Sporlan TEV.

I fitted a replacement power-head the other day, which had note with it reading.
VN power-head is a non-migrating charge that may be substituted for a Hydraulic power head.

Am I being thick?
What do they mean?
Grizzly

Think it means the charge in the bulb wont migrate to the bellows if that is the colder place, which can then result in insufficient pressure to operate the valve. Not so sure about the hydraulic part tough!

Yes my friend you are correct on that part.
It's the Hydraulic part that I am not sure about.
Grizzly

They make heads full of liquid? That would scare me a little.

Hydraulic Elements — These thermostatic elements are
specially designed double diaphragm elements which provide a pressure limiting feature without the problems associated with charge migration from the bulb when the element
becomes cooler than the bulb. The hydraulic element is
often used on chillers which require a TEV with an MOP
type charge, but experience problems with charge migration caused by cold ambient temperatures. For additional
information on the hydraulic element, contact the Sporlan
Division of Parker.
sourced from sporlans website
http://sporlanonline.com/literature-...ansion-valves/

Thanks IM.
I would of given you Reps but I have to spread the love first.
Grizzly