ms4-thc design and operation details -...

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MS4-THC DESIGN AND OPERATION DETAILS

Executive OverviewThe Microset 4 CO2 model MS4-THC is an easy-to-use indoor air quality sensor that measures temperature, humidity, and carbon dioxide (CO2) concen-trations of 0-5000 ppm. The sensor uses a SenseAir® CO2 engine to measure CO2. This document outlines important factors in ensuring accurate readings for the self-adjusting CO2 sensor. Understanding how the CO2 sensor operates will help you provide the best possible experience for your building customers.• The algorithm used to measure CO2 levels self-adjusts after the MS4-THC is installed, adapting to changes in barometric pressure or rough handling in

transit. • Alerton recommends that MS4-THC wall sensors be installed and operational for 3 weeks prior to occupancy, so the CO2 sensors have time to

adjust to the installed location and ensure accurate readings. • Alerton supports a standard 2-year warranty on the MS4-THC. SenseAir’s field studies show over 10 years of maintenance-free operation on their CO2

engine.

IntroductionCO2 concentration is one of the most important parameters monitored in modern buildings, and CO2 sensors are installed to manage indoor air quality and implement Demand Control Ventilation strategies. These sensors have been available for over 20 years, and are now required for many US building codes, schools, and LEED certification credits.

The MS4-THC CO2 SensorSensor TypeThe MS4-THC uses a nondispersive infrared (NDIR) CO2 sensor. NDIR sensors surpass other types of CO2 sensors in terms of accuracy, long-term stability, maintenance-free operation, and low power consumption.

An NDIR sensor uses an infrared light detector that measures the amount of light sent from a lamp. Before the light reaches the detector, an optical filter absorbs all wavelengths except CO2. The light that remains, or is unabsorbed by the filter, shows the amount of CO2 in the air.

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The MS4-THC uses a SenseAir CO2 engine. SenseAir is a leading manufacturer of infrared gas measurement devices. The CO2 engine in each MS4-THC has a unique serial number with traceability on all key components and is individually calibrated.


ABC Algorithm and Calibration TimeThe MS4-THC uses the SenseAir Automatic Baseline Correction (ABC) algorithm to ensure accuracy and long life. The ABC algorithm monitors CO2 levels and records the lowest CO2 reading over a period of 7.5 days.

During unoccupied periods a properly ventilated building will experience stabilized CO2 levels around 400 ppm, approximately the average CO2 level in fresh air. The CO2 engine saves the lowest CO2 level and compares it to the CO2 engine’s 400 ppm reference point. These numbers can be different due to barometric pressure, how recently the sensor was installed, transportation, or normal sensor drift due to age. The MS4-THC will make gradual adjust-ments to keep the CO2 readings within specifications, with a maximum adjustment of 50 ppm for each 7.5-day period.

Figure 1 shows how six test MS4-THCs adjusted over four days to closely track with a reference CO2 meter placed in a warehouse in active use. The vertical lines in the chart called out by i and ii indicate when the ABC for the MS4-THCs adjusted to account for environmental or installation factors. MS4-THC #3–#6 adjusted on 8/15, and MS4-THC #1 and #2 adjusted on 8/17. Units #1 and #2 were powered OFF for two days, so they did not adjust at the same time as the rest of the tested group of devices.

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Figure 1. ABC adjustment over four days.


Accuracy Testing and ResultsThe MS4-THC was tested for accuracy at a wide range of CO2 concentrations at Element, a global network of laboratories with experts specializing in materials testing. The CO2 readings were compared against a certified reference gas using a 2% accurate CO2 meter that itself was certified to an interna-tional standards authority. The MS4-THCs that were tested tracked within tolerances of ±30 ppm plus ±3% of measured value. The high and low limits of those tolerances are indicated in Figures 2 and 3 by dashed lines.

