like lcd read outs led 39 s or small incandescent or neon light bulb which may flash or vary their intensity or audible indicators like buzzers or small speakers which may produce intermittent or substantially constant tones that may vary in volume or frequency operation of the device is as follows the conduit body is adjoined to the intravenous delivery system by interconnection with the tubing below the flow rate valve as shown in fig6 preferably the placement of the conduit body is as close to the flow rate valve and as for from the catheter as possible an alternative means of placement may be by way of a pre assembled unit including the conduit body and tubing the sensing mechanism is clamped around the conduit body as shown in fig7 the catheter is then inserted into the patient and the flow of fluid is begun as the fluid flows through the conduit body fluid passes through the pressure release passages into the pressure sensing regions when the flow is normal and unobstructed the pressure in the conduit body that is inside the housing and in the pressure sensing regions will be negative and the atmospheric pressure outside will press the sheath against the housing the electrical contacts in the sensing mechanism rest directly on the sheath proximal to the pressure sensing regions when the sheath.