2 Sight Glass On The Mash Tun? What's The Purpose

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I am in a micro brewery the other day and I noticed the mash tun had two sight glasses. One seemed to be connected below the filter, the other above the filter. I still could not figure out the use of having two sight glasses on the one vessle. At first I took a guess that maybe the one below the screen was there to measure the flow rate out if you launter too fast, the sight glass below the screen would drop quicker than the one above (which I could only assume must have a filter on it).

Now I have no idea. I tried asking some of the stuff but the master brewer was not around and the other stuff had no idea.

I have been thinking about it but just dont get it. Does anybody know the reason for this setup. For those in the brissy area I am talking about the brew pup on oxford street, Bulimba. I have to say as far as a brewpub goes their pale ale was brilliant! Everything else I was sadly disappointed in.
You could be right about it being a simple early-warning for a stuck sparge. If the grain bed started to compact the glass attached below the screen would empty pretty quickly.

Just a guess though.
In effect the pressure differential would cause one sight glass to drop quicker than the other, in effect measuring suction.