Worldwide implementation of ISO 23875 has produced many questions about various aspects of the standard. The most frequently asked questions will be addressed in a series of posts.
3. The standard contains specifications for the various test instruments. The test instrument used to measure CO2 concentration has a specification of +/- 3% of reading. What should I expect to see on the certificate of conformance from the device manufacturer regarding this requirement?
The standard requires the factory to preset the CO2 threshold at 1 000 ppm. At this threshold the sensor accuracy shall be +/- 3% of the reading. An example of acceptable wording from the manufacturer on their certificate of conformance, describing sensor accuracy, would be:
Factory setting according to ISO 23875 clause 4.3.3 a), 1) shall be set to 1 000 ppm by the manufacturer of the monitoring device. At the factory threshold limit sensor accuracy is +/-3%
Sensor accuracy is +/- 3% for readings ≥1 000 ppm.
For sensor readings <1 000 ppm sensor accuracy is +/- 30 ppm or +/- 3% whichever is greater.
The focus of the standard is on operator protection. CO2 in low concentrations i.e., below
1 000 ppm does not represent a health threat to the operator. Sensor accuracy is focused on levels of CO2 which can impair the operator. The level at which this becomes a concern is at or above 1 000 ppm. Therefore, at or above this level the sensor accuracy must be +/- 3% of the sensor reading.
The standard requires the CO2 sensor resolution to be <20 seconds. This specification refers to frequency of the screen updates and not to the time its takes for the CO2 sensor to register large increases in CO2. Resolution, according to the manufactures relates to the reaction time of the sensor. The time it takes for a large change in CO2 to be recognized, stabilized, and updated on the CO2 human interface device. This misunderstanding was translated into the standard and has caused a good deal of confusion among CO2 sensor manufacturers who are not able to meet the "resolution" specification in the standard.
With this understanding, the manufacturer's certificate of conformance will state that it does not meet the resolution specification in the standard. While this misinterpretation of the word "resolution" in the standard is unfortunate, the actual device "resolution" of <3 minutes with the frequency of screen updates is <20 seconds, this does not impact the effectiveness of the sensor in reporting increases and decreases in CO2 levels. The upper alarm threshold required by the standard, of 2500 PPM, is 50% of the Occupational Exposure Limit (OEL) of 5000 PPM. Sensor performance is more than adequate for what it is intended to do, alert the operator when CO2 levels are rising to threshold levels.
4. ISO 23875 requires certification that the enclosure passed each of the four performance tests. Does the standard allow the engineer/retrofitter to determine the level of filtration required to pass the performance tests? What happens when the level of filtration required does not allow other parts of the cab air quality system to function properly, specifically, when the HVAC system air volume is compromised by a high efficiency recirculation filter?
Yes. ISO 23875 places the responsibility for choosing the engineering controls required to pass the performance tests on the engineer, retrofitter or machine owner. Filtration, as an engineering control, must be of high enough efficiency and without seal leakage to allow the system, as a whole, to pass the decay and low- pressure side leakage tests.
If the HVAC blower is not sized for the level of restriction presented by the recirculation filter, then the system will not function per design. HVAC systems with lower than design airflows typically experience high compressor head pressure, leading to premature compressor failure. Other symptoms of recirculation filtration that is too restrictive for the HVAC design are excessive amounts of intake air which cause the condensate drain pan to overflow as the system is forced to address higher than design levels of humidified air. Additional issues, are low airflow resulting in poor heat exchange resulting in higher enclosure temperatures. Each of the symptoms are caused when the HVAC design parameters are exceeded and each lead to premature HVAC failure.
When the system design is fixed and cannot be modified e.g., with a larger blower motor, and higher amperage machine alternator, then it is best not to address the air quality problem at the recirculation filter on the HVAC system. Alternatively, auxiliary systems which draw air from the enclosure, passing it through a high efficiency filter and back into the enclosure are an effective means of addressing the particular decay test without making modifications to the HVAC system. This alternative method of addressing the decay rate is both effective and necessary because most of the machines brought into compliance in the next few years will be older machines that are being retrofitted, and their HVAC systems cannot be re-engineered.
ISO 23875 is about continuously providing good operator air quality. How air quality is engineered in the cab must be both effective and flexible to allow existing machines to comply with the performance requirements in ISO 23875
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