A biological processing system comprising: a chamber having an interior for receiving and processing fluids; a sensor assembly having a sensor probe for measuring conditions in the chamber; at least one port disposed on the chamber for receiving the sensor probe and permitting the sensor probe to access the chamber interior, wherein the port comprises a stem projecting outwardly from the chamber, and the stem comprises a flange and an interior wall forming a stem passageway disposed between interior and outer openings (26; 28) and in fluid communication with the chamber interior; a probe seal for forming a seal between the port and the sensor probe and preventing fluid in the chamber from leaking past the probe seal, wherein the probe seal comprises spaced-apart flexible inner and outer skirts (42; 44), the flexible inner skirt having a wall forming a seal passageway for receiving the sensor probe and permitting the sensor probe to enter the chamber interior when the probe seal is attached to the stem; wherein the flexible inner skirt has a seal member located near the interior end of the flexible inner skirt which is inserted into the stem passageway and tapers inwardly towards the sensor probe and away from the wall forming the stem passageway for sealably compressing against the sensor probe and the stem passageway to form a leak-tight seal, wherein the flexible inner skirt permits the seal member to flex outwardly into sealing engagement with the stem passageway in response to the insertion of the sensor probe into the seal passageway; and wherein the flexible inner and outer skirts (42; 44) form a skirt channel adapted to receive the flange of the stem when the inner skirt is inserted into the stem passageway. a chamber having an interior for receiving and processing fluids; a sensor assembly having a sensor probe for measuring conditions in the chamber; at least one port disposed on the chamber for receiving the sensor probe and permitting the sensor probe to access the chamber interior, wherein the port comprises a stem projecting outwardly from the chamber, and the stem comprises a flange and an interior wall forming a stem passageway disposed between interior and outer openings (26; 28) and in fluid communication with the chamber interior; a probe seal for forming a seal between the port and the sensor probe and preventing fluid in the chamber from leaking past the probe seal, wherein the probe seal comprises spaced-apart flexible inner and outer skirts (42; 44), the flexible inner skirt having a wall forming a seal passageway for receiving the sensor probe and permitting the sensor probe to enter the chamber interior when the probe seal is attached to the stem; wherein the flexible inner skirt has a seal member located near the interior end of the flexible inner skirt which is inserted into the stem passageway and tapers inwardly towards the sensor probe and away from the wall forming the stem passageway for sealably compressing against the sensor probe and the stem passageway to form a leak-tight seal, wherein the flexible inner skirt permits the seal member to flex outwardly into sealing engagement with the stem passageway in response to the insertion of the sensor probe into the seal passageway; and wherein the flexible inner and outer skirts (42; 44) form a skirt channel adapted to receive the flange of the stem when the inner skirt is inserted into the stem passageway.