Digital panel meters provide an indication of machine and process conditions, but also contain a host of added functions. Of course, these added functions aren't free, adding not only cost but also varying degrees of complexity. That's why it sometimes makes sense to use an analog panel meter.
"Analog panel meters are a low-cost solution to provide a visual cue if the machine or process is operating as expected within a given range," says Greg Hendry, product manager for the meter and instruments division at Yokogawa Corp. of America. "Analog meters don't require external power, as they are powered from the input signal. A typical analog panel meter has an accuracy of ±2%. Common input signals are ac or dc amps or volts, with custom scales possible." Application examples, Hendry adds, would be to use the input frequency of an ac input signal to indicate ac frequency, or as a time meter.
"Use a digital panel meter when a more precise reading is required as they have an accuracy of ±0.05% or less," Hendry continues. "Digital panel meters also can incorporate control, output and communication functions — giving them added features as compared to analog panel meters, but at a higher price point."
It seems the list of digital panel meter features is limited only by the imagination and the needs of machine, robot and process skid builders. In addition to real-time control, other digital meter features include signal conversion, web server capability, and wireless communication with sensor networks.
Moore Industries makes a meter that can be used for alarming purposes, as well as signal conversion and retransmission. "Our process monitor and indicator accepts a thermocouple or an RTD signal input," notes Matt Moren, director of sales support at Moore. "Two fully independent alarms may be specified. Each may be individually configured for high and low set points and a variety of latching sequences. Additional models are available with an analog output signal (proportional to input) for retransmission, and with RS-485 communications."
Moore Industries' meter, like many panel meters, is programmed with front panel keys. This simplifies long-term maintenance because there's no programming software to keep track of, and no need to lug around a PC. This simplicity is a virtue of panel meters, particularly for basic machines with limited analog monitoring and control requirements.
Though simple to program and maintain, panel meters can control sophisticated operations such as weighing and scaling. "Our weight indicator/controller panel meters have a one-touch (no weights, no wait) calibration feature and can be used with a self-diagnostic load cell," says Rodger Jeffery, director of marketing and business development at Hardy Process Solutions.
An embedded web server allows remote connection to the meter from anywhere in the world via a web browser. "To help reduce the time and cost of integrating our meters into machine or skid control systems, we provide pre-tested PLC code," Jeffery adds. "In terms of control, the meter's pre-act weight can be adjusted in real time based on current feed conditions, simplifying control and helping to minimize over feeds."
Omega Engineering has a line of controllers that can be configured for PID or on/off control. Output options include solid-state relays, SPDT relays, pulse, and isolated programmable analog voltage/current.
Transducer inputs to the meter can be hardwired or wireless. "Our point-to-point extended range (300 ft) wireless transmitters and our high-powered NEMA-rated extended range (1,000 m) transmitters both support a wide range of Omega transducers for extended flexibility in setting up your system solution," explains Daniel Sparks, Omega product manager. "Our 1/8 DIN panel meter-controller can monitor up to eight wireless sensors. The compact instrument connects directly to an Ethernet network, or can be connected to the USB port of a computer with a USB Ethernet adapter."