Have a look at:
http://www.yorkshire-divers.com/foru...-part-1-a.html
for some alternatives.
I would spend some cash on a pO2 meter as soon as possible to be honest, as it will make your unit safer and give you more range in your diving.
Also have a look at:
Semi Closed Rebreather diving gas calculations.
for some information on what you can do with a Dolphin and adjusting the tank mix.
I have a spreadsheet that I can send you is you email me that will do the calculations in the second link, drop me an email [NOT a PM!] if you would like it.
To use the spreadsheet effectively [and safely] you
must have a pO2 monitor. If you fancy a dive with one and you can get to Stoney on Sat [14th], drop me a line and I'll meet you there.