What size of Surge tank for ET2

I have an ET2 fed by a Medo AC0110 pump.
I also have all the parts to convert it in the future to ET2.5

What is the volume of surge tank I need to build?
What material to use - my first effort to build a 2 litter surge tank using Drain pipes was a failure. The end caps flew out after 2 min eventhough I had them glued using Epoxy

PVC pipe will work if you use PVC cement, not epoxy and you pre regulate down (as you should if your pump already has a tank) before the surge tank. I am not sure of the burst pressure of PVC, but your issue was with using epoxy. PVC cement is a "weld" of sorts, good for at least 40 psi.
Drain pipe with end caps work well as 4est said using pvc cement. This cement is a solvent along with cement so it actually softens and combines with the pvc giving it a strong, water tight (or in this case air tight) bond. Be generous with the cement and use the cleaner/primer first. Also, inserting some fiber in the tube helps dampen the system better than an open tube. One small tweak, put a valve also on the bottom of the surge tube so you can drain tramp water out of the tube.

Presumably your aim is to filter out the pump noise, in which case the three things we need to know are: the frequency at which the pump operates, the INTERNAL diameter and the length of the air line.

The Air line and the surge tank form an RC filter with the flow resistance of the air line as the R (calculated according to Poiseuille's Law) and the volume of the tank as the C. The resulting time constant (RC) should be well below the period of the pump (1/2piF where F is the pump frequency). 1 octave will give you 3dB attenuation, 1 decade will give you 10 dB so if, for instance, you wish to reduce the surge to say 1% of its initial value you need two decades (eg RC = 100 x pump period)

If there's a pre regulator on the line things become very complex as this is hard to model. Probably the ebst approach is to insert a small surge tank after the pre reg and model it as a zero output impedance from the reg - this will slightly oversize the final tank but that's probably not a worry.

Mark Kelly

Actually I've just gone through the physics of this and the analogy between a rigid walled tank and a capacitor doesn't hold. I think we could derive a term for the effective equivalent capacitance but it will take a bit more work and involve differential equations.

Mark Kelly
Two 2 liter tanks with a small (1/4") hose or additional regulator (making it RCLC ?) between should do fine. If you chose the above suggestion to put batting in the tube, make sure that you have a very good filter on the output and allow it to pump air directly out for a bit to clear the system. Something like the ones used for plasma cutting were there is a internal replaceable filter that looks like a toilet tissue roll. Make sure that you adjust the tank pressure down to about 3-5x final pressure. The pump will work quieter and more effectively with less water condensation.

What are your starting and ending pressures? Original manifold or the high pressure manifold?