All units are designed to represent the three chambers under water seal concept: a collection chamber, a water-seal chamber, and a suction control chamber. Suction control is achieved either by a mechanical device without need for water (termed dry suction control) or by using a water column. Of the eight units we tested, four units used dry suction control and four units used a water column.
The RT 200 Calibration Analyzer (Timeter Instrument Corporation; St. Louis, MO) was used for measuring PDU flow rates and pressures. The RT 200 Calibration Analyzer (Fig 1) provides a digital readout of the measurements. A pneumotachometer measures gas flows with a resolution of 0.1 L/min in a range of 5 to 180 L/min with an accuracy of 1% or better of the readout. Pressures from 0 to 250 cm H2O are measured with a resolution of 0.1 cm H2O with an accuracy of 0.5%, according to the operation/service manual of the manufacturer. Our unit was last calibrated for accuracy by Onsite Calibration Service (Louisville, KY) 1 week prior to our data collection on the PDU. Calibration is suggested by the manufacturer to occur once yearly. This measurement device may be set to make all measurements at body temperature, ambient pressure, saturated with water vapor (BTPS). canadian-familypharmacy.com
Flow Measurement: A bench set-up was created as in Figure 1. The PDUs were prepared following manufacturer instructions. The suction port of the PDU was connected to wall suction through a regulator. The patient connection tubing of the PDUs was connected to the “flow outlet” of RT-200 Calibration Analyzer. Extra connection tubing was required for this, and was minimized in length (measured at 15 cm) with a diameter (approximately 2.5 cm) significantly larger than that of the connection tubing supplied with the PDU to avoid additional resistance to flow.
Figure 1. The set-up to measure PDU flow. Shown (left to right), sequentially connected, are the RT-200 Calibration Analyzer, a PDU, and a wall suction regulator.