The growth of wireless instrumentation technology has provided new ways to apply temperature measurement sensors, combined with personal computers, to collect, tabulate, and analyze data. For complex, multi-sensor applications, wireless devices provide a means to eliminate the nuisance of running multiple leads over long distances through harnesses or conduit to a control room, instrument panel, or equipment rack, while keeping track of leads. For simpler one- or two-sensor applications, it means installing the wireless sensor and setting up the receiver.
The temperature sensors most commonly used are thermocouples and RTDs (resistance temperature detectors). The various types of transmitting units available will handle nearly any type or model, and, depending on obstructions and other factors that affect transmission, can have ranges of up to 400 feet. The simplest transmitting devices, like the wireless thermocouple connectors shown in Figure 1, accept a plug-in sensor and transmit the data to a receiving device at programmable intervals of 5 seconds to 1 minute. Depending on this sampling rate, battery life can be a year or more. In addition to temperature readings, these units transmit battery status information, which helps ensure timely battery replacement and prevent unexpected sensor-down situations.
For outdoor applications or harsh environmental conditions, a wireless NEMA transmitter can be used. These devices transmit the same data as those in standard enclosures, but have a much longer battery life (up to 3 years), since they may be installed in remote or difficult-to-access locations. A weather seal protects the internal sensor connections.
The RS-232 interface has been popular in the past for sensors with built-in electronics, but RS-232 cable lengths are limited to 50 feet. For longer runs, wireless RS-232 transmitter/receiver sets are available. The RS-232 output is forwarded to the receiver, which converts it to USB protocol for connection to a PC. The PC can still communicate with the RS-232 device as if it is directly connected. This type of wireless device is suitable for both new installations and retrofits.
When making measurements in liquids or corrosive environments, the thermocouple or RTD sensing device often needs to be protected. In such cases, a probe, which encloses the sensing device in a protective tube or shell, is commonly used. For these applications, a wireless sensor probe transceiver integrates the wireless transmitter with the probe in a single unit.
The most common receivers connect to a PC through a USB port. Software displays the readings from each channel, as well as the sensor type, sensor designator or description, ambient temperature, units, signal strength, and battery condition. High- and low-temperature alarms are indicated by a blinking red reading. Data can be saved to a file for purposes of charting and analysis.
For those cases in which an analog signal is necessary to provide process feedback to a controller or PLC, receivers that include an analog output for one channel are available. A display can be added to show this reading on the unit, although it will also be displayed on the computer screen. Receivers in NEMA-rated enclosures are also available for outdoor applications or harsher indoor settings.
The most interesting new receivers do not require a host computer at all. They are built with embedded Web servers and connect directly to an Ethernet network or to the Internet with a standard RJ45 jack. Such a device has its own IP address and serves active Web pages that display realtime temperature readings and charts. It is accessed for viewing and configuration using a PC with a Web browser. Access can be restricted through password protection. Additional capabilities typically include alarm notification by e-mail or text messaging.
Web-based wireless receivers allow ad hoc monitoring using a PC from any location having an Internet connection. In addition to allowing data to be viewed from more than one location, this effectively removes any restriction on the distance of the monitoring point or control center from the temperature sensors being monitored.
For situations in which remote monitoring is desirable, but the machine or process operator needs access to the readings locally, a wireless panel meter is available. This Web-based receiver is also a wireless scanner that displays the output from up to eight sensors. It adds local machine or process control capability by means of relay contacts and voltage output sign als. Typical browser screens from a Web-based wireless receiver are shown in Figure 2. The top screen shows a chart of data that has been logged, and the bottom screen shows sensor readings. The screens are interactive and allow configuration programming of the receiver.
Many of the old limitations and complexities of point-to-point, mechanically interconnected instrumentation systems no longer apply. Control can be centralized or decentralized and data can be collected and analyzed in realtime or later using readily available personal computers.
This work was done by OMEGA Engineering, Inc. For more information, click here.