Raul,

They do work well.

I use a Milwaukee brand SMS 122 ph meter to drive my auto c02.

I have had it for over 3+ years and is still accurate. I replace the probe annually just in case and I calibrate every 2-3 months to ensure accuracy........... I would recommend this brand.

Do some web research on ph meters and you can see the variety, price, etc.

Here is a link to the meter I use:

Automated Aquarium Systems - SMS122 pH/CO2 Controller

Please note that prices on the web vary widely. Find the model you want, and then search for the best price.......

I have seem my meter from $80 - $150 usd...........

Best of luck