Just took delivery of a 312B. I am probably overthinking this (or not thinking enough), but what is the purpose of the sight glass that appears to be on the fuel tank. It is for the fuel level, right ? Nothing in the manual about it....
I hope you haven't put fuel in there. That sight glass is on the hydraulic tank, or at least so it appears to me.
Maybe I'm not understanding the question, but if that tank is indeed the fuel tank and not the hydraulic tank like Nige says, it is there to let you see the level of the fuel in the tank. Now it may be that this machine also has an actual electric gauge in the cab and this is a hold over from an earlier model with no gauge in the cab. Or may be the tank was the same as the non-cab gauge model and someone preferred to be able to visually check the fuel level without climbing into cab and turning machine on. Easy to walk by and check to see fuel is full or below half. You really need to learn what tank is for what and what kinds of oils and fluids this machine takes and where before you do anything that will hurt your wallet a lot harder than paying a few dollars for a manual. Just went back to the first post and you did mention you have a manual. In that case you need to read it page by page at least twice while looking at everything in it to be sure you understand what is what. I tried taking a quick look on SIS for the 312B to help but as usual there are so many variations of any one model number, at least 10 for the 312B, without some help in the form of an actual S/N or at the very least the Prefix, it would be a big gamble to try to answer the question. One note make sure you understand what the proper position and warm up procedure recommended before just starting to dump fluids in to any compartment. Too little oil and you destroy a pump and contaminate the complete system, too much oil and you might see the tank blowup like a balloon!
Thanks for the input guys, but I solved the mystery. It is on the fuel tank, and I believe it is part of the "electric refueling pump option". The sight glass I photographed is number 5 in the diagram. I believe the sight glass may not be installed on models, only the ones that have this option. I have yet to open the hatch to see if there is indeed the pump and hoses present, but I am satisfied with this explanation according to the diagram.
My two cents: The gauge is indeed to display fuel level. It does not display -full to empty- but lets you know when the fuel is getting low (no fuel shown in the indicator) and you need to TOP OFF the fuel. The similar looking -hydraulic- oil lever gauge is INSIDE the rear right hand door near the front.
Sight glass for fuel and only shows fuel in it for the top quarter of tank or so and aids you for fueling up,handy if you don't have a auto shut off nozzle or handpumping
The machine has an onboard pump for filling the fuel tank from an external tank? Not a bad idea.