Particle Charge
HVPS instrument
The High-Volume Particle Sampler (HVPS) instrument was acquired just prior to STEPS and installed on the T-28 under the right wing. It was specially modified by attaching a device designed and built at the New Mexico Institute of Mining and Technology consisting of 2-segmented plates installed between the main arms of the probe. Signals from these plates can be used to estimate electric charge on hydrometeor.
Figure 15: HVPS, as in Figure 13.
The HVPS operates by taking a two-dimensional shadow digital picture of the particles that pass through a 4.5 x 20.3 cm laser light plane that is normal to the direction of aircraft flight. The sheet of light is imaged onto a linear array of 256 photo-elements. The HVPS has 200 mm pixel resolution when measured across the array. In order to keep the pixel resolution along the flight path constant, the system clock frequency is adjusted according to the detected True Air Speed (TAS) by the data system itself.
The HVPS data recorded during STEPS have 800 mm resolution along the flight path. In addition to particle image recording, the HVPS recorded particle charge data. The charge probe has 8 charge amplifiers, one linked to each segment on the top and bottom plates, that are each sampled six times as a particle passes through the arms. This produces six charge readings per particle per plate segment. From these readings it is possible to estimate charge on the particle. A representation of the data obtained with the HVPS instrument is presented below.
Figure 16: Example of HVPS image and charge data, as in Figure 14.
The above figure shows a typical display using software developed at SDSM&T to display HVPS data. On the lower right corner is a display of raw counts proportional to charge induced on each of the eight HVPS electrode plates. The induced charge is recorded as counts (N). Before transforming N into engineering units, it is preprocessed for noise and offset. A calibration constant converts the induced charge into the particle charge. For one plate:
V = N * k , where k = 0.00122, N = counts, V = voltage
Qi = V * C/G, where C = 470e-12 F (capacitance), G = -23.1 (amplifier gain)
qi = Qi * 1e12 pC where qi is the induced charge for one plate.
The particle charge is then calculated as the sum of the induced charges for all 8 plates,
Qparticle = 1/.658 * S qi.