Patent Description:
Large facilities (e.g., buildings), such as commercial facilities, office buildings, hospitals, and the like, may have a fire alarm system that can be triggered during an emergency situation (e.g., a fire) to warn occupants to evacuate. For example, a fire alarm system may include a fire control panel and a plurality of fire sensing devices (e.g., smoke detectors), located throughout the facility (e.g., on different floors and/or in different rooms of the facility) that can sense a fire occurring in the facility and provide a notification of the fire to the occupants of the facility via alarms. Fire sensing devices can include one or more sensors. The one or more sensors can include an optical smoke sensor, a heat sensor, a gas sensor, and/or a flame sensor, for example.

Over time components of a fire sensing device can degrade and/or become contaminated and fall out of their initial operational specifications. For example, an output of a light-emitting diode (LED) used in an optical scatter chamber of a smoke detector can degrade with age and/or use. These degraded components can prevent the fire sensing device from detecting a fire at an early enough stage. As such, codes of practice require sensitivity testing (e.g., alarm threshold verification testing) of smoke detectors at regular intervals. However, accurate sensitivity testing on site can be impractical due to access problems and the need to deploy specialist equipment to carry out the testing. Consequently, rudimentary functionality tests are almost always done in lieu of accurate sensitivity tests which are misleading by inaccurately depicting the sensitivity of a smoke detector as being verified.

In some countries, because an accurate sensitivity of the smoke detector may not be able to be determined and/or testing is not performed, devices are required to be replaced after a particular time period. For example, in Germany, even the most advanced smoke detector must be replaced after <NUM> years, even though the device may still be performing accurately. This can create unnecessary waste which can negatively impact the environment.

<CIT> discloses a method for the automatic calibration of a smoke detector comprising: mounting the smoke detector in a channel with an aerosol flow, along with a reference smoke detector; calibrating the smoke detector with data received by the reference detector. The reference detector comprises a scattered light receiver and a scattered light transmitter defining a scattered light plane. The aerosol flow through the channel flows through the reference detector transversely to the scattered light plane.

Devices, methods, and systems for a self-calibrating optical smoke chamber, within a fire sensing device are described herein. One device includes an adjustable particle generator and a variable airflow generator configured to generate aerosol having a particular particle size and optical scatter properties at a controllable density level, a first transmitter light-emitting diode (LED) configured to emit a first light that passes through the aerosol, a second transmitter LED configured to emit a second light that passes through the aerosol, a photodiode configured to detect a scatter level of the first light that passes through the aerosol and detect a scatter level of the second light that passes through the aerosol, and a controller configured to calibrate a gain of the photodiode based on the detected scatter level of the first light, the detected scatter level of the second light, and the controllable aerosol density level.

In contrast to previous smoke detectors in which a maintenance engineer would have to manually test sensitivity of a smoke detector and replace the smoke detector if the smoke sensitivity was incorrect, the smoke detectors in accordance with the present disclosure can test, calibrate, and/or recalibrate themselves. Accordingly, fire sensing devices in accordance with the present disclosure may take significantly less maintenance time to test and can be tested, calibrated, and/or recalibrated continuously and/or on demand, and can more accurately determine the ability of a fire sensing device to detect an actual fire. As such, self-calibrating fire sensing devices may have extended service lives and be replaced less often resulting in a positive environmental impact.

These embodiments are described in sufficient detail to enable those of ordinary skill in the art to practice one or more embodiments of this disclosure. It is to be understood that other embodiments may be utilized and that mechanical, electrical, and/or process changes may be made without departing from the scope of the present disclosure.

As will be appreciated, elements shown in the various embodiments herein can be added, exchanged, combined, and/or eliminated so as to provide a number of additional embodiments of the present disclosure. The proportion and the relative scale of the elements provided in the figures are intended to illustrate the embodiments of the present disclosure and should not be taken in a limiting sense.

As used herein, "a", "an", or "a number of" something can refer to one or more such things, while "a plurality of" something can refer to more than one such things. For example, "a number of components" can refer to one or more components, while "a plurality of components" can refer to more than one component.

<FIG> illustrates a block diagram of a smoke verification test function of a self-calibrating fire sensing device <NUM> in accordance with an embodiment of the present disclosure. The fire sensing device <NUM> includes a controller <NUM>, an adjustable particle generator <NUM>, an optical scatter chamber <NUM>, and a variable airflow generator <NUM>.

