Posted By TomP May have just answered my own question: 'where it can be ensured that the gas or dust is present in a concentration below the lower flammable limit, the atmosphere is not explosive, and regulations 7, 11 and 17 do notapply.' So how to ensure this doesn't happen is now the trick.

Posted By Ian Waldram For such an application you should have, outside the lab: regulator plus relief valve sized for the regulator-failure case (regulators have small RV's, but designed to protect the pressure gauge, not any downstream equipment). That ensures any releases inside the lab will be small-scale. Inside the lab you should have no pipework joints up to the point of use, or else tested HP joints even though it's LP piping. Good practice would be to have the lab equipment inside a fume cupboard, with ventilation alarm and regular ventilation flow measurement. You can check that a worst-case leak won't exceed LEL at the design ventilation rate. If you do have any joints or potential leak points inside the lab, then there will be Zone 2's around these.