Measuring and testing – Instrument proving or calibrating – Gas or liquid analyzer
Reexamination Certificate
2000-12-29
2003-03-04
Williams, Hezron (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Gas or liquid analyzer
C702S104000
Reexamination Certificate
active
06526801
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to gas sensing equipment and calibration methods associated therewith. More particularly, the present invention relates to digitally-implemented methods for identifying and preventing “drift,” or unwanted and inaccurate changes in the readings, of the gas sensing equipment.
BACKGROUND OF THE INVENTION
Gas sensors are commonly used to measure the presence of one or more elements or components in a gas mixture. For example, a gas sensor may be used to measure the concentration of one or more pollutants in ambient air in an indoor or outdoor environment. Other examples may include flue gas monitoring, combustion processes control, fire alarms, gas leak detectors, and other applications where gas sensing is required. Such sensors are often connected to process control and monitoring equipment, or they may be connected to alarms that are triggered when concentrations of a measured gas exceed a predetermined limit, such as a desired limit based on worked protection guidelines or applicable regulations, or a health-based limit that is derived from research in the field.
A carbon dioxide (CO2) monitor, which measures the concentration of CO2 in ambient air, is one such gas sensor. Typically, CO2 monitors continuously measure CO2 concentrations in the subject environment and are connected to a ventilation control system to ensure appropriate ventilation rates. The nature of many environments, such as offices, stores, and industrial environments, results in a cyclical CO2 level in the environment at various points in the day. For example, an office area may experience its highest CO2 levels in the middle of the afternoon, when most CO2 sources (i.e., the office workers) are in the office and have been present for a period of time. Such an environment may experience its lowest CO2 levels in the wee hours of the morning, after most or all of the workers have departed from the previous day and before most or all the workers have arrived for the day.
Moderate natural background levels of CO2 exist in most environments. Thus, even when most or all CO2 sources are not present, as when all workers have departed from an office, a background level of CO2 typically remains. Thereafter, the background level may be considered to be a baseline against which measurements of concentrations in ambient air may be made.
Such background levels also exist in many environments for components other than CO2. For example, very low levels of carbon monoxide (close to 0.1 ppm) exist in most environments under normal conditions, and carbon monoxide alarms thus must be able to distinguish a measured concentration from almost zero levels of carbon monoxide.
One drawback of the prior art gas sensors is that such sensors typically experience a phenomenon known as “drift.” Drift is a slow change of the properties of the sensor components, resulting in a gradual reduction in the accuracy of the sensor. Drift may be caused by any number of external factors, such as gradual chemical changes, a build-up of foreign matter over time and aging of the sensor components. The inaccuracy of the sensor is normally associated with changes in two sensor calibration parameters: baseline (zero readings) and span (calibration point). For many types of sensors, only one type of inaccuracy may dominate. Thus, for example, for one type of sensor an inaccuracy may be caused by the sensor zero drift, while another type of sensor may be inaccurate because of span drift. For example, it is well known that non-dispersive infrared (NDIR) gas sensors often primarily exhibit zero drift, while photo-acoustic infrared gas sensors typically have span drift as a dominating factor of inaccuracy.
A typical method for reducing or eliminating drift in a gas sensor is to either construct it to have high accuracy or to manually recalibrate the sensor on a periodic basis. Gas sensor re-calibration normally requires the use of at least one calibration gas mixture. Either option leads to increased costs, and both methods can be unreliable due to equipment failure and/or human error. Moreover, many applications require unattended sensor operation for very long periods of time, and thus frequent re-calibration is not desirable.
One method of compensating for drift of a gas sensor is disclosed in U.S. Pat. No. 5,347,474, to Wong. In particular, columns 3-6 of Wong, incorporated herein by reference, disclose a method of calibrating a carbon monoxide sensor. However, we have determined that alternate methods are desirable and preferable.
Therefore, we have determined that it is desirable to provide a method of automatically compensating for drift in gas sensors.
SUMMARY OF THE INVENTION
It is therefore a feature and advantage of the present invention to provide a method of automatically compensating for baseline and/or span drift in gas sensors.
One embodiment of the present invention is a method that includes providing a processor, such as an embedded microprocessor, with gas concentration data relating to a first period of time. The method includes identifying a quiescent period, which a subset of the first period of time. The gas concentration data that relates to the first period of time includes quiescent period data relating to the quiescent period. The method also includes determining a first component concentration, as well as providing the processor with at least one additional component concentration. Each additional component concentration corresponds to a separate and distinct time period. The first component concentration and the additional component concentrations provide an initial concentration set. The method also includes selecting, by the processor, at least one valid concentration from the initial concentration set. The selected valid concentrations yield a valid concentration set. The method also includes providing the processor with a preset background value, calculating, by the processor, an estimated background value, and calculating, by the processor, a correction value. The correction value in one embodiment may equal the difference between the preset background value and the estimated background value. Alternately, the correction value may equal the ratio of the preset background value to the estimated background value.
The method also includes the step of detecting a measured component concentration and adjusting the measured component concentration, the adjustment corresponding to the correction value, to yield an adjusted component concentration.
Optionally, the quiescent period has duration, and the duration is equal to or greater than a minimum duration, and the estimated background value relates to the quiescent period data, such as an average of some or all of such data.
As an alternative option, the quiescent period has a duration, the duration is less than a minimum duration, and the first component concentration is omitted from the valid concentration set.
As an additional option, it may occur that the first component concentration does not correspond to a predetermined value, in which case the first component concentration may be omitted from the valid concentration set. In this option, it is a further option that the predetermined value may relate to one or more of the additional component concentrations.
As additional options, the estimated background concentration may correspond to the valid concentrations in the valid concentration set, such as an average of some or all of such concentrations. Further, the method may include the additional step of triggering an event or selecting a restriction if the correction value exceeds a trigger value.
In accordance with an additional embodiment of the present invention, a system for compensating for gas sensor drift includes a processor and a memory for storing data to be processed by the processor. The processor is programmed to perform steps including accepting gas concentration data relating to a first time period. The steps also include identifying a quiescent period, where the quiescent period is a subset of the
Higgins Don Q.
Kouznetsov Andrian
Maddux Brian C.
Nelson Audrey
Nisbet Jean A.
Baker & Hostetler LLP
Edwards Systems Technology, Inc.
Garber Charles D
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