The controller <NUM> can include a memory <NUM>, a processor <NUM>, and circuitry <NUM>. Memory <NUM> can be any type of storage medium that can be accessed by processor <NUM> to perform various examples of the present disclosure. For example, memory <NUM> can be a non-transitory computer readable medium having computer readable instructions (e.g., computer program instructions) stored thereon that are executable by processor <NUM> to test, calibrate, and/or recalibrate a fire sensing device <NUM> in accordance with the present disclosure. For instance, processor <NUM> can execute the executable instructions stored in memory <NUM> to generate aerosol having a controllable density level, emit a first light that passes through the aerosol, emit a second light that passes through the aerosol, detect a scatter level of the first light that passes through the aerosol, detect a scatter level of the second light that passes through the aerosol, and calibrate a gain of a photodiode based on the detected scatter level of the first light, the detected scatter level of the second light, and the controllable aerosol density level. In some examples, memory <NUM> can store the detected scatter level of the first light and/or the detected scatter level of the second light.

In a number of embodiments, the controller <NUM> can send a command to the adjustable particle generator <NUM> and the variable airflow generator <NUM> to generate particles. A particle size of the particles can be well defined and repeatable by the adjustable particle generator <NUM> to have scatter properties at different wavelengths and/or different angles be the same and/or within a particular threshold. The particles can be drawn through the optical scatter chamber <NUM> via the variable airflow generator <NUM> creating a controlled and known aerosol density level. The optical scatter chamber <NUM> can include transmitter LEDs <NUM>-<NUM> and <NUM>-<NUM> and photodiodes <NUM>-<NUM> and <NUM>-<NUM> to measure the aerosol density level by detecting scatter levels. Scatter can be light from the transmitter LEDs <NUM>-<NUM> and/or <NUM>-<NUM> reflecting, refracting, and/or diffracting off of particles and can be received by the photodiodes <NUM>-<NUM> and/or <NUM>-<NUM>. The amount of light received by the photodiodes <NUM>-<NUM> and/or <NUM>-<NUM> can be used to determine the aerosol density level.

Transmitter LED <NUM>-<NUM> can emit a first light and transmitter LED <NUM>-<NUM> can emit a second light. Photodiode <NUM>-<NUM> can detect a scatter level of the first light and/or the second light and photodiode <NUM>-<NUM> can detect a scatter level of the first light and/or the second light.

Transmitter LEDs <NUM>-<NUM> and <NUM>-<NUM>, which may be referred to herein collectively as transmitter LEDs <NUM>, can have varying LED emission levels due to, for example, manufacturing variations. As such, transmitter LEDs <NUM> may require calibration prior to use. The fire sensing device <NUM> calibrates the transmitter LEDs <NUM> by producing a known aerosol density level, as described above. The photodiodes <NUM>-<NUM> and <NUM>-<NUM>, which may be referred to herein collectively as photodiodes <NUM>, detect scatter levels and the controller <NUM> can compare the detected scatter levels with the known aerosol density level to calculate a sensitivity for each scatter path. For example, transmitter LED <NUM>-<NUM> can emit a first light and photodiode <NUM>-<NUM> and/or photodiode <NUM>-<NUM> can detect the scatter level from the first light scattering off of the particles of the known aerosol density level. The controller <NUM> can calculate a sensitivity, based on the detected scatter level and the known aerosol density level, for the scatter path of transmitter LED <NUM>-<NUM> to photodiode <NUM>-<NUM> and/or the scatter path of transmitter LED <NUM>-<NUM> to photodiode <NUM>-<NUM>. The controller <NUM> can similarly calculate a sensitivity for the scatter path of transmitter LED <NUM>-<NUM> to photodiode <NUM>-<NUM> and/or the scatter path of transmitter LED <NUM>-<NUM> to photodiode <NUM>-<NUM>. The sensitivity for each scatter path can be stored in memory <NUM>.

In some examples, the sensitivity accuracy can be improved by recalibrating a gain used to amplify the input signal of a photodiode <NUM>. For example, an amplifier gain can be increased to increase the voltage and/or current of the input signal of photodiode <NUM>-<NUM> to detect the first light from transmitter LED <NUM>-<NUM> as the first light from transmitter LED <NUM>-<NUM> weakens over time. A gain of the amplifier can be recalibrated (e.g., modified) responsive to the detected scatter level. For example, a gain of the amplifier can be recalibrated responsive to a calculated sensitivity of a scatter path being less than a threshold sensitivity.