Figure 2 shows the MS4 THC readings of the same six MS4-THC sensors at the following CO2 levels: 0, 600, 800, and 1200 ppm. The high and low tolerances (the dashed lines) get a bit farther apart at each level of ppm concentration; this is because the limits expand as the CO2 concentrations increase. The sensor readings are clearly within tolerance and track closely to the reference CO2 meter.

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Figure 2: C02 readings taken at varying ppm concentrations.

Figure 3 shows a close-up of the readings shown in Figure 2, focusing on measurements taken at the 800 ppm concentration.

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Figure 3: CO2 readings taken at 800 ppm concentration.


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Out-of-the-Box AccuracyWhen power is first applied to the MS4-THC after installation, no calibration is required. There is a warm-up period required, as is the case for all CO2 sensors. This warm-up typically takes 3 minutes, but could take as long as 15 minutes. Initial CO2 readings might be outside of the accuracy specifications due to shipping, rough handling, or barometric pressure, but the automatic baseline correction algorithm will adjust the sensor to the lowest CO2 reading, ensuring accuracy. This process typically takes 2-3 weeks, based on the sensor’s 7.5-day sampling period. The ABC algorithm will continue to monitor readings and make ongoing adjustments to the baseline calibration.

FAQsWhat can dealers do to minimize the impact of long calibration time?CO2 sensors regularly experience drift from rough handling, transportation, and general aging. For new installations Alerton recommends installing and powering the MS4-THC for 3 weeks in the customer’s building prior to occupancy to allow the ABC algorithm to adjust before comparing readings to CO2 meters. (The ABC algorithm expects the building to have CO2 background levels of 400 ppm.)

NOTE: Do not perform verification tests on sensor reading accuracy immediately after installation.

What happens if there is a loss of power?The MS4 CO2 engine will resume measuring CO2 using the last calibration or adjustment point for reference. The MS4 CO2 engine logs the lowest CO2 reading each hour.

How does barometric pressure affect the accuracy of the CO2 sensor?CO2 readings are influenced by barometric pressure. As altitude increases and barometric pressure decreases, there are fewer molecules in the air inside the air sampling space, which means there are fewer CO2 molecules for the NDIR sensor to detect, which results in readings with lower ppm values.

This is particularly important when installing the MS4-THC at higher altitudes. The change in barometric pressure will impact the CO2 readings on first power-up. No custom settings or additional calibration is necessary, because the ABC algorithm will correct for the change in barometric pressure at the 400 ppm level during its first 3 weeks of operation.

How long will the sensor be accurate?According to SenseAir, the sensors should last 15 years or more, and their field studies show over 10 years of maintenance-free operation. Alerton supports a standard 2-year warranty on the MS4-THC.

What if my space is occupied 24 hours a day?The MS4-THC is not designed for spaces that are occupied 24 hours a day unless there is a period where the CO2 levels return to background or outside air levels (approximately 400 ppm). CO2 levels must be verified with the design engineer or monitored on a building already occupied.

My MS4-THC has been installed for over 3 weeks and is still inaccurate. Why?There are a number of reasons why the CO2 sensor might report inaccurate CO2 readings.• Barometric pressure (altitude)• Rough handling• 24 hour occupancy without a period of near fresh air CO2 levels of 400 ppm

Does the MS4-THC have any installation considerations?The internal sensor relies on reading CO2 molecules within its chamber, so there should be enough clearance around the MS4-THC for air to flow through the unit. When there are obstructions or lack of airflow, the reading response time will be affected.

Locate away from:• windows• vents• any source of draft

Do not install on outside walls. If the MS4-THC is installed on drywall, ensure the wallplate does not allow airflow into the sensor from behind the wall. Installing an electrical box might help prevent this.

What is the response time of the CO2 engine?SenseAir specifies the CO2 engine with a measurement period of 30 seconds and a response time (T1/e) of less than 3 minutes due to its low-power engine and digital filter.

If this document does not address a calibration situation that you are experiencing, please contact Alerton Technical Support at [email protected] or 855-377-9530.

ms4-thc design and operation details -...

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