Marineland Labs/Aquarium Systems Hydrometers, part 1

By Steven Pro

 

I am going to take a slight departure from my regular Impressions column this month.  Instead of merely describing what I think about a particular type of equipment and the brands/models available, I will instead be detailing some product testing that I conducted.  While I believe my standard Impressions column is worthwhile, it is inherently based upon anecdote and experience.  In some instances, like power filters for example, I have handled hundreds of each brand and model.  But with protein skimmers, in some cases I have only used three or four of some models.  This limited experience can shade my opinion because of the flaws or alternatively fine performance of those units in that small sample.  If I have only used a few of a product and all those were fine, it would give me the opinion that this brand was reliable whether or not that was the case.  And of course, the opposite could be true.  I could use a few of one brand and discover those were defective for some reason, but that may not necessarily be representative of the entire line.

While it is a commonly held belief that the standard box-style, swing-arm hydrometers are inaccurate, like so much else in this hobby, this belief is based solely on anecdotal evidence and experience and not based upon scientific study.  After the release of the new and improved Marineland Labs/Aquarium Systems hydrometer, I decided to change that.  I wanted to put this product through a series of tests to evaluate its accuracy and consistency when brand new, to reevaluate both after extended use and in some cases, a bit of abuse.  I was particularly curious if the product was inaccurate and imprecise as manufactured or if they became inaccurate and/or imprecise only after use.

The first thing I should probably do is define some terms.  (The following terms and definitions come from Beckwith, Buck, & Marangoni, 1982.)  There is the true or actual value of something.  In the strictest of senses, this value can never really be determined.  It can only be approximated.  There is also accuracy, which is the maximum amount by which the result is different from the true value.  And finally, there is precision.  Precision is how close the results are to each other.  An extreme example of precision versus accuracy was given in the text book for illustration.  “If all clocks in a jewelry store are set at 8:20 but are not running, the indicated values show precision but are accurate only twice per day".  So, an instrument (or group of instruments) can be precise, consistently measuring the same value, but not necessarily accurate if those values are always incorrect.  Additionally, an instrument can be occasionally accurate, but not necessarily precise if it gives wildly varying results.  In the worst case, an instrument can be both wildly inaccurate and inconsistent, therefore neither accurate nor precise.  Ideally, one would want an accurate and precise instrument, but could settle for a precise one if the error (the actual difference between the true and measured result) was known.

Before I could begin my experiment, I had to obtain a sample group of hydrometers.  I could have just purchased a case of them through one of my wholesalers, but this would have resulted in me likely getting just one batch or lot.  This would not necessarily tell me how well this product would work, just how well this particular group did.  In an effort to lessen the likelihood of obtaining a single batch, I enlisted the help of friends and acquaintances from all over the country to purchase hydrometers off the shelf and ship them to me in their original packaging.  Additionally, I picked up a few hydrometers in my own travels around the country.  Below is a listing of all the locales I was able to obtain a hydrometer from:

 

Pittsburgh, Pennsylvania

Cleveland, Ohio

Atlanta, Georgia

Dallas-Fort Worth, Texas

Salt Lake City, Utah

Los Angeles, California

East Lansing, Michigan

Ithica, New York

Las Vegas, Nevada

Denver, Colorado

Orlando, Florida

Sacramento, California

 

To satisfy my own curiosity, I labeled all of these locations on a map of the United States that I had laying around.  From the image, one can see that while there is a definite Northeast concentration, most of the hydrometers came from all over the US.

Phase One - Initial Testing:

I first had to test these twelve hydrometers to determine if they were consistent and accurate right out of the box.  This was the most critical part of my experiment.  If they did not ‘pass’ this first round of testing, subsequent phases would be unnecessary and my article would be awfully short.

Fresh water was purified by an Aquatechnik separate stage, two resin deionization unit. Tropic Marin Pro Reef salt mix was added to obtain a salinity of 36 ppt or approximately 1.027 specific gravity at 78*F according to a Sybon Opticon Series FG100sa refractometer with automatic temperature compensation.  The refractometer was calibrated prior to taking measurements with a reference sample of pure water (< 18 MΩ-cm and 0 ± 0.01 ppt).  Initial water quality was checked on the batch mix of saltwater with Salifert test kits and was recorded as follows:

pH                   8.2

Calcium            400 ppm

Alkalinity          3.0 meq/l

Ammonia          0 ppm

Nitrite               0 ppm

Nitrate              0 ppm

Phosphate        0 ppm

I then checked the specific density of the same seawater mix with the various hydrometers I had obtained.  The values are recorded in the chart below.

As one can see from the above chart, all the hydrometers were both precise and accurate.  I must admit that I found this to be a bit of a shock!  To see such a high degree of accuracy and precision out of a $15 cheap piece of plastic astonished me.  Furthermore, I had heard all the stories and seen it for myself that hydrometers tend to be inaccurate after some use.  So, is this level of accuracy and precision due to the “new & improved” style hydrometers’ design or are all plastic hydrometers good right out of the box, but tend to drift with use?  This will be the subject of the next round of testing.

References:

Beckwith, Buck, & Marangoni. 1982. Mechanical Measurements. 3^rd Edition.  Addison-Wesley Publishing Company. Reading, Massachusetts. Pages 262-263.