Why is my Spirometry Volume Channel drifting and how can this be minimized?

Spirometry Volume is an integral calculation of the raw Spirometry Flow data.  On occasion you may see drifting in this channel either in a positive or negative deflection. This could be caused by 3 things:

  • Thermal drift
  • Initial offset drift
  • Breath dependent drift
Thermal drift can be minimized if you turn on your equipment at least 15 minutes before the experiments in order to allow the Spirometer Amplifier or Spirometer Pod time to warm up.
If your amplifier or pod has not been zeroed, you may see an initial off-shoot in the Volume channel due to rapid repeated integration of an offset in the flow signal.  This can be overcome by clicking the 'Zero' button in the Spirometer's settings dialog in LabChart when there is no flow being applied to the flow head.
Breath-dependent drift is caused by inequalities in inspired and expired volumes (expired volume is greater than inspired volume under most atmospheric conditions). So this may be seen regardless of whether the device has been zeroed or not.
A volume trace drifting from this breath-dependent drift can be corrected in two different ways:
  • Volume Correction
  • Volume Reset

Volume Correction

A great improvement results from a simple modification to the integration of flow, so as to reduce the expired volume:

V = ∫ F dt (inspiring)

V = ∫ F/k dt (expiring)

where k (>1.0) is a volume correction ratio. This method, however, over-corrects shallow panting breathing, in which most of the air breathed is dead-space air. LabChart's Spirometry Extension accounts separately for dead-space air while integrating, and uses a more elaborate correction:

V = ∫ F dt (inspiring, or expiring dead-space air)

V = ∫ F/k dt (expiring air in excess of dead-space)

Non-ideal distribution of air flow across the flow head’s mesh screen also appears to contribute to breath-dependent drift. The required volume correction factor is therefore usually larger than that predicted by the Gas Laws. This component of the drift is minimized by use of the clean bore tubing and droplet filter.


In order to accomplish a volume correction for the above, it would be best to have the Spirometry Extension installed. In this LabChart extension you can perform a manual correction, an automatic correction, or an event based baseline reset.


Manual correction

You can manually set the value of the volume ratio k in the Spirometry Volume Dialog after selecting Apply volume correction. The value of the dead-space is not critical, and the default value (400 mL) is suitable in almost all cases. Other dead-space values can be entered by typing directly into the text field.


Automatic correction

The extension can calculate a value of k automatically from a data selection in two ways; however, we will discuss the full expiration method here. For the full expiration method, the selection should include a full expiration, a period (20–60 s) of normal breathing (which may include panting), and then another full expiration. When you display the Spirometry Volume dialog, the extension finds the minimum value of volume in the first and last thirds of the selection, and calculates a value of k that makes the values equal. It is important to breathe out fully once in the first third and once in the last third of the selection: these expirations provide reference points at which it can be assumed that the lung volumes are equal. Click 'Apply' to apply the calculated value of k.


Volume Reset

If you are using the spirometry extension, the 'Reset volume to zero at the end of each expiration' checkbox in the Spirometry Volume Dialog can be used instead of the methods listed above to prevent accumulated drift by resetting the volume trace to zero at the end of each expiration. This is recommended for breath-by-breath analysis.
Volume Trace calculated with an Integral Channel Calculation and Reset
If you are not using the Spirometry Extension, you can still calculate volume from the Spirometry flow signal with the Integral Channel Calculation and applying a reset in your integral calculation by selecting 'reset each cycle' from the 'reset type:' drop down menu.
If you require further technical assistance with this or any other issue, please contact ADInstruments Technical Support.