• Product Review: Milwaukee Digital Refractometer

    Sometimes I just get an urge to buy a new measuring device for my reef's needs. I've been using a refractometer for probably a decade, and while it still works it surely is approaching the point of needing to be replaced. For the past 24 months I've seen more and more digital products come to market, and last September the new Milwaukee Digital Refractometer caught my eye.

    There are three ways to measure salinity, primarily. You can measure it with a floating or swing arm hydrometer to determine the specific gravity of the water. You can use a refractometer, which is the preferred method for accuracy. And now you can measure saltwater electronically and view the digital readout with greater ease. The most important question we hobbyists care about is "how accurate will it measure?" For the experienced reef keeper, we double and triple check our results, using calibration solution to make sure each device is set correctly.

    After seeing the Milwaukee in my facebook newsfeed, I checked Amazon's price and pulled the trigger. A different brand was nearly $200. The Milwaukee MA887 was $93 plus tax with free shipping. It arrived in no time at all.

    The digital refractometer comes with instructions, warranty card, two pipettes, and a 9v battery.

    The battery compartment's large door feels rugged, and the rubber feet will keep the MA887 stable on any flat surface.

    The access door unscrews.

    The rubber O-ring was a little loose (not holding onto the threaded cap), but I was able to carefully align it when screwing the battery compartment shut. Since this is the wet end of the tool, it needs to be well-sealed.

    Turning it on, it immediately reported the battery was 100%. Then it displayed the temperature in Celcius. Using the instructions, I changed that setting to Fahrenheit. According to the manual, the Milwaukee digital refractometer is auto-temperature compensating. If the water temperature varies too much from the temperature of the lens, one minute should pass before running a test.

    Easily calibrated to zero, one only needs distilled water or de-ionized water. Most of us have RO/DI systems on hand, and one pipette's worth will suffice to calibrate it. If the screens measures "0", it's ready. According to the manual, it needs to be calibrated often: daily, whenever a long period elapses, prior to testing, and when changing the battery. For the majority of hobbyists, calibrating it monthly should be good enough, but more often is so simple that there's no excuse not to make sure it's measuring correctly. Calibration is retained even when the device is shut off.

    Now that it's calibrated, I took a sample from my reef tank. What? That can't be right. I tested twice.

    The water sample should fill the sample well, covering the prism completely. If there is too much light (the manual mentioned direct sunlight), hold your hand over this section to shade it during the test.

    How does one find out if a test result is accurate? Measure it with a different kit or product. I took my refractometer and studied a sample through the viewfinder. The hard line was at 1.026 sg, slightly higher than 35ppt on the optical screen. It had been a while since I'd calibrated it, so I used several drops of 35ppt calibration solution to discover my refractometer was actually measuring a little bit lower than it should.

    Retesting, I found that the tank water measured 1.027 sg on the refractometer, so it was .001 less than the new digital display. I tried to get a clean picture 15 times, and this was the best of my attempts. (I don't know how they do that.)

    My next test was to take a sample from the 250g of newly mixed saltwater that has been circulating in a tall poly tank for the past week. The Milwaukee provided me with a measurement of 1.027 sg. My refractometer measured 1.026sg, again .001 lower. This tells me two things: 1) the difference was consistent between the two water samples, and 2) I need to add some freshwater to my tank to get salinity closer to natural sea water.

    A nice feature: the display will retain the last test performed, even if the device shuts off, allowing you to reference it again if you were pulled away momentarily. The unit turns off automatically if not used for three minutes.

    According to the manual, the product has a two year warranty. It states electrodes have a six month warranty, but I don't see any and wonder if that is some type of standardized statement rather than a sentence pertaining exclusively to the MA887.

    If you have a hospital tank and need to maintain hyposalinity, this will make sure you know if the water is 1.009 sg. If you have multiple tanks, each one can be measured as long as you clean the sample well carefully between tests. And if you have but one tank, you still need to make sure the salinity is correct, as well as every batch of new saltwater needed for those necessary water changes.

    Clean up is a breeze. Using a soft cloth, you wipe away the water on the prism surface using care not to scratch lens. I used the ultra soft cloth that is used for monitors and television screens, the micro mesh fabric.

    With the price being under $100, I can see more people opting to purchase this over the alternatives. Floating glass hydrometers can roll off the countertop and shatter; swing arm hydrometers are cheap and often can provided worrisome results unless kept very clean; and handheld refractometers can be somewhat hard to see unless it is pointed directly at a bright white light (try using one near a tank lit with actinics and see for yourself) and if your vision is good. Now we can add a small puddle onto an optical prism, press a button and within seconds know exactly what the salinity of the water measures. I'm impressed by how easy this was to use, how well it appears to be built, and feel good about making this purchase. You will as well.

    I have a feeling I'll be testing salinity way more often now.
    Comments 4 Comments
    1. fridmani1's Avatar
      fridmani1 -
      Did you try testing the 35PPT calibration fluid sample on the Milwaukee to see if it was really off by .001?
    1. melev's Avatar
      melev -
      That's an excellent suggestion, and I'll do that!

      Okay, here's what I got:
      PSU: 35 - The manual refers to PSU as Practical Salinity Units, btw.
      PPT: 36
      SG: 1.027

      So while my 35ppt sample appears to read a bit higher on the MA887, the temperature of the device and the solution was right at 71F. For an accurate reading, I'm pretty sure both have to be at 77F because temperature definitely has an effect based on previous reading I've done. The bottle even states on the label it is rated for 77F, if you check the picture near the bottom of this article. So until my home warms up a tad, I can't be absolutely positive that the 35ppt solution is 35ppt. It's what I bought and what I would hope is correct, since it's what I would gauge against during calibration.
    1. DaveMorris's Avatar
      DaveMorris -
      I bought this same unit about six months ago and absolutely love it. It is so simple to use and extremely accurate. Very nice write up Marc.
    1. melev's Avatar
      melev -
      Thanks Dave.