EP4724805A1

EP4724805A1 - Methods for calibrating a total volatile organic compound (tvoc) sensor

Info

Publication number: EP4724805A1
Application number: EP23944601.6A
Authority: EP
Inventors: Bhavesh S. Gupta; Bin Dong; Jie Gao; Jianguo Zhao; Yaolin LIU; Shidian XU; Qinghua Wang
Original assignee: Honeywell International Inc
Current assignee: Honeywell International Inc
Priority date: 2023-07-10
Filing date: 2023-07-10
Publication date: 2026-04-15
Also published as: CN121336110A; WO2025010578A1

Abstract

A TVOC sensor drifting downward may be calibrated by comparing a current measured TVOC concentration to an expected minimum TVOC concentration. When the current measured TVOC concentration is lower than the expected minimum TVOC concentration by at least a threshold amount, a TVOC adjustment factor is determined based at least in part on a difference between the current measured TVOC concentration and the expected minimum TVOC concentration. A TVOC sensor drifting upward may be calibrated by finding a minimum TVOC concentration corresponding temporally with a minimum CO₂ concentration, and comparing that TVOC concentration with the expected minimum TVOC concentration. A TVOC adjustment factor is determined based at least in part on a difference between the minimum TVOC concentration and the expected minimum. Subsequent measured TVOC concentrations are adjusted by the determined TVOC adjustment factor to provide a calibrated TVOC concentration.

Description

METHODS FOR CALIBRATING A TOTAL VOLATILE ORGANIC COMPOUND (TVOC) SENSOR

TECHNICAL FIELD

The present disclosure relates to methods for monitoring air quality and more particularly to methods for calibrating air quality sensors.

BACKGROUND

A variety of air quality sensors are employed within building control systems. Building system controllers rely upon indications of air quality that are provided by the variety of air quality sensors that are deployed within a space. Some air quality sensor, including Total Volatile Organic Compound (TVOC) sensors, can be tedious to manually calibrate after installation. Some air quality sensors, including TVOC sensors, can suffer from drift, meaning that the sensor provides increasingly inaccurate readings over time. What would be desirable are improved methods for calibrating air quality sensors such as TVOC sensors.

