My personal experience is the cheap ones become inaccurate over time.
they will be close but still off some.
The blue labs stuff, not the pen is what you want if you are serious and have the cash to spend.
It is blue labs or nothing for me.
As far as replacement probes go, this one is expensive but it does not have the breakable bulb on the bottom and will last longer than the bulb type.
Oh, and you can use it for soil too.
This was a really good tip given to me by a member of another site.
