Tag Archives: RTD

Using RTDs for Measuring Temperature

Much industrial automation, medical equipment, instrumentation, and other applications require temperature measurement for monitoring environmental conditions, correcting system drift, or achieving high precision and accuracy. Many temperature sensors are available for use like electronic bandgap sensors, thermistors, thermocouples, and resistance temperature detectors or RTDs.

The selection of the temperature sensor depends on the temperature range to be measured and the accuracy desired. The design of the thermometer also depends on these factors. For instance, RTDs provide an excellent means of measuring the temperature when the range is within -200 °C to +850 °C. RTDs also have very good stability and high accuracy of measurement.

The electronics associated with using RTDs as temperature sensors with high accuracy and good stability must meet certain criteria. As an RTD is a passive device, it does not produce any electrical signal output on its own. The electronics must provide the RTD with an excitation current for measuring its resistance. This requires a small but steady electrical current passing through the sensor for generating a voltage across it.

The design of the electronics also depends on whether the design is using a 2-, 3-, or 4-wire sensor. This decision affects the sensitivity and accuracy of the measurement. Furthermore, as the variation of resistance of the RTD with temperature is not linear, the electronics must condition the RTD signal and linearize it.

RTDs in common use are mostly made of platinum, and their commercial names are PT100 and PT1000. These are available in 2-wire, 3-wire, and 4-wire configurations. Platinum RTDs are available in two shapes—wire wound and thin-film. Other RTD types available are made from copper and nickel.

When using an RTD as a temperature sensor, its resistance varies as a function of the temperature, and not in a linear manner. However, the variation is very precise. To linearize the output of the RTD, the electronics must apply a standardizing curve, the most common standardizing curve for RTDs is the DIN curve. This curve defines the resistance versus temperature characteristics of the RTD sensor and its tolerance within the operating temperature range.

Using the standardizing curve helps define the accuracy of the sensor, starting with a base resistance at a specific temperature. Usually, this resistance is 100 ohms at 0 °C. DIN RTD standards have many tolerance classes, which are applicable to all types of platinum RTDs in low power applications.

The user must select the RTD and its accuracy for the specific application. The temperature range the RTD can cover depends on the element type. The manufacturer denotes its accuracy at calibration temperature, usually at 0 °C. Therefore, any temperature measured below or above the specified temperature range of the RTD will have lower accuracy and a wider tolerance.

The categorization of RTDs depends on their nominal resistance at 0 °C. Therefore, a PT100 sensor at 0 °C has a resistance of 100 ohms, while at the same temperature a PT1000 sensor has a resistance of 1000 ohms. Likewise, the temperature coefficient at 0 °C for a PT100 sensor is 0.385 ohms/°C, while that for the PT1000 is ten times higher at the same temperature

Connecting Wireless Temperature Controllers

Modern industrial temperature controllers are not just simple thermostats, as their earlier counterparts were. With the ability to control upwards of a hundred parameters, the latest industrial temperature controllers allow users to set not only temperature points, but also program alarm settings based on adjustable ramp parameters. Users can select the RTD or thermocouple they want to use for collecting data, while setting limits on the set points.

With the advent of digital temperature controllers, users can configure them with a physical interface. Although, initially, the design of some models allowed them to connect to nearby computers through a wired link, the later models of temperature controllers come with Bluetooth enabled.

Traditionally, the physical interface of temperature controllers featured two to five buttons that allowed the user to set the various parameters for the controller. With the limited three to four character LED display on the controller, the user had to either know the button combinations or refer to a manual during the process of setting up the parameters.

Connecting earlier temperature controllers to PCs through wired serial interfaces presented other problems. It required the PC to be near the controller, as the interface and cable could cover only limited distances. This meant the PC had to operate in the noise and dust of the industrial environment, reducing its operational life. Cables connecting the two were prone to electromagnetic interference, and a tripping hazard. Most modern PCs come with only USB connections, and do not have serial interfaces any more, complicating the situation further.

Bluetooth enabled industrial temperature controllers have solved the above problems with ease. Several controller can connect to one mobile device with an app using Bluetooth—a short-range connecting technology. As the user brings the mobile device within range of the controller, he or she can ping the controller to confirm the specific device to interact. The app on the mobile allows the user to interact with the controller for viewing and setting all its parameters and for reviewing any of its error messages.

With the app interface offering greater graphical flexibility, the user can read the error messages and parameter names in plain text. Moreover, he or she can access in-line help for further understanding the function of each parameter and its permissible settings.

The graphical app interface allows the user to set up the temperature controller easily. It does not require the user to page through a manual or memorize the settings. No cables or other inconvenient interfaces are necessary for using these modern mobile interfaces.

With the unprecedented growth of cloud-enabled devices and the Internet of Things, there are concerns about information security in wireless connectivity. Using Bluetooth technology in industrial interconnections has its own advantages. Bluetooth is currently unable to connect to LAN, industrial Ethernet, or to cloud services, and is therefore, secure to that extent.

Furthermore, Bluetooth technology functions over short distances, and communications are limited to within 70 feet, limiting long-range interference. Moreover, users can protect controllers with passwords. Users can select the parameters on the controller that the password will protect, and a remote user cannot change them through the app.