Video: Rapidox 3100

Video transcript

What this analyser measures, why one box can hold 3 different gas sensors, and how to connect it, calibrate it and read it. A controlled atmosphere is rarely defined by one gas. A glove box needs oxygen and moisture kept low; a combustion process needs oxygen and carbon dioxide. One instrument per gas means separate calibrations and separate logs. Up to 3 gases at the same time, chosen from 13, from oxygen and carbon dioxide to ammonia and ozone.

Plus an independent temperature channel, up to 1250 degrees. Each gas gets the sensor that suits it: infrared for percent levels of carbon dioxide or methane, electrochemical cells for toxic gases in parts per 1000000, thermal conductivity for hydrogen, a tunable laser for methane among other hydrocarbons, and a capacitance sensor for dewpoint. Oxygen itself can be measured in 3 ways. Zirconia reaches trace levels, down to 10 to the minus 20 parts per 1000000. An optical cell covers zero to 100% and barely drifts.

An electrochemical cell handles flammable gases, solvents, hydrogen and helium. Or there is no oxygen sensor at all. On the front: the display, 4 keys and PRINT, the two gas ports, and the flow gauge with its needle valve. On the back, everything that ties it to the plant: analogue outputs in voltage and current, 4 alarm relays, the thermocouple input, two serial ports, the pump switch, and a mains inlet for 90 to 260 volts. The gas needs a line in and a line out.

The sample passes through the filters it needs, in this order: particles, water, aggressive gases, solvents, and into GAS IN. GAS OUT goes to extraction, which is essential with toxic, corrosive or flammable gases. Then the signals: the thermocouple, the analogue outputs to a PLC or recorder, the alarm relays to their load, within 24 volts and half an amp, the serial port to a computer, and mains, last. Before switching on:

the case in fresh air, away from heat and dirt, with its fan clear; FEP or stainless tubing; and the outlet vented. Then set the flow, and allow a few minutes of warm-up. Oxygen is calibrated at two points, normally pure nitrogen and fresh air at 20.95%, and the measurement should lie between them. The other sensors take a zero gas and a span gas. Below 4 milliamps, the output current itself says what is happening: start-up, sensor fault, or out of range.

So the PLC can tell a fault from a genuine zero. Four alarm relays, each assigned to a gas, the balance gas or the pressure, rising or falling, with a delay against spikes, and latching when it must be acknowledged. The supplied software graphs every channel live and logs to a file that opens in Excel. For a PLC there is Modbus RTU: the plant reads the analyser, but cannot reconfigure it. It is not for oxygen above 30% unless specified

oxygen clean, and not for hazardous areas. Toxic gases need a vented outlet and a personal gas alarm. And calibrate at least once a year. Glove boxes and inert gas blanketing, heat treatment, research laboratories, and biogas, syngas, chemical, food and pharmaceutical processes. Gas analysis, from Amperis.