarine movies) But here it is for what its worth, I hope its OK to post this but there was a permission at the bottom of the page. Mods please delete as appropriate if I am in violation.

    I am looking for another solution that is more friendly in the presence of sulfuric acid.

    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 hydrometer's.

    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
    [email protected]
    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.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Specific Gravity Standards

    Just to note that the experiment described is evidently for checking a wine hydrometer. Note this portion: "Specific gravity is 1.202, which is way off your hydrometer scale." That would actually be viable for a battery hydrometer.

    That said, if you do this wrong you still won't know if your hydrometer is accurate or not.
    I went looking for info on this and as close as I could come to a readily available "test solution" is glycerin with an SG of 1.260 @ 25C. Don't know if that would help.

    It's a bit of a "Quis custodiet ipsos custodes?" situation. :roll:
  • Joe_BJoe_B Posts: 318Solar Expert ✭✭✭✭✭✭
    Re: Specific Gravity Standards

    Mr. Coot, you are awesome! Glycerine is much easier, I will check to see if bad things happen when glycerine meets sulfuric. (glad we dont use nitric in these things)
  • Blackcherry04Blackcherry04 Posts: 2,490Solar Expert ✭✭✭
    Re: Specific Gravity Standards

    How about a Qt. of Automotive Battery Electrolyte (1.260 ) at Autozone for $4.99 and Tax.
  • Joe_BJoe_B Posts: 318Solar Expert ✭✭✭✭✭✭
    Re: Specific Gravity Standards
    How about a Qt. of Automotive Battery Electrolyte (1.260 ) at Autozone for $4.99 and Tax.

    I think a pure substance would be better than a mixture for calibration. The tolerance of that might be 5% or worse.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Specific Gravity Standards
    How about a Qt. of Automotive Battery Electrolyte (1.260 ) at Autozone for $4.99 and Tax.

    Ah, America! Where you can actually go in to a store and buy something without being stared at like you're not even speaking English or being told to fill out 60 pages of government forms to purchase what is deemed a hazardous, toxic, controlled substance at a cost of $3.89 per ml. :p

    You guys have some great advantages down there. No joke.
  • Blackcherry04Blackcherry04 Posts: 2,490Solar Expert ✭✭✭
    Re: Specific Gravity Standards
    Ah, America! Where you can actually go in to a store and buy something without being stared at like you're not even speaking English or being told to fill out 60 pages of government forms to purchase what is deemed a hazardous, toxic, controlled substance at a cost of $3.89 per ml. :p

    You guys have some great advantages down there. No joke.
    lol.... yeah, we can for awhile, somebody will get a wild hair and ban it before you know it.

    You can also get Aviation Electrolyte (1.285 ) for about the same price. but you have to order it and I think it would be certified. It goes into " Gill " Aviation batteries.
  • waynefromnscanadawaynefromnscanada Posts: 3,009Solar Expert ✭✭✭✭
    Re: Specific Gravity Standards
    Ah, America! Where you can actually go in to a store and buy something without being stared at like you're not even speaking English or being told to fill out 60 pages of government forms to purchase what is deemed a hazardous, toxic, controlled substance at a cost of $3.89 per ml. :p

    You guys have some great advantages down there. No joke.

    So far here in Nova Scotia, there doesn't seem to be a problem. I once worked at a place that sold, among other things, dry-charged batteries. We got our acid from an auto supply store. Now a couple of months ago, my cousin got some brand new L-16's that were delivered, new from the factory, low on electrolyte. Again, no problem getting additional acid from a local auto parts outlet. In fact, I had to do the same with my L-16's (from a different manufacturer) when I got them almost 10 years ago.
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