I just looked on Amazon to buy a plant hydromter and there are many models. Can someone please recommend the one they like? (inexpensive)
  I've never used one. Do they register quickly or does it take a while to register? 
Thank you

I bought one at Lowe's that does moisture, light and ph. I don't really trust the ph readings, but the moisture reading works very well. Comes in real handy in large pots. Can't remember the brand and can't find it at the moment. I'll find it tomorrow and post back.

Mine is called a "Soil Master"

http://m.lowes.com/product?langId=-1&storeId=10702&catalogId=10051&productId=3028419&store=1811&view=detail&nValue=SEARCH And here's the link to it.