SUMMARY

The present disclosure relates to methods for monitoring air quality and more particularly to methods for calibrating air quality sensors. An example may be found in a method for calibrating a Total Volatile Organic Compounds (TVOC) sensor. The illustrative method includes accepting a value for an expected minimum TVOC concentration for the building space, measuring a current TVOC concentration, and comparing the current measured TVOC concentration to the expected minimum TVOC concentration. When the current measured TVOC concentration is lower than the expected minimum TVOC concentration by at least a threshold amount and in some instances for a user-configurable period of time, a TVOC adjustment factor is determined based at least in part on an amount that the current measured TVOC concentration is lower than the expected minimum TVOC concentration. Subsequent measured TVOC concentrations are adjusted by the determined TVOC adjustment factor to provide a calibrated TVOC concentration.
Another example may be found in a method for calibrating a Total Volatile Organic Compounds (TVOC) sensor. The method includes storing a plurality of CO₂ concentration values captured over a period of time, with each CO₂ concentration value captured at a corresponding time during the period of time. In some instances, the period of time may be user-configurable. Likewise, a plurality of TVOC concentration values are captured over the period of time and stored, with each TVOC concentration value captured at a corresponding time during the period of time. A minimum CO₂ concentration value is identified from the plurality of CO₂ concentration values along with a time that the identified minimum CO₂ concentration value was captured. The TVOC concentration value that was captured at or nearest the time that the identified minimum CO₂ concentration value was captured is identified and is compared with an expected minimum TVOC concentration value. When the TVOC concentration value that was captured nearest the time that the identified minimum CO₂ concentration value was captured is above the expected minimum TVOC concentration value (sometimes by at least a threshold amount) , a TVOC adjustment factor is determined and subsequent measured TVOC concentration values are adjusted using the TVOC adjustment factor to provide a calibrated TVOC concentration.
Another example may be found in a method of operating a Heating, Ventilating and Air Conditioning (HVAC) system including a CO₂ sensor and a Total Volatile Organic Compound (TVOC) sensor. The method includes monitoring CO₂ concentration values over a period of time in order to find a minimum CO₂ concentration value, each CO₂ concentration value having a corresponding time stamp, and monitoring TVOC concentration values over the same period of time, each TVOC concentration value having a corresponding time stamp. A minimum CO₂ concentration value over the period of time is found, along with the TVOC concentration value that corresponds to the time stamp for the minimum CO₂ concentration value. The TVOC concentration value corresponding to the time stamp for the minimum CO₂ concentration value is compared with a minimum TVOC threshold. When the TVOC concentration value corresponding to the time stamp for the minimum CO₂ concentration value exceeds an expected minimum TVOC concentration value by at least a threshold, a TVOC adjustment factor is determined. In some instances, the TVOC adjustment factor is applied during the next cycle. As an example, subsequent measured TVOC concentrations are adjusted down using the TVOC adjustment factor to provide a calibrated TVOC concentration. The method includes controlling operation of the HVAC system in response to the calibrated TVOC concentration.
The preceding summary is provided to facilitate an understanding of some of the innovative features unique to the present disclosure and is not intended to be a full description. A full appreciation of the disclosure can be gained by taking the entire specification, claims, figures, and abstract as a whole.
BRIEF DESCRIPTION OF THE FIGURES
The disclosure may be more completely understood in consideration of the following description of various examples in connection with the accompanying drawings, in which:
Figure 1 is a schematic block diagram of an illustrative building control system;
Figure 2 is a flow diagram showing an illustrative method for calibrating a TVOC sensor;
Figure 3 is a flow diagram showing an illustrative method for calibrating a TVOC sensor;
Figure 4 is a flow diagram showing an illustrative method for calibrating a TVOC sensor;
Figure 5 is a flow diagram showing an illustrative method for calibrating a TVOC sensor;
Figure 6 is a flow diagram showing an illustrative method for operating a HVAC (Heating, Ventilating and Air Conditioning) system;
Figure 7 is a flow diagram showing an illustrative method for calibrating a TVOC sensor when the TVOC sensor is drifting down;
Figure 8 is a graphical representation of example data showing adjusting the measured TVOC concentration when the TVOC sensor is drifting down;
Figure 9 is a flow diagram showing an illustrative method for calibrating a TVOC sensor when the TVOC sensor is drifting up; and
Figure 10 is a graphical representation of example data showing adjusting the measured TVOC concentration when the TVOC sensor is drifting up.
While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular examples described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure.

