the example mentioned above the second sensor may measure the pressure in the control line 18 proximal the well tool 10 fig1 also shows an alternative design in which the alternative second sensor 24 measures the characteristic in the tool 10 e g in the hydraulic chamber 14 the alternative second sensor 24 may be external to the tool 10 in which case the sensor 24 is hydraulically and functionally plumbed to measure the pressure in the tool 10 alternatively the sensor 10 is positioned within the tool 10 the sensors 22 and 24 are described as alternatives and only one may be used although alternative arrangements may use both sensors 22 and 24 in use the measurements from the first sensor 20 and the second sensor 22 and or alternative second sensor 24 are compared the comparison may reveal whether the supplied fluid is actually reaching the tool for example if the control line 18 is blocked the measurements between the first sensor 20 and the second sensor 22 or alternative second sensor 24 will be different if these values are substantially the same the operator can determine that the source is actually reaching the tool fig2 illustrates another aspect of the present invention in which the.