AN-2000 COMPUTERIZED ANEMOMETRY SYSTEM
Analog Signals and Anemometers:
What YOU Should KNOW
Figure 1: STANDARD ANEMOMETRY Systems
The most common configuration today (see Figure 1) is an analog instrument (usually controlled via RS-232), that produces voltage proportional to the flow. This instrument is generally connected via a signal conditioner module to the data acquisitions card via long cables. The data acquisition system is a PC (or MAC) interface card, and shares the same ground as the host computer. These typical analog instruments and PC configurations connected to the same ground result in random appearances of strange noises and spikes on low level signals, even 50Hz or 60Hz spikes on some signals. Each spike input to the system via the same ground as the sensor results in an overshoot.
The experienced user recognizes these Pulse or Square Wave" response curves. In the event that there is a train of such impulses from multiple sources and the PC ground is shared by the sensor (via the data acquisition system), all of the current spikes generated by the PC will appear in the signal.
Hardware interruptions caused by DMA refresh of the memory, writing to the disk, mouse & keyboard use; switching power supply spikes (50 - 80 kHz), and VGA refresh. These contribute to a noisy environment not conjunctive to accurate data acquisition.
Despite the fact that an analog anemometer has been specified to work up to 50 kHz, the user finds that it has a -3dB response of 20 kHz only. These systems do not process data channels simultaneously and most of the important phase information is lost. When requesting support from the supplier one is ultimately told that the problems encountered result from other system components. The same response is generally given by the supplier of the other system components. Unfortunately it is more often than not, a familiar scenario with no simple solution. These systems work appropriately when separate, on data acquisition, the DC levels of the analog system tend to fluctuate, and even movement of the mouse during sampling causes strange random spikes to appear on the data. These expensive 16/12 bit data acquisition PC cards sample data at 10 bit readable resolution only... however, when integrated they can become unstable, inaccurate and... unreliable.
USE OUR AN-2000 COMPUTERIZED ANEMOMETER TO AVOID THESE PROBLEMS.