DESCRIPTION

The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict examples that are not intended to limit the scope of the disclosure. Although examples are illustrated for the various elements, those skilled in the art will recognize that many of the examples provided have suitable alternatives that may be utilized.
All numbers are herein assumed to be modified by the term “about” , unless the content clearly dictates otherwise. The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) .
As used in this specification and the appended claims, the singular forms “a” , “an” , and“the” include the plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
It is noted that references in the specification to “an embodiment” , “some embodiments” , “other embodiments” , etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is contemplated that the feature, structure, or characteristic is described in connection with an embodiment, it is contemplated that the feature, structure, or characteristic may be applied to other embodiments whether or not explicitly described unless clearly stated to the contrary.
Figure 1 is a schematic block diagram of an illustrative building control system 10. The illustrative building control system 10 includes a TVOC (Total Volatile Organic Compound) sensor 12 and a CO₂ (Carbon Dioxide) sensor 14. While shown as separate sensors, it will be appreciated that in some instances, the TVOC sensor 12 and the CO₂ sensor 14 may both be disposed within a single housing. Moreover, while a single TVOC sensor 12 and a single CO₂ sensor 14 are shown, it will be appreciated that the building control system 10 may include a large number of TVOC sensors 12 and CO₂ sensors 14. The TVOC sensor 12 and the CO₂ sensor 14 are each operably coupled with a controller 16. The controller 16 may receive signals from the TVOC sensor 12 that are representative of the total concentration of volatile organic compounds in the air. In some cases, the total concentration of volatile organic compounds (TVOC) is a group of VOCs used to represent the entire pool of pollutants. In many cases, TVOC is not uniformly defined in terms of which VOCs are included. Many standards list different VOCs in their TVOC grouping or assign different weights when calculating the TVOC. This disclosure is intended to cover TVOC sensors that include any grouping of two or more VOCs when reporting the TVOC concentration. The controller 16 may receive signals from the CO₂ sensor 14 that are representative of the concentration of carbon dioxide. The controller 16 includes or is operably coupled with a memory 18 that may be used for storing sensor data, for example. In some instances, the controller 16 may be configured to calibrate the TVOC sensor 12.
In some instances, the controller 16 may communicate with an HVAC (Heating, Ventilating and Air Conditioning) system 20. In some instances, the controller 16 itselfmay be considered as being part of the HVAC system 20. In some instances, the TVOC sensor 12 and/or the CO₂ sensor 14 may be considered as being part of the HVAC system 20. The illustrative HVAC system 20 includes a ventilation damper 22 that controls a flow of fresh outside air into a building supplied by the HVAC system 20. In some instances, the HVAC system 20 may adjust operation of various components of the HVAC system 20 in accordance with the TVOC concentration and/or the CO₂ concentration. In some instances, the HVAC system 20 may adjust operation of the ventilation damper 22 in order to change the amount of fresh outside air, in accordance with the TVOC concentration and/or the CO₂ concentration.
Figure 2 is a flow diagram showing an illustrative method 24 for calibrating a Total Volatile Organic Compounds (TVOC) sensor (such as the TVOC sensor 12) . The method 24 includes accepting a value for an expected minimum TVOC concentration, as indicated at block 26. In some instances, the expected minimum TVOC concentration may be user-selectable. A current TVOC concentration is measured, as indicated at block 28. The current measured TVOC concentration is compared to the expected minimum TVOC concentration, as indicated at block 30. When the current measured TVOC concentration is lower than the expected minimum TVOC concentration (e.g. because of sensor drift) by at least a threshold amount, a TVOC adjustment factor is determined based at least in part on an amount that the current measured TVOC concentration is lower than the expected minimum TVOC concentration, as indicated at block 32. In some instances, the threshold amount may be selected to provide a desired level of hysteresis to reduce rapid switching of the TVOC adjustment factor. In some instances, the threshold amount may be user-selectable.
In some instances, the TVOC adjustment factor may be determined when the current measured TVOC concentration remains lower than the expected minimum TVOC concentration by at least the threshold amount for at least a first period of time. In some instances, the first period of time may be user-adjustable. As an example, the first period of time may be at least two minutes. When the current measured TVOC concentration is not lower than the expected minimum TVOC concentration by at least the threshold amount (and in some cases for at least the first period of time) , subsequent measured TVOC concentrations are not adjusted. Subsequent measured TVOC concentrations are adjusted by the determined TVOC adjustment factor to provide a calibrated TVOC concentration, as indicated at block 34. In some instances, the method 24 may further include sending one or more command signals to a Heating, Ventilating and Air Conditioning (HVAC) system to change operation of the HVAC system in accordance with the calibrated TVOC concentration.
Figure 3 is a flow diagram showing an illustrative method 38 for calibrating a Total Volatile Organic Compounds (TVOC) sensor (such as the TVOC sensor 12) . The method 38 includes measuring a current TVOC concentration, as indicated at block 40. The current measured TVOC concentration is repeatedly compared to the expected minimum TVOC concentration, as indicated at block 42. In some instances, the current measured TVOC concentration is compared with the expected minimum TVOC concentration with each new current measured TVOC concentration, as indicated at block 42a. In some instances, the current measured TVOC concentration is compared with the expected minimum TVOC concentration only after a user-selectable time period, as indicated at block 42b.
