A portable gas chromatographic detector includes two identical platinum coils mounted on identically aligned conducting posts. One coil is placed in an occluded chamber; the other coil is placed in an identical isolated chamber. Both coils have connection to legs of a bridge circuit and have substantially identical current flow passed along their length. Both chambers are preferably identically dimensioned and are large enough not to interfere with the field of the coils. The occluded chamber communicates through a short passageway with a passing classified gas stream from a chromatographic column. It has been found that the motion with respect to one coil is compensated with respect to the motion with the remaining coil so that the only differential in electrical resistance measured across the bridge circuit is that induced by the cooling property of helium on the heated coil in the occluded chamber. Cycling of the portable chromatograph includes a first short cycle. In the absence of detected helium, the instrument immediately returns to a standby state in readiness for a closely repeated cycle. Upon the detection of helium, a second and longer cycle results prolonged only by that time interval necessary for measurement of the helium and purging helium residual from the instrument collection conduits. For rich helium measurement resulting in super cooling of the occluded coil, a reverse bias is applied to the isolated coil through the bridge circuit to induce a warming current charge to the supercooled occluded coil. This enables rapid repeated measurement without prolonged warm-up intervals which would be otherwise required.
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