Hi Bart,
With the upcoming good weather the pressure at my home for bigger swimming is rising steeply. 
From experience with smaller swimming pool, I think measuring chlorine is more interesting than pH as pH doesn't tend to change that rapidly. I admit I have nu experience with such chlorine device. I did measure it manually using chlorine indicator in a small water sample. So when planning to buy such a device I would definitely check how accurate it is and also (regularly) compare it with some manually measured sample (I have some bad experience with cheap humidity sensors that are drifting).
Also automatically controlling chlorine concentration might protect your pool from going green (but doesn't seem a cheap solution). This is especially interesting when you go on vacation (if you can trust it to run during that period unattended) but alternatively you can also consider manually overdosing the pool with chlorine .... and in case you have continuously monitored the chlorine level you might exactly estimate how much you need to overdose the pool to bridge the vacation period :-).
Of course there are other things you can consider to automate / measure
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switching the pump on/off that is filtering the water
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if you have some sun panels to heat water you might consider to pump water through it when sun is shining and temperature of pool is low.
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measure the water level and deduct the water cost from the weekly allowances of the swimming pool users (
)