In a number of embodiments, a fault (e.g., an error) can be triggered responsive to the detected scatter level. For example, the controller <NUM> can compare the detected scatter level to a threshold scatter level and trigger a fault responsive to the detected scatter level being below the threshold scatter level. Another example can include the controller <NUM> comparing the detected scatter level to a previously detected scatter level and triggering a fault responsive to the detected scatter level being less than the previously detected scatter level.

Each amplifier gain can be calibrated by storing the initial detected scatter level and each amplifier gain in memory <NUM>. Over time LED emission levels of transmitter LEDs <NUM> can decrease, reducing the received light by the photodiode <NUM>, which could lead to the fire sensing device <NUM> malfunctioning.

The amplifier gain used by photodiode <NUM> for detecting scatter levels can be recalibrated as the transmitter LED degrades over time. Controller <NUM> can recalibrate the gain responsive to the detected scatter level. For example, the controller <NUM> can initiate a recalibration of the gain responsive to comparing the detected scatter level to a threshold scatter level and determining the detected scatter level is below the threshold scatter level. In some examples, the controller <NUM> can recalibrate the gain responsive to determining a difference between the detected scatter level and the initial detected scatter level is greater than a threshold value and/or responsive to determining the detected scatter level is less than a previously detected scatter level.

In a number of embodiments, the first sensing device <NUM> can further include a sensor, not illustrated. The sensor can measure ambient airflow outside of the fire sensing device <NUM>. The ambient airflow can be measured prior to the adjustable particle generator <NUM> and the variable airflow generator <NUM> generating the aerosol. If the measured ambient airflow is within a particular airflow range, the adjustable particle generator <NUM> and the variable airflow generator <NUM> can generate the aerosol.

In some examples, the fire sensing device <NUM> can communicate with a heating, ventilation, and air conditioning (HVAC) system, not illustrated, via a wired or wireless network. The wired or wireless network will be further discussed in connection with <FIG>. The HVAC system can send the current settings of the HVAC system to the fire sensing device <NUM>. The fire sensing device <NUM> including the controller <NUM> can receive the settings from the HVAC system and generate aerosol and/or recalibrate a gain based on the settings of the HVAC system. For example, the fire sensing device can generate aerosol and/or recalibrate the gain in response to the HVAC system being turned off.

<FIG> illustrates an example of a self-calibrating fire sensing device <NUM> in accordance with an embodiment of the present disclosure. The fire sensing device <NUM> can be, but is not limited to, a fire and/or smoke detector of a fire control system, and can be, for instance, fire sensing device <NUM> previously described in connection with <FIG>. The self-calibrating fire sensing device <NUM> illustrated in <FIG> can include an optical scatter chamber <NUM> including a single transmitter LED <NUM> with dual wavelengths and a single photodiode <NUM>, which can correspond to the optical scatter chamber <NUM>, the transmitter LED <NUM>, and the photodiode <NUM> of <FIG>, respectively.

A fire sensing device <NUM> can sense a fire occurring in a facility and trigger a fire response to provide a notification of the fire to occupants of the facility. A fire response can include visual and/or audio alarms, for example. A fire response can also notify emergency services (e.g., fire departments, police departments, etc.) In some examples, a plurality of fire sensing devices can be located throughout a facility (e.g., on different floors and/or in different rooms of the facility).

A fire sensing device <NUM> can automatically or upon command conduct one or more tests contained within the fire sensing device <NUM>. The one or more tests can determine whether the fire sensing device <NUM> is functioning properly, requires maintenance, and/or requires recalibration.

As previously discussed in connection with <FIG>, a fire sensing device <NUM> further includes an adjustable particle generator and a variable airflow generator, which can correspond to the adjustable particle generator <NUM> and the variable airflow generator <NUM> of <FIG>, respectively. The adjustable particle generator of the fire sensing device <NUM> can generate particles which can be mixed into a controlled aerosol density level by the variable airflow generator. The aerosol density level can be a particular level that can be detected by an optical scatter chamber <NUM>. Once the aerosol density level has reached the particular level, the adjustable particle generator can be turned off and the variable airflow generator can increase the rate of airflow through the optical scatter chamber <NUM>. The variable airflow generator can increase the rate of airflow through the optical scatter chamber <NUM> to reduce the aerosol density level back to an initial level of the optical scatter chamber <NUM> prior to the adjustable particle generator generating particles. For example, the variable airflow generator can remove the aerosol from the optical scatter chamber <NUM> after the scatter levels described herein are detected.

