DCS as DAQ?
Why do companies install DAQ systems? Just about every process plant has a control system, field transmitters, network and other hardware and software that support the main process control system. If they need to gather more data from process units, it's a simple matter to tap into the existing system and process the data through their DCS, PLC, or HMI/SCADA system.
As Larry Wells, a systems integrator at Confidential Control Systems Assessment (CCSALLC@bellsouth.net), puts it, "All my customers have full-blown DCSs. Even though from a hardware perspective buying a dedicated data logger can cost less, if the user has a DCS, it's much easier to use the DCS as the ‘data logger.' A little more expense for the I/O, but less training, less custom engineering, fewer spare parts, single service contract, etc."
So, unless a company owns a race car, why does it need a separate DAQ system? Harry Sim, CEO of Cypress Envirosystems says DAQ systems are installed for energy audits, condition-based maintenance, outright energy savings and better input variables for advanced control models, among other things. Note that process control is not on that list. This is probably because the cost of putting in sensors and transmitters on a process unit costs tens of thousands of dollars. "In talking with many industry customers, the cost of installing a transmitter into a legacy system is $4,000+, higher for regulated industries," says Sim. Such an installation involves a PLC, PC or DCS control system, and DAQ that will be incorporated in the control system. Therefore, many DAQs are used for non-control purposes, such as energy management or equipment diagnostics.
For these kinds of DAQ purposes, you may want to use temporary, wireless or non-invasive sensors. "One trend relates to ‘temporary' sensors, which are easy to uninstall and move," says Sim. "There are several drivers of this that we've observed, including energy audits. Some measurements of flow, pressure and temperature are only needed to get a good baseline, but not needed for permanent installation."
Though wireless sensors reduce wiring costs, some users are still skeptical about the technology. "In some cases, this is because they've looked at obsolete wireless technology that's not as reliable as today's equipment and mistakenly concluded that all wireless data loggers are alike," says Chris Sorensen, vice president of sales and marketing at Dickson. "Dickson invested a good deal of effort to develop wireless instruments that are reliable. Dickson's Wireless Wizard has an automatic data backup of nine days, real-time ongoing feedback to confirm that the wireless network is intact, and that the expected benefits of real-time data are there and that no more labor hours are required for data downloading. In our case, wireless technology is as reliable as hardwired models."
Figure 2. Wireless steam trap monitors.
Wireless DAQ worked for Genentech at its facility in South San Francisco, where it manufactures biotechnology-based drugs and performs R&D. The facility has about 500 steam traps. Chris Stubbs, senior director of corporate facility services, decided to install wireless steam trap monitors from Cypress Envirosystems (Figure 2) on 56 traps. The wireless monitors transmit data back to a receiver server, which connects to the plant's LAN. Any PC connected to the LAN can see the data. Between September 2008 and January 2009, the system detected 14 steam trap leaks. If undetected, Stubbs estimated the leaks would have cost $42,525. Based on this experience, the payback period was five months. For more details on the installation, see the sidebar, "DAQ Attacks Steam Leaks."
DAQs and Data Loggers
For those who don't want to integrate hardware from Vendor A and software from Vendor B over an XYZ interface, complete DAQ systems are available from many vendors. These vary from a simple battery-powered data logger that downloads into a mobile computer to a full-blown hardware and software system.
For example, Measurement Computing just released its USB-502-LCD data logger, which stores up to 16,379 relative humidity and 16,379 temperature readings. Users can plug its USB connection into a mobile computer and process the data with USB-500 Series Data Logging Application software.
A simple, inexpensive data logger can replace chart recorders, says Minoru Kobayashi, product marketing manager at Omron Electronics. "The electronic version of a paper-based chart recorder can measure a wide range of inputs, such as temperature, voltage, vibration and weight, on multiple channels," he says. "Critical factors in selecting such a data logger include range, measuring speed, how long the data can be saved, the method for saving data, price, size, portability and weight. Data transfer preferences for this type of data logger include USB memory stick, SD card and direct serial connection."
Moving up to bigger DAQ systems can cause problems. "Many end users want to purchase a complete solution from a single vendor," says Yokogawa's Stanier. "With cutbacks in corporate and plant engineering staffs, many customers look to us, not only for hardware but also for a fully designed, configured and installed system. This seems to be particularly true with small (20-500 I/O count) systems where end users know what they want, but don't feel the need to do specifications or internal project management, which would be more common on a DCS project."