George,

The dropper method with a ref KH solution is really useful.
It's easy, it's cheap and most can quickly look at any point of the day and know it's in good shape or not.

It does have some resolution issue since it's a colormetric test method.
So the best you can do there is .2-.4 pH units for color, not bad....but not that great either.

The pH probe method cost 60-100$ more.
It uses plain DI water + baking soda for the ref KH.
The holder is a pair of tubes, one is 2" ID and 1" tall, the other is a 1" ID tube about 3" long and is glued on the inside of the larger one.

The surface area increase is about 10-20X that of a drop checker.
The pH probe increase accuracy is 10-20X as well.

The 1.5" cylinder is glued to a 1.5" disc, 1/4" thick. I drill a 3/4" hole in the center of the disc's top. I next add a grommet to make the seal between the pH probe the chamber below. I then add a 1" disc to the lower part of the 1 ID x 3"L tube. This seals the lower half.

I glue two small pieces of plastic string, zip ties, whatever to suspend the 1" tube inside the air gap in the larger 3" tube.

See word doc drawing.
Note, I've applied for a patent pending.
Any non commericial use is permitted.

Regards,
Tom Barr