In some instances, the method 38 includes determining a new TVOC adjustment factor when the current measured TVOC concentration is lower than the expected minimum TVOC concentration by at least the threshold amount, as indicated at block 44. In some instances, the threshold amount may be selected to add a desired level of hysteresis. In some instances, the threshold amount may be user-selectable. In some instances, subsequent measured TVOC concentrations may be adjusted by the new determined TVOC adjustment factor to provide a calibrated TVOC concentration, as indicated at block 46. In some instances, the method 38 may further include sending one or more command signals to a Heating, Ventilating and Air Conditioning (HVAC) system to change operation of the HVAC system in accordance with the calibrated TVOC concentration.
Figure 4 is a flow diagram showing an illustrative method 50 for calibrating a Total Volatile Organic Compounds (TVOC) sensor (such as the TVOC sensor 12) . The method 50 includes storing a plurality of CO₂ concentration values captured over a period of time, with each CO₂ concentration value captured at a corresponding time during the period of time, as indicated at block 52. A plurality of TVOC concentration values captured over the period of time are stored, with each TVOC concentration value captured at a corresponding time during the period of time, as indicated at block 54. A minimum CO₂ concentration value is identified from the plurality of CO₂ concentration values, as indicated at block 56. A time that the identified minimum CO₂ concentration value was captured is identified, as indicated at block 58. The TVOC concentration value that was captured at or nearest the time that the identified minimum CO₂ concentration value was captured is identified, as indicated at block 60. The TVOC concentration value that was captured nearest the time that the identified minimum CO₂ concentration value was captured is compared with an expected minimum TVOC concentration value, as indicated at block 62.
When the TVOC concentration value that was captured at or nearest the time that the identified minimum CO₂ concentration value was captured is above the expected minimum TVOC concentration value (noting a drift up in the TVOC sensor output) , several actions are taken, as indicated at block 64. The actions include determining a TVOC adjustment factor, as indicated at block 64a. The actions include adjusting subsequent measured TVOC concentration values using the TVOC adjustment factor to provide a calibrated TVOC concentration, as indicated at block 64a. In some instances, the method 50 may further include sending one or more command signals to a Heating, Ventilating and Air Conditioning (HVAC) system to change operation of the HVAC system in accordance with the calibrated TVOC concentration.
In some instances, the TVOC concentration value captured at or nearest the time that the identified minimum CO₂ concentration value was captured must be higher than the expected minimum TVOC concentration value by at least a threshold amount before determining the TVOC adjustment factor and adjusting subsequent measured TVOC concentration values using the TVOC adjustment factor to provide the calibrated TVOC concentration. In some instances, the threshold amount may be selected to provide a desired level of hysteresis. In some instances, the threshold amount may be user-selectable. In some instances, the TVOC concentration value captured at or nearest the time that the identified minimum CO₂ concentration value was captured must remain above the expected minimum TVOC concentration value by at least a threshold amount for at least a threshold period of time before determining the TVOC adjustment factor and adjusting subsequent measured TVOC concentration values using the TVOC adjustment factor to provide the calibrated TVOC concentration.
Figure 5 is a flow diagram showing an illustrative method 68 for calibrating a Total Volatile Organic Compounds (TVOC) sensor (such as the TVOC sensor 12) . The method 68 includes monitoring CO₂ concentration values over the period of time in order to identify the minimum CO₂ concentration value, where each CO₂ concentration value has a corresponding time stamp, as indicated at block 70. TVOC concentration values are monitored over the period of time, where each TVOC concentration value has a corresponding time stamp, as indicated at block 72. A TVOC concentration value that has a time stamp that is nearest the time stamp of the minimum CO₂ concentration value during the same period of time is found, as indicated at block 74.
The TVOC concentration value that has the time stamp that is nearest the time stamp of the minimum CO₂ concentration value during the period of time is compared with the expected minimum TVOC concentration value, as indicated at block 76. When the TVOC concentration value that has the time stamp that is nearest the time stamp of the minimum CO₂ concentration value during the period of time is higher than the expected minimum TVOC concentration value by at least a threshold amount, the method 68 includes determining the TVOC adjustment factor and adjusting subsequent measured TVOC concentration values using the TVOC adjustment factor to provide the calibrated TVOC concentration. In some instances, the period of time includes a rolling period of time that restarts on a periodic basis. As an example, the rolling period of time may include at least 24 hours.
Figure 6 is a flow diagram showing an illustrative method 80 of operating a Heating, Ventilating and Air Conditioning (HVAC) system (such as the HVAC system 20) including a CO₂ sensor (such as the CO₂ sensor 14) and a Total Volatile Organic Compound (TVOC) sensor (such as the TVOC sensor 12) . The method 80 includes monitoring CO₂ concentration values over a period of time in order to find a minimum CO₂ concentration value over that period of time, each CO₂ concentration value having a corresponding time stamp, as indicated at block 82. The method 80 includes monitoring TVOC concentration values over the same period of time, each TVOC concentration value having a corresponding time stamp, as indicated at block 84. A minimum CO₂ concentration value over the period of time is found, as indicated at block 86. The TVOC concentration value that corresponds to the time stamp for the minimum CO₂ concentration value is found, as indicated at block 88.
