________Application Note ________
October 2000

APPLICATION NOTE
Telaire's ABCLogic™ Self Calibration Feature

Introduction
Use of CO2 to control ventilation is one of the fastest growing areas in building control. A primary reason for this growth is that the technology has now matured to the point where it is inexpensive, easy to use and reliable. Telaire leads this trend with a number of unique innovations in its sensor design. This article explains a key patented feature of Telaire's technology called ABCLogic™ that enables CO2 sensors to automatically calibrate themselves once installed in the field. This feature eliminates one of the biggest concerns surrounding CO2 sensing… sensor drift and maintenance. ABCLogic™ is a standard feature of all Telaire commercial grade CO2 controllers (models: 8001, 8002, 8007, 8008, 5010, 5011, 6003, 6004). All sensors with ABCLogic™ have a Lifetime Calibration Guarantee where Telaire will provide a free factory calibration for the sensor if it is found to be out of calibration.

Virtually all gas sensors are subject to some sort of drift. The degree of drift is partially dependent on the use of quality components and good design. But even with good components and excellent design a small amount of drift can still occur in the sensor that may ultimately result in the need for a sensor to be recalibrated. Generally, recalibration involves a maintenance person visiting each sensor in a building and performing a 5 minute to 20 minute recalibration routine using gas bottles and plastic tubing. The calibration process is simple but it can turn into a significant expense if recalibration is required frequently. If the wrong choice of sensors is made, the expense of sensor maintenance may wipe out any potential energy savings that could come from CO2 based demand controlled ventilation.

Approximately 7 years ago, Telaire developed and patented the concept of a self-calibrating sensor. The first products manufactured with this feature (originally called TEMA) are still working in the field today and accurately measuring CO2. Most of these sensors have never been field calibrated.

What Causes Sensor Drift
As discussed before, sensor design and components have a lot to do with drift. At Telaire our experience with building five generations of sensors for over ten years has allowed us to select the components and refine the design so that the primary contributor to sensor drift is the normal and unavoidable aging of the infrared light source within the sensor. In the Telaire sensor the natural drift of the sensor is very gradual at a few ppm per month with the greatest drift occurring in the first few months of operation. This drift can be up or down. Telaire's self-calibration feature called ABCLogic™is designed to correct all sensor dirft including aging of the light source.

How It Works
Outside levels of CO2 are generally very low at around 400 to 500 ppm. Inside buildings people are the major source of CO2. When a building is unoccupied for 4 to 8 hours CO2 levels will tend to drop to outside background levels. This is especially the case if the building operational schedule includes a pre-occupancy purge of fresh air into the building prior to the start of the day.
ABCLogic™ which stands for "Automatic Background Calibration" utilizes the computing power in the sensor's on-board microprocessor to remember the lowest CO2 concentration that takes place every 24 hours. The sensor assumes this low point is at outside levels. The sensor is also smart enough to discount periodic elevated readings that might occur if for example a space was used 24 hours per day over a few days. Once the sensor has collected 14 days worth of low concentration points it performs a statistical analysis to see if there has been any small changes in the sensor reading over background levels that could be attributable to sensor drift. If the analysis concludes there is drift, a small correction factor is made to the sensor calibration to adjust for this change.

The figure below shows CO2 concentrations as they might occur over 14 days in an office space with peak concentrations occurring in the morning and afternoon of each day. The dotted line is drawn through all the low points for each day as compared to an assumed background of 400 ppm. If a statistically relevant change in the data shows a shift above or below background, a slight adjustment is made to sensor calibration as shown by the solid level line. Every day the sensor looks at the past 14 days worth of data and determines if a calibration adjustment is necessary. A similar type of approach of nightly calibrations is often used for other HVAC equipment such as VAV boxes.

Measuring Inside-Outside Differential
When using CO2 to measure and control for ventilation it is most important to consider not the absolute ppm levels but the differential concentration between inside and outside concentrations. In general terms the higher the differential the lower the ventilation rate. One of the additional benefits of ABCLogic™ is that the sensor is calibrated to outside levels without having the expense and trouble of placing a sensor in the outside air. The sensor assumes that the lowest level is 400 ppm. Any readings above this level are related to the differential.

Applications for ABCLogic™
It is important to note that ABCLogic™ is designed for use in applications where spaces are periodically unoccupied for 4 hours per day or more so that indoor concentrations can drop down to typical outside levels. Use of a pre or post occupancy ventilation purge is recommended as a good engineering practice to deliver proper air quality and can help ensure that the space regularly reaches background conditions. If a purge is not used it is recommend that the occupancy schedule for the building operate the HVAC system for an hour or so after normal occupancy ends to allow for CO2 levels to be reduced to background. Some building operators may also want to use CO2 as the control parameter for activating the system.

If a Telaire sensor is being applied in a application that is unlikely to see regular background levels of CO2 then the ABCLogic™ feature should be deactivated. The manual that comes with the Telaire sensor explains how this can be accomplished. For these applications Telaire recommends using a Precision Grade dual beam sensor that uses another slightly more expensive method of maintaining long-term calibration (models 8101 and 8102).

Commissioning Sensors with ABCLogic™
When first installed CO2 sensors with ABCLogic™ , the sensors will use the first 14 days of operation to calibrate themselves to local background levels. Installers or building operators should not be concerned if readings on the sensors appear unusually high or low or if a lack of agreement between sensors is found. The sensor you receive from Telaire has gone through a complete quality check and has received a factory calibration that has verified the sensor is working properly. Each sensor will calibrate itself to its environment over the first 14 days of operation.
Reasons that sensors in the same building may read differently immediately after installation include:

The graph below shows the behavior of three Telaire CO2 sensors with ABCLogic™installed in the same space over their first 14 days of operation. As can be seen in the initial few days the sensor readings where at times quite different and in some cases over 100 ppm apart. However, by the 10th day all sensors agreed very closely in their readings. This graph is typical of the behavior of all Telaire sensors that utilize ABCLogic™.

Readings Of Three Sensors With ABCLogic™
Over 14 Days Of Initial Operation In An Office

Long Term Performance of ABCLogic™
Telaire has also conducted long term tests of CO2 sensors with ABCLogic™. The graph below shows almost three years of testing where a gas of a known concentration (980 ppm) was flowed to sensors operating in the ambient air of Telaire's test lab. This test was performed every 7 to 15 days. As can be seen from the data, all sensors have maintained calibration relative to a reference gas to well within the ± 75 ppm specification of the sensor throughout the duration of the test period. This long term testing provides fundamental proof that ABCLogic™ can eliminate the need for calibration over extended periods of time. Two years ago Telaire introduced a first in CO2 sensing, a Lifetime Calibration Guarantee. Further testing has now confirmed that ABCLogic™ should be able to maintain sensor calibration over the lifetime of the sensor (typically 15 years).

Long Term Calibration Verification Of 3 Sensors With ABCLogic™