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 occasions 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 dependant drift
Thermal drift can be minimised if you turn on your equipment at least 15 minutes before your experiments in order to allow your Spirometer amp or Spirometer Pod time to warm up.

If your amp or pod has not been zeroed, you may see an initial off-shoot in the Volume channel due to rapid repeated integration.  This can be overcome by pressing zero on the Spirometer input amplifier in LabChart or LabTutor.

Breath-dependant drift is caused by inequalities in inspired and expired Volumes (Expiredvolume is greater than inspired volumeunder most atmospheric conditions). So this may be seen regardless of whether the device has been zeroed.


Drifting Volume Trace

This 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. The 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 tubing and droplet filter.

In order to do a Volume correction, it would be best to have the Spirometry extension installed.  Here you can do either a manual correction or an automatic correction.

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. 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 checkbox in the Spirometry Volume Dialog prevents 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 corrected with a reset

If you are not using the extension, you can still apply a reset in your integral calculation by selecting 'reset each cycle' from the reset type drop down menu.