The adjustable particle generator can include a reservoir to contain a liquid and/or wax used to create particles. The adjustable particle generator can also include a heat source. The heat source can be a coil of resistance wire. A current flowing through the wire can be used to control the temperature of the heat source and further control the number of particles produced by the adjustable particle generator. The heat source can heat the liquid and/or wax to create airborne particles to simulate smoke from a fire. The particles can measure approximately <NUM> micrometer in diameter and/or the particles can be within the sensitivity range of the optical scatter chamber <NUM>. The heat source can heat the liquid and/or wax to a particular temperature and/or heat the liquid and/or wax for a particular period of time to generate an aerosol density level sufficient to trigger a fire response from a properly functioning fire sensing device <NUM> without saturating the optical scatter chamber <NUM>. The ability to control the aerosol density level can allow a smoke test to more accurately mimic the characteristics of a fire and prevent the optical scatter chamber <NUM> from becoming saturated.

As previously described, the detected scatter levels from the smoke test can be used to determine whether fire sensing device <NUM> requires maintenance and/or recalibration. For example, the fire sensing device <NUM> can be determined to require maintenance and/or recalibration responsive to a calculated sensitivity, calculated using the detected scatter level and the known aerosol density level, being outside a sensitivity range.

In some examples, the fire sensing device <NUM> can generate a message if the device requires maintenance (e.g., if the sensitivity is outside a sensitivity range). The fire sensing device <NUM> can send the message to a monitoring device (e.g., monitoring device <NUM> in Figure <NUM>), for example. As an additional example, the fire sensing device <NUM> can include a user interface that can display the message.

The fire sensing device <NUM> of <FIG> illustrates transmitter LED <NUM> and photodiode <NUM>. Transmitter LED <NUM> can emit a first light and a second light. In some examples, the first light can have a first wavelength and the second light can have a second wavelength. For example, transmitter LED <NUM> can include an infrared (IR) LED with a first wavelength and a blue LED with a second wavelength. Having two or more different wavelengths can help the fire sensing device <NUM> detect various types of smoke. For example, a first wavelength can better detect a flaming fire including black aerosol and a second wavelength can better detect water vapor including white non-fire aerosol. In some examples, a ratio of the first wavelength and the second wavelength can be used to indicate the type of smoke.

As shown in <FIG>, photodiode <NUM> can receive a scatter of the first light and/or the second light from transmitter LED <NUM>. Photodiode <NUM> can detect a scatter level of the first light and/or a scatter level of the second light. In a number of embodiments, photodiode <NUM> can be a transmitter LED.

Transmitter LEDs <NUM>, can have varying LED emission levels due to, for example, manufacturing variations. As such, transmitter LEDs <NUM> may require calibration prior to use. The fire sensing device <NUM> can calibrate the transmitter LED <NUM> by producing a known aerosol density level, as described above. The photodiode <NUM> can detect scatter levels, which can be compared with the known aerosol density level to calculate a sensitivity for each scatter path.

In some examples, the sensitivity accuracy can be improved by modifying a gain used to amplify the input signal of photodiode <NUM>, as previously described herein. A gain of photodiode <NUM> can be recalibrated responsive to the detected scatter level, as previously described herein.

<FIG> illustrates an example of a self-calibrating fire sensing device <NUM> in accordance with an embodiment of the present disclosure. The fire sensing device <NUM> of <FIG> can include an optical smoke chamber <NUM> including a transmitter LED <NUM>-<NUM>, a transmitter LED <NUM>-<NUM>, and a photodiode <NUM>. Fire sensing device <NUM>, optical smoke chamber <NUM>, transmitter LED <NUM>-<NUM>, transmitter LED <NUM>-<NUM>, and photodiode <NUM> can correspond to fire sensing device <NUM>, optical scatter chamber <NUM>, transmitter LED <NUM>, and photodiode <NUM> of <FIG>, respectively.

