Critical Orifi

Cut Away View of Critical Orifice in 3/8" Barbed Nipple with Hose Pressed Over Assembly
(click on image for larger view)

Applications

• Maintaining Positive Flow Rate Control in/with Air Sampling Instruments

Features

• Wide Range of Flow Rates from 2 lpm to 28.3 lpm
• Fabricated from Stainless Steel
• Available in General Purpose Set
• Custom Flow Rates Available under Specific Calibration Program

As used in air sampling, the critical orifice is a simple metal hole inserted downstream of a sampling device and upstream of the sampling pump. Its purpose is flow regulation. When the pressure downstream of a critical orifice is about 50% less than the pressure immediately upstream, the velocity in the orifice reaches the speed of sound and no further increase in flow rate will occur (under suction condition) that is, the flow becomes "CRITICAL". Devices of this type have long been used for controlling the flow rate through filter holders, cascade impactors, bubblers, etc.

While the critical orifice is an attractive device for maintaining a constant flow rate through a sampling instrument, certain limits should be appreciated. The suction pump must have the ability of maintaining a negative pressure of 16 inches of mercury at the desired flow rate. This can be ascertained by consulting the manufacturer. If the restrictions stated above are strictly observed, then the critical orifice represents the most economical effective and trouble-free technique for maintaining constant air flow through a device.

BGI offers numerous sizes (flow rates). The flow rate is stamped on the unit and intended to represent LPM (±15%) through a 37mm disk of 0.8mm membrane. When used with different resistances, the flow rate should be checked with a rotameter or soap film meter. We also offer these nine sizes or any size in our range, custom calibrated to your requirements. These components may be purchased individually or cased as shown.