I guess my bottle is empty since when i purge i don't see any more mist. And wen i squeeze i barely get any boost. However, nitrous pressure seems to be around 550psi on a warm bottle. Took out the bottle, didn't seem as light as i thought it should. Opened up the valve to see if anythin would come out, juss cold air. Is there still nitrous in that damn thing or what? Should I juss completely release all that pressure in there till it's at 0psi? or juss go get it filled from how it is now? it's a 10lb bottle.. empty, it should be 14lbs right? thanks fellas

Most nitrous companies recommend a bottle pressure around 900-950 PSI I could go into more detail about that but yea its time for a refill if the bottle is warm and thats the reading your getting.But there is a chart at the link below for you to check about that temp vs bottle pressure

Nitrous pressure should be kept consistent to obtain optimum performance. All nos systems are designed to work best with a bottle pressure of 900-950 PSI. Pressure is determined by the amount of nitrous contained in the bottle and temperature. The chart below shows what happens to nitrous at various temperatures. Accordingly, it's a good idea to use an NOS bottle heater and/or blanket. Likewise, maintaining consistent line filling is important...which is why most racers use an NOS purge valve to release nitrous vapor from the feed line. This provides liquid nitrous to the inlet of the solenoid(s) for better consistency and repeatability.

Bottle Heaters
NOS systems are calibrated for optimum performance with a bottle pressure of 900-950 psi. The pressure will change with temperature (see chart below). Heater kits are thermostatically controlled to keep the bottle near 85°F to provide correct pressure. Kits available for the most popular-sized bottles, with both 12-volt and 110-volt heaters offered.

The Relationship Between Bottle
Temperatures and Nitrous Pressure
The lower the ambient temperature, the lower the resultant bottle pressure leading to a potential fuel rich condition. Although usually not harmful to the engine, loss of optimal power can occur. On the other hand, very high ambient temperatures can lead to leaner burning conditions and loss of optimal performance as well as possible damage to engine components. NOS gauges are an excellent way to monitor problems before they can occur.

all taken from this link
http://www.holley.com/HiOctn/ProdLine/Products/NOS/NOSCA/BttlHeat.html

When you get your bottle refilled make sure its been in the freezer for a few hours. This will alow a more complete refill.

should i let all the air out of it first?

yes

Originally posted by ECLiPSE56
should i let all the air out of it first?

the shop refilling "should" empty the entire bottle before they refill it.