Analysers
September 24, 2020

What impact do Gas Standards on Flow or Volume measurements?


Feedback from biomedical engineers has shown us that many measurement errors of flow are caused by a mismatch between the gas standards used by the device under test (e.g. a medical ventilator) and the gas flow analyzer used for flow and volume testing.

Keep in mind both devices are measuring the same gas flow but each device can be set to apply an offset to the displayed values. Those offsets are convenient for the operator but they can be a challenge for the biomedical engineer.

The most commonly used gas standards for gas flow measurement are:

STP: Standard Temperature and Pressure – the device does a conversion based on a dry gas flow, at 21.1°C, and at an ambient pressure of 1013.25 mbar.

ATP: Ambient Temperature and Pressure – the device does a conversion based on gas flow at current humidity and temperature and local ambient pressure.

BTPS: Body Temperature and Pressure – the device does a conversion based on gas flow saturated with moisture (100% RH) at body temperature (37°C) and flowing at local ambient pressure.

However, there are many more gas standards in the industry and our analyzers can be set to apply any one of seventeen different standards.

Each manufacturer can stipulate which gas standard their device uses.  BTPS is often used in the ventilation environment.

 Here’s an example from the Swiss city called Buchs which is 450 meters above sea level (960mbar) at a sunny day with a temperature of 28°C and a relative humidity of 50%. The gas flow source, in this example, provides a steady flow of 100 liters per minute (LPM.)

When using the following gas standard, the measurement on a gas flow analyzer is:

STP: displays 100 LPM

ATP:  displays 110.19 LPM

BTPS: displays 119.04 LPM

The error can be 19% or in this example 19.04 liters per minute.  Which is why it is essential to select the same gas standard on each device before comparing their displayed measurement values…lesson learned!

Comments


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M. R. Ardakani

December 7, 2022

Hi
It is stated in savina 300 ventilator(drager) that:
Medical gases from cylinders or from a central gas supply
system are dry (approx. 0 % rel. humidity) and are
supplied by the ventilator at 20 °C (68 °F) and
1013 mbar (1013 cmH2O). Measured values for
flow and volume under these conditions are
characterized as NTPD. The difference between values measured as NTPD and BTPS is approx. 12 % at a pressure of
1013 mbar (1013 cmH2O).
Example: 500 mL tidal volume NTPD become
564 mL BTPS when warmed to 37 °C (99 °F) and
humidified to 100 % rel. humidity.
Savina 300 controls tidal volume in such a way that
the set tidal volume value is applied under BTPS
conditions in the lungs.
The expiratory measurement is performed on the
basis of saturated gases at 30 °C (86 °F).

According to the above, now a basic question arises:
To perform quality control tests of ventilators, which gas standard should be used in the flow analyzer? Comparison between volumetric quantities on the ventilator and volumetric quantities measured by the flow analyzer, when it seems logical that both devices use the same standard? How can we find out what gas standard each ventilator uses to measure the volume quantities in order to adjust the gas standard of the flow analyzer based on it???
Thank you very much in advance for guiding me and rescuing me from a dilemma.


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