The TVOC concentration value corresponding to the time stamp for the minimum CO₂ concentration value is compared with a minimum TVOC threshold, as indicated at block 90. When the TVOC concentration value corresponding to the time stamp for the minimum CO₂ concentration value exceeds an expected minimum TVOC concentration value by at least a threshold (e.g. because of TVOC sensor drift up) , a TVOC adjustment factor is determined, as indicated at block 92. Subsequent measured TVOC concentrations are adjusted down using the TVOC adjustment factor to provide a calibrated TVOC concentration, as indicated at block 94. Operation of the HVAC system is controlled in response to the calibrated TVOC concentration, as indicated at block 96. In some instances, the HVAC system includes a ventilation damper, and controlling operation of the HVAC system in response to the calibrated TVOC concentration includes opening or closing the ventilation damper. As an example, the period of time may include at least one day.
Figure 7 is a flow diagram showing an illustrative method 98 for calibrating a TVOC sensor (such as the TVOC sensor 12) when the TVOC sensor is drifting downward, meaning that the TVOC sensor is measuring TVOC concentrations that are lower than a minimum expected TVOC concentration. The method 98 includes a step of measuring the TVOC concentration (represented as [TVOC] ) , as indicated at block 100. An adjustment factor is initially set equal to zero, and a calibrated TVOC concentration is equal to the measured TVOC concentration, as indicated at block 102. A lowest expected TVOC concentration is set, and a measured time is set, as indicated at block 104. The lowest expected TVOC concentration represents the lowest TVOC concentration that is expected to be in the building space. The measured time may be user-adjustable, and may be set equal to 2 minutes, for example.
The TVOC concentration is measured again, as indicated at block 106. At decision block 108, a determination is made as to whether the calibrated TVOC concentration has remained lower than the lowest expected TVOC concentration by a threshold amount for at least the measured time. Ifnot, control reverts to block 106. If so, control passes to block 110, and the adjustment factor is set equal to the lowest expected TVOC concentration minus the calibrated TVOC concentration. At block 112, the calibrated TVOC concentration is set equal to the measured TVOC concentration plus the adjustment factor. Control then reverts to block 106, and the process continues.
Figure 8 is a graphical representation 114 showing the results of the method 98. TVOC concentration (in parts per billion, or ppb) is labeled on the Y axis while time is labeled on the X axis. A plot line 116 shows the measured TVOC concentration, a plot line 118 shows a calibrated TVOC concentration, and a baseline 120 indicates the lowest expected TVOC concentration. In this example, the lowest expected TVOC concentration is set equal to 8 ppb. A point 122 shows where the measured TVOC concentration (shown by the plot line 116) has dropped below the baseline 120 (e.g. because of sensor drift down) . At that point, an adjustment factor is calculated and is added to the measured TVOC concentration to obtain the calibrated TVOC concentration, as indicated by the plot line 118. Going forward, it can be seen that the calibrated TVOC concentration (shown by the plot line 118) generally follows the measured TVOC concentration (shown by the plot line 116) , but is higher from the measured TVOC concentration by an amount equal to the adjustment factor. The adjustment factor will remain constant until the measured TVOC concentration (or the calibrated TVOC concentration) once again drifts below the baseline 120, sometimes by at least a threshold amount and for at least a threshold amount of time.
Figure 9 is a flow diagram showing an illustrative method 124 for calibrating a TVOC sensor (such as the TVOC sensor 12) when the TVOC sensor is drifting upward, meaning that the TVOC sensor is measuring TVOC concentrations that are consistently higher than a minimum expected TVOC concentration. The method 124 includes measuring the TVOC concentration (indicated at [TVOC] ) , as indicated at block 126. An adjustment factor is set equal to zero and a calibrated TVOC concentration is set equal to the measured TVOC concentration, as indicated at block 128. A minimum TVOC concentration is set, a measured time is set and a threshold value is set, as indicated at block 130. In some instances, the measured time and the threshold value may be user-selectable or user-definable.
The TVOC concentration is measured, as indicated at block 132. At decision block 134, a determination is made as to whether the elapsed time is greater than the measured time. If not, control reverts to block 132. If so, control passes to block 126 and the TVOC concentration corresponding to a minimum CO₂ concentration during the measured time period is found. In some instances, it is assumed that the TVOC concentration in the air will hit a minimum TVOC concentration at the same or roughly the same time as the CO₂ concentration bottoms out. At decision block 138, a determination is made as to whether the calibrated TVOC concentration is greater than the minimum TVOC concentration plus a threshold value. Ifnot, control reverts to block 144, where the timer is reset equal to zero. Otherwise, control passes to block 140, and an adjustment value is set equal to the calibrated TVOC concentration minus the minimum TVOC concentration. At block 142, the calibrated TVOC concentration is set equal to the measured TVOC concentration minus the adjustment value. Control then passes to block 144, and the process continues.
Figure 10 is a graphical representation 114 showing the results of the method 124. TVOC concentration (in parts per billion, or ppb) and CO₂ concentration (in parts per million, or ppm) are labeled on the Y axis while time is labeled on the X axis. A plot line 146 shows measured CO₂ concentration values and a plot line 148 shows measured TVOC concentration values. A time stamp 150 corresponds to a minimum CO₂ concentration over a period of time 149, and the TVOC concentration at that same time stamp 150 is found. At the time stamp 150, the TVOC concentration is found to be above the expected TVOC concentration indicated by the baseline 120. An adjustment factor is calculated, and a calibrated TVOC concentration, indicated by a plot line 152, can be seen generally following the measured TVOC concentration (as indicated by the plot line 148) , but is just lower than the measured TVOC concentration to account for the upward drift of the TVOC sensor.
Having thus described several illustrative embodiments of the present disclosure, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, arrangement of parts, and exclusion and order of steps, without exceeding the scope of the disclosure. The disclosure’s scope is, of course, defined in the language in which the appended claims are expressed.

