Wpowokal, I think its good we are talking about this, others can read and see different points and decide what works for them, not a one sided recommendation!
Congratulations on taking great care of a battery, 20 years is not easy.
Sandia National Laboratories, I think that is where I got my numbers.
I have 8 golf cart batteries, only 3 years old, not used a hydrometer, yet. Only used for back up, not daily cycled! So not a good example of battery care. (not the sealed LAB battery I just fired).
Not willing to pay 12% of the battery cost on a tool do not need. If I cycle the battery daily I would think differently! Yet If a tool comes along capable of doing the job cheaply then I am game. Wasted more time than its worth looking, but that is part of the fun.
In the Navy I took care of a 1Mw battery, took daily SG readings on two cells of a 120's battery.
For fun we would look at the logs and predict the SG reading. 3 digits.
After an equalizing charge took all cell data, added water etc. then compared all cells to our reference cells. Took forever! Had to lay on top of the battery to take readings.
As you mention the relative accuracy is what's important.
If I take a reading today, will I get the same tomorrow? (same conditions).
Your readings sound like they did not agree with manufactures... typical.
And, support my point. Un-necessary accuracy is wasting money if there is a cost to it. The cheap Hydrometer will give the same accuracy from the start.
Function of the float is fixed once manufactured, adjust your readings based on knowledge of your battery and work from there, much as you have been doing.
Why pay $60 when a temperature compensated one is only $10.50.
Just so you know I will wait to hear from Bob if its any good!-)
Have fun,
Scott.