To Float, or Not To Float, that is the question for the Hydrometer

As it is now the middle of the winter, and getting very cold, concern naturally arises about the quality of the anti-freeze in the solar hot water system.  It has been over a year in service and the hot summer temperatures might have degraded the anti-freeze properties of the propylene glycol/water mixture.  If cold temperature outside causes the solar working fluid to freeze, it will ruin some very expensive solar panels.

The tool for checking out the quality of the anti-freeze is called a hydrometer.  This uses the specific gravity of the working fluid as an indicator of the level of freeze protection.

The most common hydrometers are used for testing the anti-freeze in car radiators.  However, those hydrometers will not work for the solar panels.  Cars use highly toxic ethylene glycol which has a different specific gravity than the non-toxic propylene glycol used in the solar panels.  The hydrometer must say on it that it is for propylene glycol or the reading will be wrong.

As a good starting point for any shopping these days, I surfed on over to Amazon.com.  There I found the Thexton 107 Cold-Chek for about $15 with mostly 5 star reviews.

 The basic working principle is easy enough, suck up some fluid into the device and see how many disks float.  If one disk floats, the fluid is good to +20°F, if all five disks floats, the fluid is good to -50°F.  To get started, I tried some water as shown on the left side of the picture below.  As expected, none of the disks floated indicating no freeze protection at all.
 

Next I removed a small amount of solar working fluid from the fill/drain port on the solar hot water system.  As can be seen on the right side of the image above, all the disks float and I should be good to -50°F which is very reassuring .

I will try to remember to come back and check this again next year.

One slight problem with the hydrometer was that it was easy to get false readings.  Air bubbles can attach themselves to the disks and cause them to float.  This made my measurements on the water totally unreliable.  After playing with it for a while, I discovered the vigorously tapping the side of the plastic housing caused the air bubbles to rise and then the disks would give a true reading.  I learned that a disk floating may be an error, but if you get it to sink, that is the correct reading.

Next post, I will talk about my worryingly low pressure in the solar working fluid.