Maintaining liquid level inside a vessel of boiling liquid

[rollingstein]

What's a good way to maintain liquid level inside a vessel of liquid that's heated via a jacket & has a feed pipe. Feed pipe is approx. 1/2 inch dia & expected flow in is about 300 Litres / hr

Would a ball cock work with all that boiling & bubbling happening around it?

I prefer a purely mechanical device if at all possible.

 

[russ_watters]

https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=boiler level switch

 

[rollingstein]

https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=boiler level switch

Thanks Russ! That's very useful. I think I'll get one of those.