Instrument Description 3-5
Analog Inputs
The CPC can monitor the analog voltages from two external sources via
the analog input BNC connectors on the back panel, labeled Analog Input
1 and Analog Input 2. The input voltage range for these ports is 0 to 10
volts. On the 3772 the analog voltages can be displayed on the LCD
display and saved to the removable Flash Memory Card or a computer.
Voltages from external pressure, flow, or temperature transducers can be
correlated to particle concentration in real time.
Amplification must be supplied by the user to bring low voltage signals to
the appropriate 0 to 10 volt range for best resolution.
DMA/Analog Output and Pulse Output
The DMA/Analog Output port provides an analog 0–10 V signal linearly
proportional to particle concentration. This particle concentration is
corrected for coincidence and equals the concentration displayed on the
front panel of the CPC and the concentration saved to the Flash Memory
Card or computer. Refer to Chapter 4 for details. In addition, on the 3772
this port can be configured by the Aerosol Instrument Manager
®
software to
provide the ramped voltage signal needed when the 3772 CPC is used as
part of the Scanning Mobility Particle Sizer
TM
(SMPS
TM
) spectrometer.
Although this port on the 3771 is also labeled DMA/Analog Output, the
DMA function is not available for the 3771.
Pulse Output port provides a 5-volt (50-ohm termination) digital pulse for
each particle detected. This enables you to use your own counting
electronics hardware or provides a particle trigger for special applications.
The width of the pulse depends on both the shape of the photodetector
pulse and the trigger-level of the pulse threshold. Typical (nominal) pulse
widths are 350 nanoseconds (see Figure 3-4) for the 3772/3771 CPC. To
provide accurate pulse counts, use a counter that is capable of counting
pulses with a width of 50 nanoseconds or less.
Particle concentrations calculated based on the particle counts from the
counting electronics hardware are not corrected for particle coincidence.
Thus, the concentration obtained this way might be slightly lower than the
displayed concentration when particle concentration is high. Refer to
Chapter 6 “Particle Counting” for coincidence correction for pulse output.
The Pulse Output is a way to get raw particle count information. This
information is also available through serial command. Using the SSTART,2
command, described in Appendix B, you can read raw, uncorrected,
particle counts. TSI recommends using the SSTART,2 command for raw
counts as then all the information is shipped which is used to calculate the
corrected concentration, and there are no issues with the counters ability to
accurately count the pulses.
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