Dion recommended saving my money and buying a hygrometer from Xikar as they utilize them to calibrate their units.
I would shoot Dion an e-mail if you have questions. Not sure if he monitors this forum. He is really responsive and easy to communicate with. As a matter of fact, he answered an e-mail for me just yesterday with a question I had about my LCD screen going blank.
The safety relay is worth every penny if you value your snakes and home.
I use 2 Herpstat 4's now and will most likely be adding another unit in the future.