plug vs. viable?

I hate to sound like a complete moron here, but can anyone tell me the difference between a sperm plug and viable sperm? Ok I know that viable sperm is "the good stuff" but is a sperm plug just old, not potent sperm? Or is it immature?

Also, I'm curious to know...for those of you that have your own rack systems/incubators, what kind of thermostat do you use, and what is the big difference between on/off thermostats and proportionals?

Here is a good thread on the plugs.

http://www.ball-pythons.net/forums/s...ad.php?t=18791

Herpstat for me all the way. Proportional maintains the temps via small trickles of power to heating source based on settings. On/Off is just that, its On to heat up the source and off when it reaches the temps based on settings. Most folks would agree to get a proportional type over on/off. Something about enclosure efficiency lol :) Hope it helps.

I second the proportinal stats. Some of the on/off ones will "click" when they come on or go off which annoys me.I have always used helix but recently bought a new rack and I bought a herstat ND instead of another helix and it has became my preference. Check out spyderrobotics.com