Question 24

For each of the following terms related to pressure monitoring, provide a definition and outline their role: zeroing, levelling, and calibration.

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College Answer

Zeroing: is a process which confirms that atmospheric pressure results in a zero reading by the measurement system. Intermittent confirmation ensures the absence of baseline drift (relatively common with disposable transducers), where atmospheric pressure no longer reads zero, resulting in aberrant results.

Levelling (or establishing the “zero reference point”): is a process which determines the position on the patient you wish to be considered to be your zero.  Transducers are placed at a point level with this point (often utilising fluid filled tubing).  Usually this is chosen as the midaxillary line (in a supine patient) or it could be the phlebostatic axis.  Significant errors in measurement may occur if readings using different zero reference points are used (eg. Cerebral Perfusion Pressure).

Calibration: is a process of adjusting the output of a device to match a known input value. Verification of calibration requires using a gold standard (eg. mercury or water manometer), and usually a simple two-step procedure (eg. confirming that zero = zero and 100mmHg =

100mmHg),  which  assesses  linearity  of  the  system.  The  calibration  of  disposable transducers is preset, and cannot be altered.


This question relies on the candidate's ability to generate definitions for terms which are in ubiquitous use. Unfortunately, this also means that their definition is frequently grasped intuitively, and never formally taught. LITFL have a good page on this material.

One can come up with a variety of definitions for these terms. Below, one can find a few non-canonical definitions. However, if this question appears again, one would be strongly advised to quote the college definition verbatim. The examiners wrote them in this way for a reason.


"Zeroing"can also be defined as "the use of atmospheric pressure as a reference standard against which all other pressures are measured".

"Levelling"can be defined as "the selection of a position of interest at which the reference standard (zero ) is set".

"Calibration"can be defined as "an adjustment of system gain to ensure the proper response to a known reference value".