Claims

A method for calibrating a Total Volatile Organic Compounds (TVOC) sensor, the method comprising:

accepting a value for an expected minimum TVOC concentration;

measuring a current TVOC concentration;

comparing the current measured TVOC concentration to the expected minimum TVOC concentration;

when the current measured TVOC concentration is lower than the expected minimum TVOC concentration by at least a threshold amount, determining a TVOC adjustment factor based at least in part on an amount that the current measured TVOC concentration is lower than the expected minimum TVOC concentration; and

adjusting subsequent measured TVOC concentrations by the determined TVOC adjustment factor to provide a calibrated TVOC concentration.
The method of claim 1, further comprising:

repeatedly comparing the current measured TVOC concentration to the expected minimum TVOC concentration;

determining a new TVOC adjustment factor when the current measured TVOC concentration is lower than the expected minimum TVOC concentration by at least the threshold amount; and

adjusting subsequent measured TVOC concentrations by the new determined TVOC adjustment factor to provide a calibrated TVOC concentration.
The method of claim 2, wherein repeatedly comparing the current measured TVOC concentration to the expected minimum TVOC concentration occurs with each new current measured TVOC concentration.
The method of claim 2, wherein repeatedly comparing the current measured TVOC concentration to the expected minimum TVOC concentration occurs after a user-selectable time period.
The method of claim 1, wherein the expected minimum TVOC concentration is user-selectable.
The method of claim 1, wherein when the current measured TVOC concentration is not lower than the expected minimum TVOC concentration by at least the threshold amount, not adjusting subsequent measured TVOC concentrations.
The method of claim 1, wherein the TVOC adjustment factor is determined when the current measured TVOC concentration remains lower than the expected minimum TVOC concentration by at least the threshold amount for at least a first period of time.
The method of claim 7, wherein the first period of time is user-adjustable.
The method of claim 7, wherein the first period of time is at least two minutes.
The method of claim 1, further comprising sending one or more command signals to a Heating, Ventilating and Air Conditioning (HVAC) system to change operation of the HVAC system in accordance with the calibrated TVOC concentration.
A method for calibrating a Total Volatile Organic Compounds (TVOC) sensor, the method comprising:

