Schofield, R. K.

English

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Sectional Proceedings of the Royal Society of Medicine Vol. XXXIpage 1 443section of aneixtheticsPresident-I. W. MAGILL, M.B.[December 3, 1937]The Errors of Flow-meters and the Advantages of a New Typeof ConstrictionBy R. K. SCHOFIELD, M.A., Ph.D.Rothianmsted Experimental Station, Harp ndenIF the closed (carbon-dioxide-absorption) method of administering anaestheticgases is to be used to full advantage, flow-meters are needed that can register ratesas low as 10 c.c. per minute with a fair degree of accuracy. The purpose of this paperis to survey very briefly the problems which must be solved in the production of suchinstruments, to indicate the errors to which the various types are prone, and to pointout the advantages of a new type of constriction.In all flow-nmeters the gas generates a pressure difference between the two sides ofa constriction through which it is made to pass. The instruments fall into two maingroups the fixed-pressure type and the fixed-constriction type.The fixed-pressure type. In this, the pressure difference is fixed, and the size ofthe constriction varies A-ith the flow rate. The constriction may, for instance, bethe clearance between a bobbin and the wall of a tapering tube, in which case thefixed-pressure difference is due to the weight of the bobbin. There are several alterna-tive arrangements embodying essentially the same idea. The problems that arise are:(1) How to eliminate friction that would make the pressure difference greater forrising than for falling flow rates;(2) How to prevent an enlargement in the clearance, due to wear, permitting anincreasing flow to pass in excess of that registered.Great ingenuity in design and care in manufacture are required to solve bothproblenis simultaneously. It is doubtful whether there is any instrument yet availablein this group of which it may confidently be said that when it registers 50 c.c. pernminute one may be sure that not more than 60 c.c. and not less than 40 c.c. is actuallypassing. The best may perhaps do this when new, but we do not know how greatthe error may be after they have been in use for some time. Some of the instrumentsin use at the present time can only be described as dangerously inaccurate.'Thefixed-constriction type.-In this group, the constriction is fixed and the pressuredifference is measured. Here the problems are:(1) To choose a suitable constriction and reduce to a minimum the risk of itsproperties being altered (e.g. by dirt, moisture, or corrosion).(2) To provide a convenient and sufficiently accurate means for measuring thepressure difference.Again the difficulty is the simultaneous solution of the two problems. A Bourdonpressure gauge is convenient and sufficiently accurate, provided the pressures arehigh enough, but the very fine constrictions which have been used to obtain thenecessary pressure differences have proved to be undesirably susceptible to partialblockage. A water depression manometer is much more sensitive and so permits theuse of wi-ider constrictions, but as hitherto constructed it is larger and more trouble-some to transport and also less easy to read. It is quite possible, however, that thesedrawbacks which have hitherto made " wet " flow-meters unpopular may be con-siderably reduced by improvements in design.A new type of constriction.-With the fixed constrictions hitherto employed thepressure difference varies roughly as the square of the flow rate. Thus in a water1 Errors amounting to several hundreds per cent. bave been reported, and the author has beeninformed that cyclopropane " dissolves " vulcanite bobbins.MAR.-ANAZ8TH. 1444 Proceedings of the Royal Society of Medicine 2depression flow-meter the depression increases nearly fourfold for a twofold rise inflow rate. Consequently the markings for the smallest flow rates are crowded againstthe zero, as shown on one side of the figure. It is, however, possible to get an evenlvspaced scale, such as that shown on the other, by using as constriction a set oflong capillaries arranged in parallel. The gas passes through a constriction of thiskind in " stream-line " motion, and the pressure difference, which is -controlled by theviscosity of the gas, is directly proportionalAto the flow rate (very nearly). Themotion through a hole or jet is "turbulent ", and the pressure difference is duemostly to the necessity of giving a high kinetic energy to the gas as it rushes throughthe constriction; hence its dependence on the density and the square of the velocity.By using a constriction that gives direct proportionality between pressure dif-ference and flow rate, in conjunction with a water manometer, full use can be made of- 050- 100- 150- 200- 250-300_0~-20-300100150200250- 300the fact that a manometer has a true zero. With this arrangement the smallestflow rate gives a proportional movement of the liquid surface from the zero mark.With dry flow-meters there is always " threshold " flow below which the instrumentdoes not register. In the fixed-pressure type this tends to