Installing Depth/Temp Gauge - Questions

So I picked up an older, unused depth gauge/ water/air temp gauge from a guy on the Moomba board over the winter. I'm going to install it soon but I was just looking at it the other day and I can't tell which sensor is the air and which is the water. It didn't come with instructions or anything. One sensor is cylindrical and made of steel, the other is a flat piece of plastic. Both are about 2.5 inches long. Which is the water sensor? Also, anyone installed one and have advice as to how/where to install the water sensor?