CO2 measurement
How is Carbon Dioxide Rebreathing measured?
The method developed by CPSC for measuring carbon dioxide rebreathing makes use of a mechanical machine that breathes in and out like an infant. (Reference 1) Carbon dioxide gas is metered into the lung at a rate representing infant metabolism and allowed to reach equilibrium. A CO2 analyzer is used to measure the CO2 concentration in the breathing circuit.
A newly developed Thermal Rebreathing Analyzer produces measurements that correlate reasonably well with the CPSC method.
CO2 Method Schematic
The CO2 method makes use of CO2 gas, a breathing machine, sample pump and CO2 analyzer. The probe is the same 3-inch hemisphere used for airflow testing.
new Rebreathing Analyzer
Self-containedd Portable CO2 Rebreathing Measurement Kit
The standard method and apparatus for measuring rebreathing makes use of a breathing machine, carbon dioxide gas and a carbon dioxide analyzer. Carbon dioxide is metered into a mechanically actuated lung and connected to an infant breathing model. With normal breathing, some CO2 is exhausted to the atmosphere with each exhaled breath. With bedding applied to the model’s face, rebreathing may occur and the concentration of CO2 will tend to rise in the model as it would in a live infant.
Soft sleep surfaces and infant products are known to be a potential factor for the occurrence of respiratory hazards. These hazards include airflow resistance and carbon dioxide rebreathing. The breathing model and CO2 analyzer provide a method of measuring the relative tendency of surfaces to retain exhaled breath and return it to the infant, leading to reduced oxygen and excessive CO2 delivery, a hazard.
The breathing machine models a sleeping infant. The respiratory rate and tidal volume are fixed and the probe models the face of an infant. Carbon dioxide is metered into the operating lung at a rate that models an infant’s rate of metabolism, resulting in a system that breathes oxygen in and carbon dioxide out, just like a live infant.
When the probe is applied to a sleep surface, the level of CO2 rebreathing can be measured accurately and repeatably.
Breathmeter is based on a design developed at the US Consumer Product Safety Commission in 1979. (see references below) Carleton, Porter and Donahue published a paper describing the breathing machine and analyzer. Breathmeter uses the same breathing rate, tidal volume, residual volume and measurement technique as Carleton. The probe was developed By Dr. Erin Mannen’s BabiLab at Boise State University. The removable “nose” on the probe permits rebreathing measurements on all surfaces.
Thermal Rebreathing Analyzer
New Development - US Patent 11,191,450
The standard method and apparatus for measuring rebreathing makes use of a breathing machine, carbon dioxide gas and a carbon dioxide analyzer. Carbon dioxide is metered into a mechanically actuated lung and connected to an infant breathing model. With normal breathing, some CO2 is exhausted to the atmosphere with each exhaled breath. With bedding applied to the model’s face, rebreathing may occur and the concentration of CO2 will tend to rise in the model as it would in a live infant. The standard method is useful, but suffers from several drawbacks:
- high cost of equipment
- Need to calibrate CO2 analyzer frequently
- Requires CO2 gas supply
Use of the standard methodology also requires a high level of expertise on the part of the operator to achieve reliable measurements. It is complicated.
New Concept
The Thermal Rebreathing Analyzer is a new method and apparatus for evaluating bedding materials for their relative effect on re-breathing, without using carbon dioxide. By equipping the breathing model with a heater to warm the air exhaled from a head-form probe, carefully measured temperature differences may be used to evaluate the level of re-breathing. (Temperature difference as a proxy for CO2 concentration – both a measure of rebreathing)
Just as CO2 from the exhaled breath can be stored in bedding and re-breathed, the same is true for warm air exhaled into the bedding and then returned to the “baby” warmer than ambient temperature. The extent of re-breathing is indicated by the difference between the temperatures of the inhaled breath and ambient air temperature. When there is no re-breathing, the inhaled breath temperature will be equal to ambient temperature. As re-breathing increases, so will the temperature of the inhaled breath, relative to ambient temperature. In other words, elevation in inhaled air temperature above ambient is a measure of re-breathing.
The sensor is a very accurate differential thermometer. The output is differential temperature in degrees Celsius.
Thermal Rebreathing Analyzer
Correlation with the CPSC method
At the current stage of development, the thermal rebreathing analyzer produces repeatable measurements that correlate well with measurements performed on the same materials using the CPSC method.
The analyzer is portable and does not require a source of CO2, making it available on a daily basis to evaluate materials during product development.
Current status
The Thermal Rebreathing Analyzer produces measurements that correlate reasonably well with the CPSC method. The thermal method is able to differentiate hazardous from safe sleep surfaces.