I used several over the years, noticed the cheapers ones usualy have a rather slow reaction time and or susceptible to electrical interfearance. So putting them in the water, i takes 5 minutes for the reading to stabelize, gets boring after a while waiting that long. If it keeps fluctuating it like is the pump in the tank, or even the light cables and even maybe your cellphone interfearing with the reading.
The glass ball sensor has a fluid, over time this fluid can change in consistancy or even dry completely out if stored wrong. There are pens around which can be refilled.
The ones that can't, if reading is off and can't be calibrated anylonger the fluid is compromised and are ready for the trashbin. SO for the long term, a refillable one is longer lasting.
I have 2 cheap PH pen type meters, they seem fine with new batteries and calibrated, but seem to lose it, as the batteries go down, which really means you need 2 big pots of calibration solution, and recalibration too often. The TDS meter also shown seems fine, indicating 0-1 with my RO water, but I can't prove that's correct either. Another case of buying cheap and Chinese, a couple of weeks ago I noticed that one of my tanks had droped from 25c to 19c, Alarm bells started, failed heater, check it, OK! put the therometer in another tank and it's showing about 6-7Deg lower, new battery problem solved.
Have pH controler and pen. pH pen cost about £20 off Amazon, great buy IMO. Has claimed accuracy of O.01pH !!!
But very handy for setting up BBS rates and PH drops, I don't get in the actual pH value as they all calibrated slightly differently and give slightly different readings, but it is on a log scale pH so no surprise really.
PH controler handy as can move it around tank easy on long cable and do easy spot checks that same pH drop in many different areas of tank.
Tho must note: The Milwaukee doesn't have a ground pin, well it must have but it'sbuild in and very low current, this makes it very susceptible to electrical interfearance... I had to place it inline away from any electrical source to eliminate any disturbance.. Because it's an SMS with a build in Relai, disturbane cause the relai to shake ratle and roll like mad..
The Hanna equiptment also is very high end, some come with a seperate Ground pin to eliminate this, can't be disturbed, but have a relative high working voltage and current between pin and probe for that, it's measurable in the tank water with a multimeter between 1,5 and 3 volts and even can feel the current flow in you ground yourself en stick a finger in the water. If you can feel a tickling sensation try to imagine what the fish feel swimming near it.. In the long term i doubt it is heathy for them, these high current grounded devices are not realy designed for aquarium use, this is in my opinion horticulture stuff only. Physicaly grounding an aquarium is a questionable practise anyway, any small electrical leak will start flowing affecting fish with sensation. So actualy using any type of permanent electronic measuring device in an aquarium it means the tank is permanently grounded to the electrical grid, simply because without any grund the device wont work. Than with using this type of device it is adviceabe to regularly check for power leakages with a multimeter between tank water and ground.. If the tank isn't grounded electricity can't flow and can't cause sensation. But again such a device grounds the water permanently than it becommes rather important. This is something you can't see with your eyes, nor maybe feel, but your fish might and not realy like it. It just shouldn't be there..