Particle counter
Inventors
Matsuda, Tomonobu • SHIMMURA, Masaki • SAITOU, Mitsuaki • Yamakawa, Yuki
Assignees
Publication Number
US-10705010-B2
Publication Date
2020-07-07
Expiration Date
2039-03-01
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Abstract
A particle counter includes a detector that receives, using a light receiving element, interference light between scattered light and reference light, generates a detection signal corresponding to the interference light, and amplifies the detection signal using an amplifier; a counting unit that performs counting of the particle, based on the detection signal in a measurement period for measuring particle; and an optical path length variable unit that causes the optical path length of at least one of a first optical path and a second optical path to be changed at a predetermined rate, wherein the predetermined rate is set based on a flow velocity of the fluid so as to slow a change in a phase difference between the scattered light and the reference light and to make the frequency of the detection signal lower by changing the optical path length.
Core Innovation
The invention relates to a particle counter that measures particles in a fluid, such as liquids or gases, by detecting interference light created between scattered light from particles and reference light. The particle counter includes a detector that receives this interference light using a light receiving element, generates a detection signal corresponding to the interference light, and amplifies this detection signal. It also includes a counting unit that counts particles based on the detection signal during a measurement period. Furthermore, the particle counter incorporates an optical path length variable unit that changes the optical path length of at least one of the first optical path (associated with the first light and scattered light) and the second optical path (associated with the reference light) at a predetermined rate.
The predetermined rate of changing the optical path length is set based on the flow velocity of the fluid to slow down the change in phase difference between the scattered light and the reference light. This adjustment reduces the frequency of the detection signal by altering the optical path length, enabling more accurate particle counting.
The problem being solved is that when the flow velocity of the sample fluid is high, the frequency of the detection signal due to the interference light becomes high, resulting in a decreased amplitude level of the detection signal because of the frequency characteristics of the semiconductor light receiving element and the amplifier. This amplitude decrease can cause failure to accurately count particles. The invention addresses this by dynamically adjusting the optical path length to lower the frequency of the detection signal, thus maintaining amplitude and enabling accurate counting even at higher flow velocities.
Claims Coverage
The patent includes one independent claim which covers a particle counter with several inventive features.
Particle counter with optical path length variable unit for frequency reduction
A particle counter that includes: a light source emitting light; a light superimposing unit that spatially superimposes two lights; an irradiation optical system providing a detection area by irradiating fluid with a first branched light; a detection optical system causing scattered light different from the irradiation optical axis to enter the light superimposing unit; a reference optical system causing second branched light to enter as reference light; a detector that receives interference light between scattered and reference light, generating and amplifying a detection signal; a counting unit counting particles based on the detection signal during a measurement period; and an optical path length variable unit that changes the optical path length of at least one of a first or second optical path at a predetermined rate set based on flow velocity of the fluid. This rate slows the phase difference change between scattered and reference light to lower the frequency of the detection signal by changing the optical path length.
Optical path length variable unit with movable reflective surface and slider
The optical path length variable unit includes a fixed reflective surface arranged in the first or second optical path, and a slider moving a movable reflective surface during the measurement period to change the optical path length at the predetermined rate.
Reciprocation operation of movable reflective surface and measurement period synchronization
The slider causes the movable reflective surface to reciprocate by moving it from a reference position during the measurement period and returning it during a non-measurement period. The counting unit performs particle counting only during the measurement period and not during the non-measurement period.
Predetermined rate setting based on frequency characteristics of components
The predetermined rate of changing the optical path length is set further based on the frequency characteristic of the light receiving element and the amplifier, ensuring the detection signal frequency remains within acceptable response and amplification limits.
These inventive features collectively provide a particle counter that controls the optical path length dynamically to reduce the detection signal frequency based on the fluid flow velocity, ensuring accurate particle counting by mitigating amplitude reduction associated with high-frequency signals and leveraging movable reflective surfaces operated by a slider synchronized to measurement periods.
Stated Advantages
Enables accurate particle counting even at relatively high flow velocities by reducing the detection signal frequency and preventing amplitude level decreases due to frequency characteristics of the light receiving element and amplifier.
Improves signal-to-noise ratio by filtering and differential detection of interference light components.
Allows adjustment of the optical path length dynamically during measurement period enabling compatibility with various flow velocities and fluid refractive indices.
Enhances detection sensitivity to smaller particles through interference detection, amplifying scattered light intensity effectively compared to direct scattered light detection.
Documented Applications
Measuring particles suspended in fluids, including liquids such as chemical solutions and water, and gases such as air.
Particle counting in both liquid-borne and airborne environments with the fluid flowing through a flow passage in a flow cell.
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