Temp fluctuation in incubator

I have noticed a fluctuation in my temps of my newly setup incubator(large soda cooler). During the day temps are around 89 and in the evening temps drop to 87.4. Is this fluctuation acceptable?

The more accurate the better. What are you using to regulate the heat source?

Helix DBS-1000. I have added water bottles and have not had temp spikes again. I have another thread I started asking about temp spikes. It is here. Comment on either thread!

http://ball-pythons.net/forums/showt...ed-some-advice

Thanks
Dave