True Fermentation Temp in Water Submerged Carboy

[clint404]

I'm wondering my true wort temp with a submerged carboy.

I do 2.5-3.5 gallon batches and I've chilled each one this way with swapping ice bottles out during the first few days of active fermentation then stop swapping and finally pull it from the water after a week or so raise temps to 70amb towards the end.

I keep my water temps 58-62f and I know water being cooler than the ambient atmosphere surely keeps the wort cooler. I'm wondering how far off my wort temp is from my water measurements.

My guess is 2-3°F higher but someone is set me straight I know.

 

[WoodlandBrew]

When I've measured the temperatures they stayed within 1 degree of each other the whole time. Water conducts heat much better than air.

Details here:
http://www.woodlandbrew.com/2012/09/swamp-cooler.html

 

[clint404]

Thanks!