Operating Principle

SprayWatch uses a high-quality, fast shutter CCD camera to create digital images of the spray. This means, that instead of just one or two photodetectors looking at a point-like measurement volume, there is an array of hundreds of thousands small photodetectors covering the measurement volume of about 34x27x25 mm. In most cases this is enough to cover the whole width of a thermal spray.

Digital images taken with short exposure times to stop the high-speed particles are processed on a dedicated PC computer by Oseir developed software algorithms to give the measurement data for the operator. The algorithm identifies individual particles in the images and measures their position, direction and velocity. Simultaneously, the camera takes another image using a longer exposure time to measure the average particle temperature in the spray.

Figure 1. Measurement volume of the SprayWatch system

Particle velocity is measured by time-of-flight method. The length of the particle traces on the CCD detector is measured by the image processing algorithm and is then converted to velocity by dividing by the known camera shutter time. The method is accurate (accuracy 0.1 - 1 m/s depending on the velocity range) and does not require recalibration.

Figure 2. The blue and orange lines indicate particles detected in an image taken from a typica industrial plasma spraying process. The green lines indicate the flow lines of the particle spray.

Temperature of the particles is measured by two-color pyrometry. An optical double-stripe filter (pat. pending) is integrated in the camera and covers a part of the CCD detector. The spray is imaged on the CCD in two different wavelengths so that the lateral distribution of the average particle temperature in the spray can be calculated using calibration information.

Figure 3. Lateral distributions of particle count (blue), velocity of partilces (green) and average particle temperature (red) are measured on-line. The orange curve on the right is the intensity distribution of the spray.

Conclusions

The large measurement volume makes SprayWatch extremely sensitive to even the smallest changes in particle properties. This is necessary in order to detect failures in the spraying equipment such as electrode wear and nozzle clogging in the earliest possible phase.

The 12 bit high resolution CCD camera together with high quality optics and efficient data processing gives the system unmatched accuracy, sensitivity and repeatability. Based on solid-state digital technology, the system is almost maintenance free.