storing a plurality of CO₂ concentration values captured over a period of time, with each CO₂ concentration value captured at a corresponding time during the period of time;

storing a plurality of TVOC concentration values captured over the period of time, with each TVOC concentration value captured at a corresponding time during the period of time;

identifying a minimum CO₂ concentration value from the plurality of CO₂ concentration values;

identifying a time that the identified minimum CO₂ concentration value was captured;

identifying the TVOC concentration value that was captured nearest the time that the identified minimum CO₂ concentration value was captured;

comparing the TVOC concentration value that was captured nearest the time that the identified minimum CO₂ concentration value was captured with an expected minimum TVOC concentration value;

when the TVOC concentration value that was captured nearest the time that the identified minimum CO₂ concentration value was captured is above the expected minimum TVOC concentration value:

determining a TVOC adjustment factor; and

adjusting subsequent measured TVOC concentration values using the TVOC adjustment factor to provide a calibrated TVOC concentration.
The method of claim 11, further comprising:

monitoring CO₂ concentration values over the period of time in order to identify the minimum CO₂ concentration value, where each CO₂ concentration value has a corresponding time stamp;

monitoring TVOC concentration values over the period of time, where each TVOC concentration value has a corresponding time stamp;

finding a TVOC concentration value that has a time stamp that is nearest the time stamp of the minimum CO₂ concentration value during the same rolling period of time;

comparing the TVOC concentration value that has the time stamp that is nearest the time stamp of the minimum CO₂ concentration value during the period of time with the expected minimum TVOC concentration value; AND

when the TVOC concentration value that has the time stamp that is nearest the time stamp of the minimum CO₂ concentration value during the period of time is lower than the expected minimum TVOC concentration value by at least a threshold amount, determining the TVOC adjustment factor and adjusting subsequent measured TVOC concentration values using the TVOC adjustment factor to provide the calibrated TVOC concentration.
The method of claim 12, wherein the period of time comprises a rolling period of time that restarts on a periodic basis.
The method of claim 12, wherein the rolling period of time comprises at least 24 hours.
The method of claim 11, wherein the TVOC concentration value captured nearest the time that the identified minimum CO₂ concentration value was captured must be lower than the expected minimum TVOC concentration value by at least a threshold amount before determining the TVOC adjustment factor and adjusting subsequent measured TVOC concentration values using the TVOC adjustment factor to provide the calibrated TVOC concentration.
The method of claim 11, wherein the TVOC concentration value captured nearest the time that the identified minimum CO₂ concentration value was captured must remain lower than the expected minimum TVOC concentration value by at least a threshold amount for at least a threshold period of time before determining the TVOC adjustment factor and adjusting subsequent measured TVOC concentration values using the TVOC adjustment factor to provide the calibrated TVOC concentration.
The method of claim 11, further comprising sending one or more command signals to a Heating, Ventilating and Air Conditioning (HVAC) system to change operation of the HVAC system in accordance with the calibrated TVOC concentration.
A method of operating a Heating, Ventilating and Air Conditioning (HVAC) system including a CO₂ sensor and a Total Volatile Organic Compound (TVOC) sensor, the method comprising:

monitoring CO₂ concentration values over a period of time in order to find a minimum CO₂ concentration value, each CO₂ concentration value having a corresponding time stamp;

monitoring TVOC concentration values over the same period of time, each TVOC concentration value having a corresponding time stamp;

finding a minimum CO₂ concentration value over the period of time;

finding the TVOC concentration value that corresponds to the time stamp for the minimum CO₂ concentration value;

comparing the TVOC concentration value corresponding to the time stamp for the minimum CO₂ concentration value with a minimum TVOC threshold;

when the TVOC concentration value corresponding to the time stamp for the minimum CO₂ concentration value exceeds an expected minimum TVOC concentration value by at least a threshold, determining a TVOC adjustment factor;

adjusting subsequent measured TVOC concentrations down using the TVOC adjustment factor to provide a calibrated TVOC concentration; and

controlling operation of the HVAC system in response to the calibrated TVOC concentration.
The method of claim 18, wherein the HVAC system comprises a ventilation damper, and controlling operation of the HVAC system in response to the calibrated TVOC concentration comprises opening or closing the ventilation damper.
The method of claim 18, wherein the period of time comprises at least one day.