A dip tube (also known as an outage tube) provides vapor space above liquefied gases in a sampling cylinder so that if expansion occurs with an increase in temperature, the pressure is not significantly increased. Basically, the length of the dip tube is used to determine the filling capacity of the cylinder.
Outage is expressed as a percent of the total cylinder volume, based on the ratio of the vapor space to the total length of the cylinder with a maximum available outage of 50%. The dip tube is welded directly to the male inlet of the valve and is cut to a length ranging from one inch to 5 ¼ inches (available in ~1/8 inch increments).
• Dip Tube Length = Cylinder Length x Percent Outage (as a decimal)
Typically, the ideal dip tube length will provide a ~20% void within the cylinder based on the cylinder length and the physical (expansion) properties of the sample. To calculate for this length, determine the overall length of the cylinder and multiply by 0.20 to obtain the desired dip tube length in inches. Note that this is an approximate calculation (+/-20%) since several cylinder design variables will influence the actual volume.
Restek offers 10, 20, 30, 40, and 50% outage options. A 10% outage would translate to a 90% fill capacity, 20% would equate to 80% fill, 30% would be a 70% fill, etc.
The following chart provides suggested dip tube lengths for a ~20% outage (~80% fill) based on Restek supplied sample cylinder dimensions.
To calculate a ~10% outage, multiply the cylinder length by 0.10. For example, Restek catalog #22925 (1000cc High Pressure Cylinder) has a length of 10.9 inches. 10.9 inches x 0.10 = 1.09 inches (or about 1 1/8 inch). Multiply by 0.30, 0.40, or 0.50 respectively for 30, 40, or 50% outage values. Again, keep in mind that all these calculations and the associated dip tube lengths are approximate.
We offer two types of dip tube materials: stainless steel and Sulfinert-treated stainless steel. Stainless steel is recommended for general purpose applications, while Sulfinert is superior for the collection and analysis of low-level sulfurs in the ppb range. Make sure to match the dip tube (stainless steel or Sulfinert) to the sample cylinder material.
Placing an order for a Restek supplied dip tube is easy. Simply contact our US Customer Service Team or your local Restek representative. Specify dip tube length in inches or percent outage when ordering. Note that when ordering a treated dip tube, the end of the part will not be Sulfinert treated after we cut the tube to length.
To learn more about dip tubes, refer to ASTM D3700 (Standard Practice for Obtaining LPG Samples Using a Floating Piston) and ASTM 1265-05 (Standard Practice for Sampling Liquefied Petroleum (LP) Gases Manual Method) as well as the link below.
In summary, appropriate dip tube selection is a critical part of a successful liquefied petroleum gas sampling. Review your method specifications, select the dip tube length based on cylinder volume and sample composition, and determine the best material based on sample inertness requirements to ensure successful sampling.