Influence of residual oxygen in inert atmosphere

Very often the question arises as to the amount of residual oxygen in the system. This can very easily be determined by measuring the combustion rate of activated carbon at 700°C in the thermobalance. 

Outgassing of constructional components (oxygen adsorbed on parts of the measuring cell) and dead volume Switch on the flow of protective gas to the microbalance several hours before measurement. Purge the vacuum connection as well. Only open the furnace briefly to insert the sample. Active replacement of air through slight evacuation to about 1 kPa. Then flood with the desired purge gas (if necessary twice). 

Ingress of atmospheric oxygen due to leaks A possible cause of leakage is the furnace seal which could be damaged or dirty. 

Ingress of atmospheric oxygen due to back diffusion at the purge gas outlet Attach a long narrow tube to the outlet (this functions as a diffusion baffle). 

For example, given a purge gas flowrate of 100 mL/min a 5 xg/min weight loss corresponds to 90 ppm concentration of oxygen. A routine check might require the weight loss to be less than, say, 10 xg/min see Table 3.2. 

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