increase with wear, and isa serious source of error.It is a comparatively simple matter to make an accurate manometer flow-meter,using a set of parallel tubes as constriction, since Nature has provided such a con-striction ready made in a stick of cane (not bamboo but " solid " cane). A pieceabout the size of a pencil can be selected to give a water depression of 2 in. for 100 c.c.per minute. Thus a 6-in. depression tube will register flows up to 300 c.c. per minuteand will give one-tenth of an inch depression for 5 c.c. per minute. One depressiontube could be given two scales, one for high and one for low sensitivity if two canes,one of which would be cut out for high sensitivity, were connected in parallel. Inthis way the scale could be extended to, say, 6 litres per minute, without lengthening3 Section of Anwsthetics 445the instrument or generating excessive pressure differences. There would be noreal difficulty in making an instrument on this principle that wA-ould give an error notexceeding ± 5 c.c. per minute below 100 c.c. per minute, or ± 5%0 for higher readings.Blockage of the cane is the only possible source of error. The risk of this seems tobe very slight in view of the many hundreds of channels through which the gas maypass, but it is a point that will have to be checked under practical conditions.An instrument using a cane or equivalent svstem of metal tubes and a manometerof improved design seems most likely to combine the needed accuracy with moderatecost, as no highly specialized technique is involved in its construction. The possi-bility must not, however, be overlooked that a similar constriction might provesatisfactory in conjunction with a sensitive Bourdon gauge.Needforfrequent checking.-In conclusion it should be stressed that all flow-metersthat are used in closed circuit work should be periodically checked. It is to be hopedthat instruments will soon be available that are of such proved dependability thatthe checking need only be done at long intervals, but as matters stand at present thechecking should be frequently done. Testing apparatus is cheap to assemble andcomparatively easy to use.The Continuous-Flow Administration of CyclopropaneBy LAWRENCE H. MORRISTHE method of continuous-flow administration of cyclopropane was excellentlydescribed by Burford in 1936.An anaesthetic apparatus capable of measuring very small flows of gas is essential,but there are no flow-meters which measure accurately flows below 100 c.c. per minutedown to abouit 10 c.c. per minute. A special apparatus was therefore constructed.Apparatus.-The apparatus is comnposed of three parts a footpiece, an uprightrod, and a head; these fix together by means of male and female joints and also fitinto a small box container. The head has three yokes to take nitrous oxide, oxygen,and cyclopropane cylinders. Nitrous oxide flows at the rate of 1-10 litres per minute,oxygen at 100 c.c. - 1 litre per minute, and cyclopropane at 10-300 c.c. per minute.The flows are measured by means of calibrated gauges, which are, in fact, pressuregauges with a fixed restriction valve at the outlet. When the pressure is increased,the flow of gas increases and it is really this pressure, wrhich is calibrated, and not theactual flow of gas.Disadvantage.-This restriction type of flow-meter appears to have one disad-vantage, in that the rate of flow of the gases falls (it never increases), and so does notcorrespond to the reading show-n on the dial. This occurs gradually after some hoursof use and for no accountable reason, although it may be due to very small particlesof dirt or moisture from the gauges and the gases clogging the restriction valves, butthere is no definite proof of this. The restriction valve must therefore be reset fromtime to time in order to overcome this disadvantage. As this apparatus is purelyexperimental, no emergency valves for a fast flow of oxygen have been fitted, but Ihave always had an oxygen cylinder at hand and have, so far, not had cause to use it.Technique.-In conjunction with this apparatus I use a Waters to-and-fro soda-lime canister.To induce, I start oxygen flowing alone at 1 litre per minute. After fixing theface-mask, and with the bag only slightly distended, I set cyclopropane flowingimmediately at 300 c.c. per minute, which is the highest limit of flow at my disposal.I feel, however, that it would make induction easier and quicker in certain cases, ifthis limit was higher, say 400 or 500 c.c. per minute but not more-for in mostcases 300 c.c. suffices. After enough of the mixture has passed into the bag to preventit from collapsing completely at the end of an inspiration, the rate of oxygen flow isreduced to 350-400 c.c. per minute, cyclopropane remaining at 250-300 c.c. perminute. When the desired level of anaesthesia is reached, cyclopropane is then cut

The errors of flow-meters and the advantages of a new type of constriction

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The errors of flow-meters and the advantages of a new type of constriction

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