Different losses from different mash tuns. Why?

[gobuchul]

I have started do a lot of single infusion, no sparge mashing.
When I am doing a large 40l batch, I use a 50l SS insulated mash tun and a 30 litre coolbox type mash tun.
Today I got some strange results from the quantity of wort from each tun.

In the large one I mashed in 40 litres of water with 6kg of grain.
In the 30 litre tun I had 25 litres of water and 3.5 kg.
The grain bill was 9kg of lager and 500g of carapils. The carapils was spread roughly 60/40 between the 2.

When i came to drain the wort I only got 17 litres from the 30 litre tun and I got 32 litres from the 50 litre tun.
That works out at about 32% loss for the smaller one and only 20% for the larger one.

Any suggestions?

[vacant]

If the 30ltr coolbox has the drain tap on the side .... you need a small piece of tube on the end of the tap to extend it below the base of the mash tun, you need tubing from the back of the tap to as close as possible to the base of the mash tun if passing through a mesh filter or you need cuts in a copper pipe filter to be restricted to the underside of the filter as far as possible so when finishing draining the syphon effect is kept until air finally gets in to break it. All tubing/connections have to be air tight to prevent the syphon from being broken.

Of course bottom drain is simpler.

You could line your mash tun with a net curtain, do the mash and drain, lift out the grain and tip the rest into your boiler - but you might as well BIAB those 25 ltr and scrap the 30 ltr coolbox.