Measuring, controlling, regulating, signalling, checking and monitoring apparatus and instruments, namely, gas chromatography apparatus, gas detectors for detecting the presence of gas, gas sensors for measuring gas concentration, gas sensing electrodes, leak detecting devices for water or gas in the nature of leak detectors, gas pressure indicating gauges, pressure sensors, flame detectors, thermal exhaust flow detectors, flue gas monitors, thermo-electric safety devices in the nature of saf...