Posted By garyh I don't really understand your question but would state that in my experience you should ALWAYS measure the Oxygen level before entry; there should be sufficient passive or active ventilation. If in any doubt measure the oxygen level during the entry preferably using an alarmed device. Suggest you read the excellent HSE publication "Safe Work In Confined Spaces L101 ISBN 0 7176 1405 0". It should answer your questions. Could I offer one piece of advice - if you are in ANY DOUBT re a confined space entry, stop the entry and sort it out.

Posted By Allan Rhodes I suspect that you are not sure what you want but you may need the volume of a confined space for ventilation reassons so you can specify the correct ventilation requirements You may also need the volume of a space to calculate the amount of say asphyxiant that could be released from pipes or pressurised cylinders You do always check for oxygen first because if you are also checking for flammables then the explosive sensor may not reliably detect any explosive gases if the oxygen levels are generally less than 6% by volume.