Flowmeters

The flowmeters of the anaesthetic machine are the measuring devices above all others upon which anaesthetists rely in the course of their work. The majority are content with the Aowmeters that are supplied with modern anaesthetic machines, whilst others consider that the accuracy could be better at certain rates of flow. The purpose of this article is to improve communications between the clinician and the engineer, so that the former may know what is technically possible and the latter what the clinician really wants. It was possibly the desire for simplicity and ‘nothing concealed to go wrong’ which created the ‘Water Sight Feed’ meter about fifty years ago. Having gained this feature the clinician had to accept the disadvantages of this device. It was possible to obtain only an approximate idea of actual flow rates but, with experience, it was possible to estimate fairly accurately the proportion of gases being administered. The ‘Dry Flowmeter’ later introduced by Coxeter had the tremendous advantage of instantaneous reading in terms of flow rate. This device also had its weaknesses. Its float had a ‘stick-and-slip’ action which limited its accuracy and the tube was fragile because of its column of holes. In current practice the variable area flowmeter using a ‘free float’ is used most frequently. It consists of a tapered bore glass tube mounted vertically with the small end at the bottom and an indicator inside the tube. This indicator, called the float, is lifted by upward flowing gas to a height in the tube which is related to the flow rate. Like its predecessors it has its limitations as discussed in the next paragraph, but against these it has design possibilities not exploited to the limit. The most commonly used variety of meter has a plumb bob float (figure 1) or the so-called skirted type (figure 2). This latter type is developed from the plumb bob variety with the advantage of quicker