Choosing the Right Device for Specific Monitoring Needs
Data Collectors, Recorders, Transmitters
Monitoring may either be local (at the location of measurement) or remote (at a switch-box, local control room, or central control station). Depending on the parameter, automatic or manual control might be inserted, or perhaps the measurement is simply logged for reporting. Based on the environment and specific requirements, transmitters, recorders and data-collectors are utilized.
If local power (perhaps 120V AC, or 24V DC) is available at the measuring-site, then a 4-wire transmitter is used to convert the local sensor-signal to a high-level voltage or current which can be transmitted to the central station. This type of transmitter is termed four-wire because two wires are needed for the power-source and two wires for the transmission signal. Because power is available, this type of signal-conditioner is typically multi-functional - it may have local display of the measurement and high-low limit-alarms, etc. If the output signal is to fed to a local recorder, then it is usually voltage - e.g., 0-10V DC - which can conveniently be fed to several devices (alarms, displays, controls) in parallel. If the signal is to be transmitted to a remote station (perhaps up to a few miles) then a current output is more useful because the (unknown) resistance of the leads is immaterial, up to several hundreds of ohms. 4-20mA is the typical remote transmission signal.
If a measurement is remote - water-level in a remote tank, for example - and local power is not available at the site, then a "2-wire transmitter" can be used. With this type of device, the signal-conditioning circuitry is run on the same pair of wires as the output signal, operating on 4mA. As the measured signal (tank-level, for example) varies from 0 percent to 100 percent, then the output signal is modulated from 4-20mA. The power-supply to operate the 2-wire transmitter is located at the central station - typically 24V or 48V DC. Each transmitter is connected with two wires to this central power source, and each return lead (varying over the range 4-20mA) is typically connected through a resistor (say 250 ohms) and the voltage drop across that resistor (1-5V DC) represents the measured signal. So, with several 2-wire transmitters connected, each 1-5V DC signal would be generated at a bank of resistors near the voltage source in the central station. From there, the signals may be fed to multiplexers, recorders, controllers, computer data-acquisition systems, etc.
What if the remote-site has no power and no wire connections at all?
This is when "data collectors" are useful. These could be battery-operated chart-recorders, or the equivalent electronic recorders which take time-stamped measurements at pre-determined intervals. The readings may be stored for future readout at the central station, or perhaps read out periodically via a telephone modem connection to the site.
For example, one type of data-collector product travels on railcars along with fresh strawberries, monitoring the temperature during the entire time of shipment.
The most common recorders are, of course, paper-chart-recorders. Here the input signal (perhaps 0-10V, 1-5V, 4-20mA) moves a pen which marks a moving paper that is controlled to move at a desired speed. The chart provides a permanent record of the input signal. Several inputs may be fed to a single multi-point recorder, with each channel being marked with different color inks. The charts may be circular (usually 24-hour cycle) or linear, rolled into a take-up mechanism, or simply streaming out the end. The total time over which the recording is available is dependent on the speed of the chart and the length of the chart-paper. If the recorder runs out of paper, then the recorder is useless. However, provided the recorder is operating correctly, with paper, it presents an excellent permanent time-record of the monitored signal.
The problem with conventional paper-recorders is that the recording takes place irrespective of the input. If the measured variable is stable, the recording is nothing but an endless straight-line. On the others hand, if the variable changes when the recorder has run out of paper, then the recording is lost. "Smart" recorders might provide paper-movement only when the variable changes - but then the time-axis would be meaningless and the recorder would require a simultaneous imprint of time as the paper moves. This tends to make smart paper-recorders impractical.
The availability of inexpensive processing power and memory has brought the introduction of "paperless" recorders - products which provide all the functions of conventional recording, but without paper. Monitored data is stored in electronic memory, and displayed on a PC screen in any of numerous formats - bar-graphs, pie-charts, linear or circular chart equivalents.
Both the time and signal axes can be easily expanded or contracted at will, providing significant utility that is almost impossible with a conventional recorder. For example, a stable measurement can be reviewed at 1-inch-per-hour, while a fast-changing signal can be viewed at 11-inch-per-minute; a large variation can be viewed with 100 percent full-scale Y-axis, while a small-variation can be viewed with a greatly expanded scale.
When the applicable view is selected, a paper-printout provides a permanent record. And, of course, a paperless recorder of this type can be extended to several channels being monitored at the same time.
The wide availability of personal computers in a variety of configurations - desktop, laptop, hand-held, etc. - has signaled the advent of a new type of recording: networked I/O devices connected to PCs. Networked I/O data acquisition provides data-collection and storage when the PC is not connected, but transfers all the data to the PC when connected, for convenient storage on the typical gigabyte-capacity hard disk of the PC. The stored data is time-stamped and available for recall in any convenient format.
Networked I/O channels can be extended to literally hundreds of I/O points, vastly extending the concepts of multi-channel recording. Further, once the measurement is in the computer, calculated combinations are an easy extension - e.g., flow is the square root of differential pressure.
Therefore, several calculated combinations of monitored parameters can be stored and displayed as "virtual point" measurements. The era when a chart-paper measurement is transferred manually to a spread-sheet is on its way out - today, the measurement is automatically fed into an Excel spread-sheet and the chart-display is available in any one of a number of formats through the "chart-wizard."
In the water and waste business environment, data collectors, transmitters and recorders are all product facets and extensions of the basic technology of signal monitoring, measurement, display, storage and control.
James J. Pinto is the president of Action Instruments, Inc. in San Diego, Calif.