Since O2 analyzer location in reflow oven is fixed, which cause the sampling airtube length is different zone to zone because each reflow zone sampling distance to O2 analyzer location. In theory, while the longest tube analysis result (flush time + sampling time) is able to be measured accuracy, the rest ones should be OK . Pls check your reflow oven tube length and tube diameter to get tube volume (theory), calculate O2 analyzer flowmeter and verify the result, add a little bit buffer will be enough. If the sampling tube is too long due to original setting up there, you should reduce tube to a reasonable length there and retest to verify the result.Flush time: switch to 2nd zone and time to cleaning existing air in the pipe. Sampling time: exactly sampling air read out time.