As previously discussed in connection with <FIG>, a fire sensing device <NUM> can sense a fire occurring in a facility and can automatically or upon command conduct one or more tests contained within the fire sensing device <NUM> to determine whether the fire sensing device <NUM> is functioning properly, requires maintenance, and/or requires recalibration.

As previously discussed in connection with <FIG>, a fire sensing device <NUM> further includes an adjustable particle generator and a variable airflow generator, which can correspond to the adjustable particle generator <NUM> and the variable airflow generator <NUM> of <FIG>, respectively. The adjustable particle generator of the fire sensing device <NUM> can generate particles which can be mixed into a controlled aerosol density level by the variable airflow generator. The aerosol density level can be a particular level that can be detected by an optical scatter chamber <NUM>.

As previously described, detected scatter levels can be used to determine whether fire sensing device <NUM> requires maintenance and/or recalibration. For example, the fire sensing device <NUM> can be determined to require maintenance and/or recalibration responsive to a sensitivity, calculated using the detected scatter level and the known aerosol density level, being below a threshold sensitivity.

In some examples, the fire sensing device <NUM> can generate a message if the device requires maintenance (e.g., if the sensitivity is below a threshold sensitivity). The fire sensing device <NUM> can send the message to a monitoring device (e.g., monitoring device <NUM> in Figure <NUM>), for example. As an additional example, the fire sensing device <NUM> can include a user interface that can display the message.

The fire sensing device <NUM> of <FIG> illustrates transmitter LED <NUM>-<NUM>, transmitter LED <NUM>-<NUM>, and photodiode <NUM>. Transmitter LED <NUM>-<NUM> can emit a first light and transmitter LED <NUM>-<NUM> can emit a second light. Transmitter LED <NUM>-<NUM> and/or transmitter LED <NUM>-<NUM> can be located at particular angles from photodiode <NUM> to detect various types of smoke. For example, transmitter LED <NUM>-<NUM> can be located approximately <NUM> degrees from photodiode <NUM> and/or transmitter LED <NUM>-<NUM> can be located approximately <NUM> degrees from photodiode <NUM>.

As shown in <FIG>, photodiode <NUM> can receive the first light from transmitter LED <NUM>-<NUM> and/or the second light from transmitter LED <NUM>-<NUM>. Photodiode <NUM> can detect a scatter level of the first light and/or a scatter level of the second light.

Transmitter LEDs <NUM>, can have varying LED emission levels due to, for example, manufacturing variations. As such, transmitter LEDs <NUM> may require calibration prior to use. The fire sensing device <NUM> can calibrate the transmitter LED <NUM>-<NUM> and/or transmitter LED <NUM>-<NUM> by producing a known aerosol density level, as described above. The photodiode <NUM> can detect scatter levels, which can be compared with the known aerosol density level to calculate a sensitivity for each scatter path, as previously described herein. In some examples, the sensitivity accuracy can be improved by modifying a gain used to amplify the input signal of photodiode <NUM> responsive to one or more detected scatter levels.

<FIG> illustrates a block diagram of a system <NUM> including a self-calibrating fire sensing device <NUM> in accordance with an embodiment of the present disclosure. Fire sensing device <NUM> can be, for example, fire sensing device <NUM> and/or <NUM> previously described in connection with <FIG>, <FIG>, and <FIG>, respectively. The system <NUM> can further include a monitoring device <NUM>.

The monitoring device <NUM> can be a control panel, a fire detection control system, and/or a cloud computing device of a fire alarm system, for example. The monitoring device <NUM> can be configured to send commands to and/or receive test, calibration, and/or recalibration results from a fire sensing device <NUM> via a wired or wireless network. For example, the fire sensing device <NUM> can transmit (e.g., send) the monitoring device <NUM> a message responsive to the fire sensing device <NUM> determining that the fire sensing device <NUM> requires maintenance and/or requires recalibration. The fire sensing device <NUM> can also transmit a message responsive to calibrating the fire sensing device <NUM>, failing to calibrate the fire sensing device <NUM>, recalibrating the fire sensing device <NUM>, failing to recalibrate the fire sensing device <NUM>, detecting a scatter level at the fire sensing device <NUM>, and/or failing to detect a scatter level at the fire sensing device <NUM>.

