By Dr. Ahmed Mostafa Assist. Prof. of anesthesia and I.C.U. Flowmeters By Dr. Ahmed Mostafa Assist. Prof. of anesthesia and I.C.U.

Classification 1. Constant-pressure variable-orifice (Rotameter): The ball or bobbin is supported by the flow of gas through a tube (Thorpe tube) whose bore (orifice) is tapered. Near the bottom of the tube, where the diameter is small, a low flow of gas will create sufficient pressure under the float to raise it in the tube.

Classification 1. Constant-pressure variable-orifice (Rotameter): As the float rises, the orifice of the tube widens, allowing more gas to pass around the float. The float will stop rising when its weight is just supported by the difference in pressure above and below it.

Classification 1. Constant-pressure variable-orifice (Rotameter): If flow is increased, the pressure under the float increases, raising it higher in the tube until the pressure drop again just supports the float's weight. This pressure drop is constant regardless of the flow rate or the position in the tube and depends on the float weight and tube cross-sectional area.

Classification 1. Constant-pressure variable-orifice (Rotameter):

Classification 2. Electronic flowmeters: Available in recent anesthetic machines. In these new machines there must be a backup conventional auxiliary oxygen flowmeter which is used in case of failure of the electronic type.

Classification 2. Electronic flowmeters:

Factor affecting the performance of the rotameter The viscosity and density: Flowmeters are calibrated for specific gases, as the flow rate across a constriction depends on the gas's viscosity at low laminar flows and its density at high turbulent flows.

Factor affecting the performance of the rotameter 2. Sticking: The bobbin may stick into the wall of tube. To minimize the effect of friction: 1- The flowmeter tube should be kept vertical.

Factor affecting the performance of the rotameter 2. Sticking: The bobbin may stick into the wall of tube. To minimize the effect of friction: 2- Floats are designed to rotate constantly by: Small slot around the top of the bobbin. The ball is used.

Factor affecting the performance of the rotameter 2. Sticking: The bobbin may stick into the wall of tube. To minimize the effect of friction: 3- Coating the tube's interior with a conductive substance grounds the system and reduces the effect of static electricity.

Factor affecting the performance of the rotameter 2. Sticking: The bobbin may stick into the wall of tube. To minimize the effect of friction: Dust filter to minimize dirt in the flow tube.

Factor affecting the performance of the rotameter 3. Accuracy: It is within ± 2-2.5%. To increase the accuracy: Avoid sticking. Reading: From the upper surface of the bobbin or the central equator of the ball.

Factor affecting the performance of the rotameter 3. Accuracy: Dual taper design: This allows single flowmeter to read both high and low flows and allow easy adjustment of the bobbin or the ball on the desired flow.

Factor affecting the performance of the rotameter 3. Accuracy: Some flowmeters have two glass tubes, one for low flows and another for high flows, the two tubes are in series and are still single flowmeter to read both high and low flows.

Factor affecting the performance of the rotameter 3. Accuracy: Back pressure effect of the ventilator witch increase the resistance in front of the flowmeter. Some flowmeters are now pressurized and calibrated to work at increased pressure of several bars.

Factor affecting the performance of the rotameter 4. The safety: Leaks: Should a leak develop within or downstream from an oxygen flowmeter, a hypoxic gas mixture can be delivered to the patient. To reduce this risk one of the following should be done:

Factor affecting the performance of the rotameter Oxygen flowmeters are always positioned downstream to all other flowmeters (nearest to the vaporizer).

Factor affecting the performance of the rotameter A channel is present at the outlet of oxygen flowmeter to deliver it separately away from the nitrous oxide. Electronic flow control and measurement. The amount of pressure drop caused by a flow restrictor is the basis for measurement of gas flow rate. In these machines oxygen, nitrous oxide, and air each has a separate electronic flow measurement device in the flow control section before they are mixed together.

Factor affecting the performance of the rotameter Oxygen/Nitrous Oxide Ratio Controller: linkage of the N2O flow to the O2 flow helps ensure a minimum oxygen concentration of 21– 25%. The oxygen/nitrous oxide ratio controller links the two flow valves by one of the following methods:

Factor affecting the performance of the rotameter Oxygen/Nitrous Oxide Ratio Controller: Mechanically: Chain link is present between the O2 flow controller and the N2O controller.

Factor affecting the performance of the rotameter Oxygen/Nitrous Oxide Ratio Controller: Mechanically:

Factor affecting the performance of the rotameter Oxygen/Nitrous Oxide Ratio Controller: Pneumatically: Where a pneumatic mixing valve is present.

Factor affecting the performance of the rotameter Oxygen/Nitrous Oxide Ratio Controller: Electronically: Either by: Minimum oxygen flow: The oxygen flow valves are usually designed to deliver a minimum flow of 150 mL/min when the anesthesia machine is turned on.

Factor affecting the performance of the rotameter Oxygen/Nitrous Oxide Ratio Controller: Electronically: Either by: The Quantiflex mixer flowmeter: One dial is set to the desired % of O2 and the total flow rate adjusted independently.

The Quantiflex mixer flowmeter

Auxiliary Oxygen Flowmeter: Is a self-contained flowmeter with its own flow control valve, flow indicator, and outlet. It usually has a short tube with a maximum flow of 10 L/minute and a barbed fitting on the outlet. It is usually mounted on the left side of the machine. This can be used to supply oxygen to the patient without turning ON the anesthesia machine or in case of failure of electronic type flowmeters.

Oxygen Flush Valve It allows direct communication between the oxygen high-pressure circuit and the low-pressure circuit. Flow from the oxygen flush valve enters the low- pressure circuit bypassing the flowmeters and vaporizers. The entire oxygen flow of 45 to 75 L/min is delivered to the common gas outlet at a high pressure of 50 psi.

Oxygen Flush Valve Barotrauma. Hazards: Barotrauma. Patient awareness due to dilution of the inhaled anesthetic. Back pressure from the breathing circuit closes the check valve airtight, and major low- pressure circuit leaks can go undetected.

Oxygen Flush Valve

Oxygen Flush Valve

Oxygen supply failure devices 1. Alarm devices: These are pneumatic or electronic devices activated by oxygen pressure. They give audible and visual signals when the pressure falls below a certain threshold value.

Oxygen supply failure devices 2. Fail-safe system: The fail safe device ensures that whenever oxygen pressure is reduced and until flow ceases, the set oxygen concentration shall not decrease at the common gas outlet. In addition, the loss of oxygen pressure results in alarms, audible and visible, at 30 psi pipeline pressure. 3. O2/N2O Oxide Ratio Controller: Discussed before.

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Thank you Dr. Ahmed Mostafa