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  1. #1
    BobL is offline Member: Blue and white apron brigade
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    Feb 2006
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    Default Measuring soluble oil dilutions using a refractometer

    Both my my mill and lathe have a compressed air powered mister lube system but my lathe still has a working soluble oil coolant/lube set up that I tend to still use for things like knurling. The other machine that has soluble oil coolant/lube is my little bandsaw. I typically replace the oil and clean the tank every few years and in between when teh tank level gets low I just top up with either water, or water+oil depending on what looks right in terms of how milky or clear the mixture is.

    At work we used a hand held refractometer to measure the coolant/lube dilution so I have used one before. I could still borrow one from work but repeated borrowing could be problematic as the borrowing relationship is getting a tad thin and would rather leave future opportunities for borrowing intact for something more important. So I decided to fork out and buy my own refractometer. This is the one I bought.
    https://www.ebay.com.au/itm/23287176....c100275.m3474

    It looks like this - the measurement units are "Brix" - usually some sort of factor is determined to covert Brix into a dilution but once you get used to using it you just work in Brix. A drop of fluid is placed on the clear blue window, a transparent plastic flap is then swung over and into the window to flatten the drop out and the unit is held under a bright light and the Brix reading (line between visible and blue and white aresa) is made by looking into the little telescope window .
    Screen Shot 2022-01-30 at 12.11.56 pm.png
    It's not the cheapest one around but it does have the relatively narrow range (0-10 Brix) required for measuring the typical dilutions used for soluble oils.
    Some refractometers will go to up to 40-50 Brix, but above about 10 Brix this is of no value for soluble oils and also means you have reduced measurement sensitivity between 0 and 10 Brix which is where you need it

    To check it out I made up a batch of small amounts of 4 known dilution ratio lube/water mixtures and measure the Brix rating on these.
    The nominal working dilutions for my oil (ROCOL) are between ~40:1 and 20:1
    The 4 dilutions I made were 43.5:1, 23.3:1, 26.6:12 and just for fun 9.9:1
    (Pure water should have a zero Brix reading - which it did but I did not include this in my calibration - I just forgot)
    Then I made a graph of Brix reading versus Dilution (Left hand vertical axis - blue) and inverse dilution (right hand vertical scale - blue) like this

    Screen Shot 2022-01-30 at 12.10.54 pm.png


    The red line (inverse dilution) being nominally linear indicates its all working reasonably well.
    The blue line clearly shows some kind of Inverse relationship which is also correct.
    So to meet the required between 20:1 to 40:1 dilution ratio, what I want to see are readings between about Brix 3 and 4.

    Sticking to the red dots/line you can see the oil in my BS lube tank (Green dot measures at 1.2 Brix ) while that from the lathe tank (purple dot) is 1.5 Brix, are below requirements.
    Interestingly the Lathe mix still looks milky/white enough but the Brix meter says its too dilute.
    The BS mix looks brownish and much more watery, but is nearly the same Brix.
    Both tanks need a good clean out and new fluid anyway.

    The other machine lubes etc that end up in the tank may effect the Brix reading but if you are careful about how you collect a sample those oils should sit on the top of the mix and not be caught up in any sampling.

    If anyone around Perth wishes to borrow the refractometer to check their own fluids out, just PM me.

  2. #2
    Join Date
    Aug 2011
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    Default

    Hi Bob,

    As usual, a thorough scientific approach to an interesting topic.

    Thanks for taking the time to post your results. Very interesting.

    Simon

    Sent from a galaxy far far away
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    Default

    Thanks Simon,

    RE: very scientific, maybe not compared to what I have done in the past but I think it serves its main purpose of indicating what Brix range device is needed. Other soluble oils may be slightly different but it won't be by that much.

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