In a number of embodiments, the fire sensing device <NUM> can transmit data to the monitoring device <NUM>. For example, the fire sensing device <NUM> can transmit detected scatter levels. In some examples, the monitoring device <NUM> can receive messages and/or data from a number of fire sensing devices analogous to fire sensing device <NUM>.

The monitoring device <NUM> can include a controller <NUM> including a memory <NUM>, a processor <NUM>, and a user interface <NUM>. Memory <NUM> can be any type of storage medium that can be accessed by processor <NUM> to perform various examples of the present disclosure. For example, memory <NUM> can be a non-transitory computer readable medium having computer readable instructions (e.g., computer program instructions) stored thereon that are executable by processor <NUM> in accordance with the present disclosure. For instance, processor <NUM> can execute the executable instructions stored in memory <NUM> to generate aerosol, emit a first light that passes through the aerosol, emit a second light that passes through the aerosol, detect a scatter level of the first light that passes through the aerosol, detect a scatter level of the second light that passes through the aerosol, and recalibrate a gain of the photodiode based on the detected scatter level of the first light or the detected scatter level of the second light. In some examples, memory <NUM> can store previously detected scatter levels, the detected scatter levels, and/or scatter specification ranges.

In a number of embodiments, the controller <NUM> can send a command to the fire sensing device <NUM> to recalibrate a gain of a photodiode (e.g., photodiode <NUM> in <FIG> and <FIG>) of the fire sensing device <NUM>. In some examples, the command can include a gain setting for the photodiode. The controller <NUM> can determine a gain setting based on the detected scatter level received from the fire sensing device <NUM>. The controller <NUM> can compare the detected scatter level with a scatter level range, previously detected scatter levels, and/or detected scatter levels of a different fire sensing device. The fire sensing device <NUM> can recalibrate the gain of the photodiode based on the comparison.

In a number of embodiments, the monitoring device <NUM> can include a user interface <NUM>. The user interface <NUM> can be a GUI that can provide and/or receive information to and/or from a user and/or the fire sensing device <NUM>. The user interface <NUM> can display messages and/or data received from the fire sensing device <NUM>. For example, the user interface <NUM> can display an error notification responsive to a detected scatter level being outside of a scatter specification range.

The networks described herein can be a network relationship through which the fire sensing device <NUM>, the monitoring device <NUM>, a sensor, and/or an HVAC system communicate with each other. Examples of such a network relationship can include a distributed computing environment (e.g., a cloud computing environment), a wide area network (WAN) such as the Internet, a local area network (LAN), a personal area network (PAN), a campus area network (CAN), or metropolitan area network (MAN), among other types of network relationships. For instance, the network can include a number of servers that receive information from and transmit information to fire sensing device <NUM> and monitoring device <NUM>, via a wired or wireless network.

As used herein, a "network" can provide a communication system that directly or indirectly links two or more computers and/or peripheral devices and allows a monitoring device <NUM> to access data and/or resources on a fire sensing device <NUM> and vice versa. A network can allow users to share resources on their own systems with other network users and to access information on centrally located systems or on systems that are located at remote locations. For example, a network can tie a number of computing devices together to form a distributed control network (e.g., cloud).

A network may provide connections to the Internet and/or to the networks of other entities (e.g., organizations, institutions, etc.). Users may interact with network-enabled software applications to make a network request, such as to get data. Applications may also communicate with network management software, which can interact with network hardware to transmit information between devices on the network.

In the foregoing Detailed Description, various features are grouped together in example embodiments illustrated in the figures for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the embodiments of the disclosure require more features than are expressly recited in each claim.

Claim 1:
A self-calibrating fire sensing device (<NUM>, <NUM>, <NUM>), comprising:
an adjustable particle generator (<NUM>) and a variable airflow generator (<NUM>) configured to generate aerosol at a controllable density level within the self-calibrating fire sensing device (<NUM>, <NUM>, <NUM>);
a transmitter light-emitting diode, LED, (<NUM>-<NUM>, <NUM>, <NUM>-<NUM>) configured to emit a light that passes through the aerosol within the self-calibrating fire sensing device (<NUM>, <NUM>, <NUM>);
a photodiode (<NUM>-<NUM>, <NUM>) configured to detect a scatter level of the light that passes through the aerosol; and
a controller (<NUM>) configured to calibrate a gain of the photodiode (<NUM>-<NUM>, <NUM>) based on the detected scatter level of the light and the controllable density level of the aerosol.