upset the collapsible tube may be provided down stream of the barostat device and drip chamber this collapsible tube may be in the form of a 35 centimeter length of lay flat tubing not shown approximately one centimeter wide made from 0 15 millimeter thick plasticised medical grade pvc material and is connected to the outlet of the drip chamber 23 and conveys the fluid to the infusion tube and so on to the patent care is taken to make the tube so that it will collapse completely when the pressure inside becomes less than outside for example if the edges are folded they must be pressed absolutely flat or if they are formed by welding these welds must be free from beads that could hold the internal surfaces slightly apart this long form of the collapsible tube is used to measure the back pressure from the veins as previously described when it is employed there is no need for the short form of the collapsible tube 26 in the pocket 28 immediatey down stream of the barostat device 19 in the administration set shown in fig3 and 4 the reservoir 30 barostat device 31 in the air tight chamber 32 and the short collapsible tube 26 in the pocket 34 are made by the same methods as are described in the second embodiment and shown.