1304 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



I — 



fig 16. Rotameter of Shipley and Wilson. .4, contact holder; 



B, one of the contacts to induction and compensating coils, 



C, detecting assembly; D, rubber cap for removing air bubbles; 

 E, coils; F, protecting sleeve; G, outflow spout; H, soft -iron 

 float wire; /, brass float; J, metering chamber; A', conical 

 metering portion; L, support; M, float rest at zero flow; N, 

 float guide; 0, inflow spout. [From Shipley & Wilson (121).] 



rod moves into it, the inductance of the coil increases, 

 and this is recorded continuously by means of a 

 bridge circuit, rectifier, and galvanometer. Since the 

 lift of the float is proportional to the flow of blood, 

 the record can be calibrated in terms of flow rate. 



The theoretical basis of the rotameter may chiefly be 

 derived from mass inertia of the streaming fluid ac- 

 cording to the Bernoulli effect. The fluid streaming 

 upward is accelerated in the ring slot around the 

 float. The fluid reaches its maximal velocity not in 

 the plane of the slot, but at a somewhat higher level. 

 This velocity difference causes a pressure difference to 

 develop between the levels below and above the float. 

 Other effects, such as eddy formation, may play an 

 additional role. The pressure difference is augmented 

 by friction due to viscosity of the fluid. By the action 

 of the total pressure difference on the cross-sectional 

 area of the float, a force is brought about which lifts 

 the float, a force is brought about which lifts the float. 

 In steady states the lifting force must be in balance 

 with the float's weight diminished by its buoyancy. 



Since the weight minus buoyancy remains constant, 

 the force must be constant also. This is achieved by 

 the fact that the ring slot area increases with flow rate 

 by elevation of the float to a higher level where the 

 tube diameter is larger. 



Since the rotameter should be independent of 

 viscosity (due to changes in temperature or hemato- 

 crit), the frictional force must be kept minimal as 

 compared to the inertia force. This can be achieved 

 either by increasing the inertia force by special 

 shaping of the float (121) or by diminishing the fric- 

 tional force by using large ring slot areas and floats of 

 light weight (60). 



The relationship of volume flow to galvanometer 

 deflection can be made linear if the electrical settings 

 are adjusted. Since the response to changes of flow is 

 slow, only mean flows are recorded. However, large 

 pulsations, such as occur in arteries, are not averaged 

 correctly, particularly by units which use heavy brass 

 floats. This results in the recording of lower mean 

 values than are actually present (60), and it may be 

 useful in such cases to diminish the amplitudes of the 

 flow pulsations by an air chamber arranged upstream 

 from the rotameter (122). 



THE ELECTROTURBINOMETER 



The Potter electroturbinometer, originally built for 

 technical purposes, has been applied by Sarnoff et al. 

 116, 117) for the registration of aortic blood flow in 

 dogs. It consists of a stainless-steel turbine which is 

 driven by the blood stream. The turbine is suspended 

 within a Lucite tube by spring clips. The necessity of 

 using thrust bearings is avoided by shaping the rotor 

 in such a way that the stream generates, in addition to 

 rotation, a hydrodynamic force which acts in an up- 

 stream direction. The rotor contains a permanent 

 magnet which induces, by its rotation, an alternating 

 voltage in a pickup coil outside the tube. The fre- 

 quency of that voltage is proportional to the rotational 

 speed; by means of a counting and integrating 

 electronic system, an output signal is obtained, 

 the strength of which is a measure of the number of 

 turbine revolutions per time unit. Two models of 

 different sizes are described, the smaller of which 

 responds to flow from about 0.5 to more than 4.5 

 liters per min. The calibration curve shows a slight 

 bend and is, in the case of blood, independent of 

 temperature from 22 C to 40 C and of the hematocrit 

 down to 22 per cent. Although the instrument is un- 

 able to follow the instantaneous changes of the aortic 



