Measuring Specific Gravity

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Rngr275
Rngr275 Solar Expert Posts: 127 ✭✭
in Off Grid Solar & Battery Systems #1
I have read about needing to adjust the SG reading based on the temperature. But I got to thinking about this ( sorry about that:p).
Does the specific gravity read low or high based on temperature? Meaning:Say batteries are at 50F, Trojan says subtract 0.004 for every 10F below 80F. Lets say I measure the SG and it reads 1.260 ( temp is at 50F). Do you subtract 0.12 (3 x 0.004) from the 1.260 or is the SG reading low because of the temperature and if the battery was really at 80F the SG would be reading 1.272?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Measuring Specific Gravity
    Rngr275 wrote: »
    I have read about needing to adjust the SG reading based on the temperature. But I got to thinking about this ( sorry about that:p).
    Does the specific gravity read low or high based on temperature? Meaning:Say batteries are at 50F, Trojan says subtract 0.004 for every 10F below 80F. Lets say I measure the SG and it reads 1.260 ( temp is at 50F). Do you subtract 0.12 (3 x 0.004) from the 1.260 or is the SG reading low because of the temperature and if the battery was really at 80F the SG would be reading 1.272?

    It's a fluid and behaves as such: heat it up and it expands, becoming less dense and therefore giving a lower density reading (specific gravity) than what would indicate the actual concentration of acid. Cooling has the opposite effect.

    Altitude affects it similarly, although people keep disagreeing with me on that even though I've checked it at 750 feet and 3200 feet. :roll:
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  • Rngr275
    Rngr275 Solar Expert Posts: 127 ✭✭
    #3
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    Re: Measuring Specific Gravity

    So to be clear... If I measured the SG @ 1.260 and the temperature of the battery/acid was 50F the I need to subtract the 0.012 from the reading? I beleive this is correct and is what I have been doing. My confusion came when I considered that the cold acid could be making the reading of the hydrometer lower by the 0.004 per 10F. But now that I consider that cooling concentrates the acid... t
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: Measuring Specific Gravity

    No, add.
    With higher temperature the indicated SG will be lower than the actual density (as a percentage of acid content).

    More acid in the electrolyte = higher density. For any given percentage of acid (including none) if you raise the temperature of the fluid it becomes less dense so the hydrometer reading will falsely read lower than it would at the standard temperature.

    So when you apply the compensation factor you are adjusting the hydrometer's reading to reflect the actual percentage of acid in the electrolyte.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #5
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    Re: Measuring Specific Gravity
    Rngr275 wrote: »
    So to be clear... If I measured the SG @ 1.260 and the temperature of the battery/acid was 50F the I need to subtract the 0.012 from the reading? I beleive this is correct and is what I have been doing. My confusion came when I considered that the cold acid could be making the reading of the hydrometer lower by the 0.004 per 10F. But now that I consider that cooling concentrates the acid... t

    You are correct... The "standard" is ~77F reference temperature for electrolyte... If the electrolyte is cold, it tends to be more dense. If it is hot, it tends to be less dense. So, "we" just pick a handy number (around room temperature) to call that "standard" conditions and "correct" any readings not done at standard conditions would be "corrected" to the standard.

    There is nothing magic about 77F... It could have been 32F or 95F. Just a value so that we can estimate the ratio of sulfuric acid to distilled water with a consistent method.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    #6
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    Re: Measuring Specific Gravity
    No, add.
    With higher temperature the indicated SG will be lower than the actual density (as a percentage of acid content).


    Now I'm confused.

    in Rngr's example, the solution temp is lower than the Standard.

    My understanding was that if the solution temp was lower than the standard (for example, Standard temp 80F, solution temp 50F) then the adjustment factor would be subtracted from the Hydrometer reading to obtain the true SG. This is because the solution is more dense than the Standard and the Hydrometer is reading higher than the true SG. Conversely, if the solution temp is higher than the Standard, then the correction factor is added to the Hydrometer reading.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
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    Re: Measuring Specific Gravity

    Apologies: someone around here keeps forgetting whether he's dealing with Celsius or Fahrenheit. I think it's me. :blush:
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #8
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    Re: Measuring Specific Gravity

    Rybren,

    As the battery cools down, the acid doesn't become more concentrated. It is still the same concentration (e.g. 30% acid/ 70% water). However, both water and acid become more dense. Therefore, the density (that's what the hydrometer measures) becomes higher. Say, it was 1.250 before the battery cooled down, but is now 1.260. You use your hydrometer and measure 1.260. To correct this to 25C and go back to 1.250, you need to subtract the correction (e.g. 1.260 - 0.010 = 1.250).

    Similarly, if the battery gets warmer, everything gets less dense. Now, if you do the measurement, you get lower value, say 1.240. To correct it back to where it was at 25C, you need to add someting (e.g. 1.240 + 0.010 = 1.250).

    If hydrometer would measure concentration directly (not density), you wouldn't need the correction.
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  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    #9
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    Re: Measuring Specific Gravity

    NorthGuy

    Thanks. In my head, that's what I thought I said (:p).
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #10
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    Re: Measuring Specific Gravity
    Cariboocoot wrote: »
    More acid in the electrolyte = higher density. For any given percentage of acid (including none) if you raise the temperature of the fluid it becomes less dense so the hydrometer reading will falsely read lower than it would at the standard temperature.

    So when you apply the compensation factor you are adjusting the hydrometer's reading to reflect the actual percentage of acid in the electrolyte.

    A complete and rigorous treatment of the whole SG versus temperature problem involves making it clear whether the SG is being measured relative to a reference of water at the standard temperature or water at the measurement temperature.
    Depending on the background assumption, the SG of pure water (your 0% example) will or will not also require temperature correction. :-)
    Such Fun!
    AFAIK, the commonly accepted practice is to state the SG with a reference of water at the standard temperature rather than water at any single universal reference temperature. <sigh>
    SMA SB 3000, old BP panels.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #11
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    Re: Measuring Specific Gravity
    Rybren wrote: »
    NorthGuy

    Thanks. In my head, that's what I thought I said (:p).

    That definitely is :D
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