Introduction
Flowmeters are used to assess the flow rate of gases that pass through it. These delicate instruments are always calibrated for one gas only in an anesthetic system. The flowmeter system consists of a control valve and flow meter sub-assembly. A needle valve is used as the control value and is responsible for controlling the flow of gas passing through the flow meter. Aggressive handling of these instruments can lead to breakage or false readings. Thus, flow meters should always be operated by hand and with care.
Principle
Modern anesthetic systems are usually equipped with a tapered glass tube that consists of a bobbin or a ball to measure the flow rate. The readings from the flowmeters are observed from the bobbin or sphere floating on the stream of gas. The scale is marked directly on the flow meter tube or is present to the right of the tube. Since the annular space increases more rapidly than the internal diameter as we move upwards in the tube, the gradations are closer together at the top of the scale. Another type of flowmeter, called the turret-type flow meter can be seen in older anesthetic machines. These flowmeters, unlike the modern ones, have gas flowing out from the bottom of the instrument.
The control valve knobs usually are color-coded. However, some systems have a fluted knob for oxygen flow.
Mode of Operation
The control valves are opened, and this leads to the flow of gases through their flow meters. Compared to the ball flowmeters, the bobbin flowmeters provide more accurate readings. Readings in the bobbin flowmeter are read from the top of the bobbin, while in the ball flow meter the reading is taken from the middle of the ball.
The sequencing of the flow meters is also crucial. When using multiple flowmeters, it should be ensured that the oxygen gas flow meter is always mounted downstream. The downstream mounting of the oxygen flow meter ensures that the subjects do not get a hypoxic mixture in case of leakage.
Precautions
Flowmeters may not function as expected if they are not placed vertically. Bobbin and ball in the flowmeter may stick to the tube due to static electricity or dirt. Back-pressure from other components of the anesthetic system may affect the values. Care must be taken to ensure the valve is properly opened for optimal gas flow without any undue resistance. Ensure that the valve is not closed too tightly, as this may result in damaging the inlet.
References
Flecknel, P. (2009). Laboratory Animal Anaesthesia. Elsevier.
Gurudatt C (2013).
The basic anaesthesia machine. Indian J Anaesth. 57(5):438-45. doi: 10.4103/0019-5049.120138.
