Topics - Gas Density Balance

Gas Density Balance The gas density balance was originally described in the early 1950s and was the first in-line gas chromatography detector to be invented. In its original form it consisted of a Wheatstone network of tubes drilled out of a high conductivity copper block. The net-work of tubes were self balancing and were so arranged that, in the center of the block, the carrier gas from the column passed up one tube and pure carrier gas flowed up another tube beside it. The two tubes were connected at the center through a cylindrical convection chamber about 2 cm in diameter and 3-4 mm wide. A looped heater was situated in the chamber and twin thermocouples consisting of two copper wires 0.001 in OD with 2 mm of constantan wire of the same diameter arc welded between them placed above the heater wires. It was arranged that the thermocouple junctions were situated about 2 mm from the heater wires and thus, with no solute present were heated by convection to the same temperature When a solute was present in one tube, due the increases in vapor density of the tube contents, a pressure difference developed across the joining tube. This resulted in a flow of gas across the convection chamber, displacing the convection currents away from one thermocouple onto the other. This resulted in a potential developing across the copper wires which was amplified and fed to suitable recording device. Because the detector responded to the vapor density of the solute, the device could also be used to determine the molecular weight of a solute. The sensitivity of the detector was about 5 x 10-7 g/ml. The detector did not become very popular because it was so difficult to make. In fact, this constructional difficulty helped to provoke the development of other detectors.