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From your description it sounds as if your spirometers may use a range of different measurement techniques. Some, such as the "Gold standard" traditional Vitalograph, measure volume directly, while others use pneumotachographs (flowheads), rotating devices or orifice plate systems to measure flow rate and then integrate that to get volume. Pneumotachographs generally have a linear flow/pressure response but have a finite resistance to airflow, which should be below a certain value to be acceptable in this application, and are sensitive to contamination. They tend to follow the flow signal quite well. Rotary devices also measure air flow but can suffer from overshoot when the flow rate changes quickly or inertia when the rate of change is slow. They tend to have lower resistance to airflow than pneumotachographs. Orifice plate devices have very little resistance to airflow and will not suffer much from contamination but need electronic correction of the flow/pressure response and will not be very stable at low flow rates. Each technique offers a different resistance to the person using the equipment, which might affect your readings, and offers a slightly different response depending on the mix of rapidly or slowly changing flow rates. My guess would be that when you use your syringe to check calibration, you will apply it to each device in a way that will produce a sudden increase in flow which will plateau and continue for a couple of seconds and which will terminate suddenly when you hit the end stop. Your devices are calibrated to this step type of flow signal. This is not however what a person does when using the equipment. There is an explosive initial burst which immediately tails off and might continue for a few seconds at a very small flow level before terminating. You could try to simulate that with a syringe if you want to assess which, in the real world, gives consistently best results. There are other factors which could be at play of course but these are some of the issues which manufacturers have to try and deal with when designing equipment of this type. You could try P&A tel 01204 676180 as an independent testing and calibration company. From:- G M Instruments Ltd Unit 6, Ashgrove Ashgrove Road Kilwinning KA13 6PU Scotland UK Tel + 44 (0)1294 554664 Fax + 44 (0)1294 551154 WWW.GM-INSTRUMENTS.COM ~~~~~~~~~~~~~~~ Please remove this footer before replying. For list archives and documents, go to http://www.jiscmail.ac.uk/lists/occ-health.html for list archives For jobs in Occupational Health, go to http://uk.groups.yahoo.com/group/OHJobs/