Flowmeters and the Low-Pressure System

KEYWORDS

flowmeters

rotameter

laminar-turbulent flow

rotameter configuration

Similar to vaporizers, flowmeters are one of our interfaces with the anesthesia machine. Here is something that we can actually see in action. But there is more than meets the eye, as we will see. Flowmeters are part of the low-pressure system of the pneumatic part of the machine along with the high- and intermediate-pressure systems discussed in earlier chapters.

LOW-PRESSURE SYSTEM

Technically, the low-pressure system is everything that is downstream of the flow control valves. This includes the flowmeters themselves, vaporizers, and one-way valves (to decrease pumping; discussed in the vaporizer Chapter 6) and ends at the common gas outlet (which is the end of the pneumatic part of the machine and the beginning of the ventilator part of the machine).

FLOWMETERS AND THEIR PARTS

Flow Control Valve

The flow control valve is simply the thing attached to the knob we turn to adjust flow through the flowmeters. On most machines, flow control still uses a mechanical valve that is directly attached to one of the three knobs that are used to manipulate the flowmeters. However, some machines use electronic controls as the interface between the operator and the flowmeters.

With electronic flow control valves, the user may dial in the fraction of inspired oxygen (FiO₂) that is desired and which other gas to use to dilute the oxygen (nitrous oxide or air). The mixture is controlled by pressure sensors and may have input from the gas monitor or oxygen analyzer to maintain the concentration of oxygen that has been selected. The total fresh gas flow (FGF) with electronic flow control valves is set by yet another controller, instead of the standard way of controlling FGF via flow control valves with flowmeters. Electronic flow control valves thus allow a very exact control of oxygen concentration but with the penalty of needing electrical or battery power, something that traditional flow control valves and flowmeters do not need to function.

Control Knobs

You have probably noticed that the control knobs for the three different gases are color coded. The oxygen button and flowmeter are always on the right (more on that later). In addition, the oxygen knob is bigger and juts out more than the other two knobs. It is also fluted, so it feels different. That is all, of course, so when you need to increase the oxygen flow, you will with time know which one to change simply by touch and feel and not have to divert your eyes from the patient or monitors; you will instinctively know the most important of the three knobs by touch and not inadvertently change the air or nitrous oxide. All three of the knobs have some sort of guard around them to decrease the chance of the knobs being accidentally adjusted if something bumps into them. On standard flowmeters, turning the knob counterclockwise increases gas flow, and turning the knob clockwise decreases flow.

Machines with electronic flow control interfaces will not have the kind of knobs just described. Instead, they have touchpads to indicate which gas you want to change and a common knob that is used to change flow in whatever gas you have pushed the button for (Figure 5-1).

Figure 5-1 Simple schematic of a flowmeter with labeling. (Reproduced with permission from Morgan GE, Mikhail MS, Murray MJ. Clinical Anesthesiology. 2nd ed. New York, NY: McGraw-Hill; 2002. Figure 4-4.)

Flowmeter

Although most of us would think of the flowmeter as the control knob and the glass tubing; to be specific, the flowmeter is actually the part that is controlled by the knob, where we see the bobbin going up and down. Another name for a flowmeter is a rotameter.

You can go into the physics of flowmeters as deeply as you want. For our discussion, we will not delve too deeply. A flowmeter tube of the standard type is called a Thorpe tube. This kind of tube is made out of glass to keep a static electricity charge from building up on the bobbin inside and causing the bobbin to stick to the wall of the flowmeter. This would not allow you to accurately know what the actual flow would be.

The term variable orifice is also used. All that means is that the tube is tapered inside, being narrow at the bottom and gradually increasing in diameter toward the top. The annular ring is the little circular space that is between the indicator (bobbin) and the inside wall of the flowmeter; this is where the gas actually passes around the bobbin. The gas flows up the tube and causes the bobbin to travel with it until the pressure of the upward force of the gas is equal to the gravitational force on the bobbin (Figure 5-2).

Figure 5-2 Illustration of an annular ring, the air space surrounding the bobbin or ball (blue arrows). (Reproduced with permission from Morgan GE, Mikhail MS, Murray MJ. Clinical Anesthesiology. 2nd ed. New York, NY: McGraw-Hill; 2002. Figure 4-10B.)