What is 2nd Stage Cracking Pressure?

[dflaher]

Likely a "dumb question of the day" for many of you, but I recently purchased a Dive Rite XT4. Its manual says an Allen wrench is required to adjust a micro spring which "independently adjusts the spring preload and cracking pressure".

What is cracking pressure, and is this something i need to do before I take it on its first dive?

 

[Jim Lapenta]

It is not something you need to do before your first dive. IF it was set properly by the dealer you bought it from.
Cracking pressure is the point where the reg opens and delivers air to the diver. It's measured in inches of water on a magnahelic. It can also be checked in a sink or bucket using a ruler.
That you asked this question says until you take a regulator service course, don't turn anything that requires a tool.

 

[arkstorm]

Cracking pressure is the amount of inhalation effort that it takes to cause the regulator to deliver air.

Too low a cracking pressure will make the reg more prone to free flows. Too high a cracking pressure will make it feel like you are sucking too hard at the start of each breath to get the air to start flowing.

On some second stages you will see a knob which adjusts cracking pressure on the fly.

On your reg, it appears from what you say, it is a a fixed cracking pressure that should be tuned properly by a trained service tech.

 

[Jim Lapenta]

Cracking pressure is the amount of inhalation effort that it takes to cause the regulator to deliver air.

Too low a cracking pressure will make the reg more prone to free flows. Too high a cracking pressure will make it feel like you are sucking too hard at the start of each breath to get the air to start flowing.

On some second stages you will see a knob which adjusts cracking pressure on the fly.

On your reg, it appears from what you say, it is a a fixed cracking pressure that should be tuned properly by a trained service tech.

Not fixed. It is adjustable and has what other mfgs refer to as a "micro-adjust" on the inhalation adjustment knob. It can be used to fine-tune the effort and prolong seat life by adjusting spring pressure without removing the hose and turning the orifice. It's designed to be adjustable by the user once they understand how and what it does.

 

[Nemrod]

When a regulator is serviced with a new seat/poppet or is new from the factory the LP seat will take a set after a few dives and may begin to slightly free flow or bubble. The micro adjuster will initially be set per the SM at a neutral position. Rather than using the LP orifice (course adjustment) the micro adjust can be used instead (fine adjustment) to then fine tune the regulator.

Cracking pressure is the force needed to offset the second stage poppet from the LP volcano orifice and begin demand flow, typically about 1.2 to 1.6 inches of water. The three variables that affect the regulators work of breathing (WOB) are;

1. Cracking effort
2. Venturi (sustained) effort
3. Exhaust effort

People tend to fixate on cracking force/effort because it is easy to measure and intuitive to understand. But it is only one of the parameters of WOB and not really the most important.

The legendary G250, no micro adjust:





The new goodness G260 with micro adjust: