CHAPTER 1: DISCUSSION OF PROBES

HOW DO PMS OAP-2D PROBES WORK?

When the light incident on one or more of these diodes falls below some pre-set relative threshold, the probe activates. It starts to scan the output voltages of each diode in the array at regular intervals. Each scan results in a 32-bit binary number being stored in a small memory buffer in the probe. Each bit in the number represents one of the diodes. A "1" indicates an unshadowed diode, and "0" a shadowed diode. This scanning continues until all elements are again indicating incident light above threshold, at which time the probe records some housekeeping data and then goes back into a "wait" state. In this way, a shadow of a particle passing by the array is recorded as a sequence of 32 bit numbers followed by a terminator.

(Recently available "gray-scale" probes distinguish four gray levels, as opposed to on/off as does the 2D-C employed on the T-28.)

The time between scans is established by the probe based on the instantaneous true airspeed of the aircraft on which the probe is mounted. The time interval is computed such that the probe moves through the air a distance equal to the effective separation of the photodiodes in the array, from scan to scan. This gives truly proportioned particle shadows.

As an example, the 2D-C probe normally used on the SDSM&T T-28 has its optics set so that the effective distance between diodes is 25 micrometers. At a true airspeed of 100 m/s (typical for the T-28), the aircraft moves 25 micrometers in 0.25 microseconds, so the time interval between scans will be 0.25 microseconds in these typical circumstances.

While the probe is in a wait state, it is accumulating a running count of the number of array scans it could be making. This accumulation is proportional to the time interval between particles. This number is recorded with each particle shadow group, indicating the elapsed time between the particle and the one preceding it. This elapsed time is required to calculate particle and mass concentrations from the image data, as its summation over a series of images defines the distance the aircraft traveled while it recorded the series of images.
 
 

Figure 1. Schematic diagram of 2D-C probe optical path.

The probe contains two internal memory buffers able to store 1024 slices. Each slice is 32 bits, or 4 bytes, making each buffer 4K bytes. In many installations, including that on the SDSM&T T-28, at most one memory buffer can be dumped to the aircraft data system each second. If the first buffer fills in less than one second, the second will begin to fill. A single particle image may, in fact, begin in one buffer and finish in the other. If this second buffer also fills to the 512th slice before the first buffer is dumped, the probe stops accepting new slices. It resumes after the first buffer is emptied to the data system. The T-28 2D-C probe gives no indication in its data that such a gap in data acquisition occurred or how long the gap was (other probes may). In some cases, the juxtaposition of two partial particle shadows at the 512th slice is obvious; in others, it is not.

The same data acquisition system (manufactured by PMS itself) was used with the PMS 2D-C and FSSP probes on the T-28 from 1975 through 1988. The image data stored by this system consisted of 1024-slice records of particle shadows and also include the time the record was written to the acquisition system, making each record 4100 bytes. In 1989, this system was replaced by one manufactured by Science Engineering Associates (SEA). It outputs a somewhat different data structure, but included on the buffers are 4,096 bytes of particle shadow data plus various housekeeping variables such as start time, buffer duration time, true air speed, and shadow or factors. On both systems, the individual particle shadows are bounded by a time slice that indicates the elapsed time between particles. The time a given series of particles was actually encountered is not recorded.

The precise time a given particle was encountered can be uncertain by over one second, but not more than two seconds, given the known time a buffer was dumped and the elapsed time between particles. (An exception to this could occur if the elapsed time between two consecutive particles exceeded the number that could be represented by a 32- or 24-bit binary integer. The T-28 probe uses all 32 bits on the time slice to represent the interparticle elapsed time. Others may use only 24. Assuming time units of .25 µsec, 32 bits would allow 1,073 seconds to elapse before an overflow condition occurred. However, some probes use 8 synchronization bits and only 24 bits to record the elapsed time. In this case, an overflow could occur if the interparticle elapsed time exceeded 4.2 seconds.)

Most PMS OAP-2D probes experience occasional electronic problems that result in less than clean particle shadow data. The T-28's 2D-C is no exception. Problems that have been encountered over the years include "sticky" bits in which one or more bits in successive 32-bit slices remain on or off, irrespective of the stream of particles passing by (see Fig. 2). Sticky bits can interfere with the images and with the time bars that separate shadows and, in some cases, make elapsed times and boundaries between particles uninterpretable.

Another common electronic artifact in the T-28 2D-C probe data is a series of empty images separated by a bar indicating zero elapsed time (see lower portion of Fig. 2). The probe occasionally will generate dozens of buffers filled with these empty frames in quick succession, filling the data tape with garbage and greatly limiting the time the probe has to record real particle shadows. If the buffers fill with garbage in tens of milliseconds, the probe is then almost always waiting to dump garbage to the data system at the end of the current second and not accepting real particle shadows.

Another problem which was encountered when using a P probe in 1991 was that fairly frequently the first half of buffers were afflicted with a "measles" pattern of erratically shadowed bits. The analysis program can be geared to work with only the second half of 2D buffers, which appears to provide enough data by itself to make the situation acceptable, albeit less than desired. In 1994, software was developed to clean up data afflicted with "measles". Some information on the smallest particles is lost when this is done.

The PMS OAP-2D probe is generally capable of collecting very valuable data despite these occasional problems. It is particularly useful for indicating particle shapes. Its limitations must be borne in mind, however. In particular, it must be understood that the probe samples discontinuously and can only accept data at a finite rate. This renders PMS OAP-2D probes less than ideal for inferring spatial distributions and particle number and mass concentrations in cloudy regions where buffers are filling in much less than one second.

One final note: The 2D-C flown on the T-28 has always been oriented so as to shadow particles from the side. On other aircraft, OAP-2D probes are sometimes oriented vertically so as to observe the particles from above.

Fig. 2: Hardcopy of two 2D-C image buffers produced by the program IMAGE. Note how the bars between particle shadows are missing a bit. Unless this stuck bit information was declared in the setup menu, PRELIM2D would find no particles in either of these buffers. Buffer #195 (the lower one) is an example of a buffer full of artifact image frames with no elapsed times.

Introduction
Chapter 2 - Particle Shadow Classification
Chapter 3 - PRELIM2D Program
Chapter 4 - TOUCH2D Program
Chapter 5 - ANALYZE and ANELAP
References