tip of the sensor application_number US 60001200 A description referring to the drawings the apparatus comprises a tubular probe 1 comprising a microporous sheath which permits the transfer of csf into a gel a filling the probe a number of sensors are housed within the tubular probe one of these is a ph sensor 3 sensor 3 comprises a length of optical fibre having a mirrored distal end 10 to reflect light back towards the proximal end 11 longitudinally of the optical fibre several holes 12 are laser drilled through the optical fibre in a number of random directions normal to the longitudinal axis of the fibre these holes are filled with a gel containing a phenol red dye which undergoes a colour change with change in ph a colour change over the ph range from about 6 8 to 7 8 is desirable the colour shade of the phenol red indicator is determined by passing a light beam along the optical fibre and measuring the absorption spectrum of the reflected beam after calibration the absorption spectrum of the reflected beam gives a measure of the ph of the csf as indicated in fig1 the tubular probe may also include other sensors such as a co 2 concentration sensor pco 2 4 a partial oxygen pressure sensor po 2 6 and a thermocouple 5.