by Matt Migliore

Emerson Process Management’s Rosemount 848T temperature transmitter is among the latest generation of devices capable of supporting multiple sensor inputs. It can transmit up to eight RTD or thermocouple sensor signals.

As temperature transmitter technology continues to advance, the devices are getting easier and easier to use, while at the same time bringing cost-cutting opportunities and added visibility to the temperature measurement process as a whole. Customers want better temperature measurement at a lower price, which, in turn, is pushing suppliers to work hard to increase their price-performance ratios.

The Modern Temperature Transmitter
A key enabling trend along this line is the move from analog to digital temperature transmitters. Much like the instrumentation market in general, temperature transmitters have evolved over the past 10-20 years from analog devices that could only accept a single, specific input to smart, digital devices that are capable of accepting multiple inputs from a variety of sensor types. This capability allows the temperature transmitters of today to, among other things, handle with a single device what would have previously required multiple instruments. As such, users can add measurement points into their process at minimal cost and with relative ease.

“Microprocessors are bringing more horsepower to the transmitters, so you’re getting more and more capability,” says Michael Cushing, pressure and temperature product manager for Siemens Energy & Automation (www.siemens.com). With this added power, Cushing says temperature transmitters provide a lot of inherent functionality that can be quite useful to the end-user. For example, many of today’s temperature transmitters can perform a Delta T calculation directly within the transmitter, as well as create custom curves based on the sensor input to enhance the accuracy and stability of the temperature measurement. Cushing says he expects temperature transmitters to continue to evolve in this way, adding such options as averaging and density calculations.

According to Cushing, the ability to accept multiple sensor inputs is also a key evolutionary step within the temperature transmitter segment. He says that since temperature is a relatively simple measurement, users can realistically employ a single transmitter to multiplex up to eight sensor points. In such a scenario, the end-user would effectively be eliminating the cost of deploying seven additional transmitters (i.e., one per sensor), which is what would have been required in an analog environment.

Loren Engelstad, director of Rosemount temperature engineering for Emerson Process Management (www.emersonprocess.com), also believes the multi-input capability of modern temperature transmitters holds significant value for the end-user. In particular, Engelstad says dual-input transmitters with an auto-backup option are a good fit for applications where availability is a concern.

With a dual-input, auto-backup transmitter, the temperature measurement process can be configured to fail over to a backup sensor if the primary sensor goes down. This enables the temperature measurement to stay online so maintenance can be scheduled in an orderly fashion. Taking it a step farther, Engelstad says that many of today’s temperature transmitters are able to relay diagnostic data back to the control room so the user has a better feel for sensor health and can replace the sensor before it fails.

Going forward, Engelstad says the depth and breadth of diagnostic data available to the user will grow exponentially. “Today we have statistical process monitoring within our device,” he says. “And we can potentially use this information to help understand key characteristics of the entire process, not just temperature.”

Engelstad also sees wireless as a key feature for temperature transmitters. With wireless, he says users can now avoid having to go out into the field to gather temperature data from measurement points that are too expensive to direct wire. “With wireless, you can send that information back to the control room,” he says, thus eliminating time, inconvenience and potential safety concerns from the process.

And while the functionality available to the end-user is growing more advanced, the user interface for temperature transmitters figures to become more simplistic. “End-user are trying to do more with less people, so I think what you’re going to see is more ease of use of the devices so the user can get the device installed without having to be an expert,” says Engelstad.

Consider the Lifecycle Costs
While there is a lot of added functionality available in today’s temperature transmitter technologies, it does not come without a price. However, suppliers make the case that a feature-rich temperature transmitter will generally pay for itself many times over during the course of its operation.

“Often a customer will specify a lower-tiered temperature transmitter based solely on price considerations,” says Patrick Cupo, marketing manager for pressure and temperature products at Invensys Process Systems (www.ips.invensys.com). “After a few months of operation, he regrets not purchasing a higher-tiered product. The higher-tiered product had the one diagnostic he needed to prevent the unplanned shutdown. The cost of the shutdown exceeded the additional cost of the higher-tiered temperature transmitter.”

To prevent such a scenario, Cupo says users should have a clear understanding of their temperature measurement requirements, including whether or not the application calls for monitoring or control, the necessary accuracy, safety issues, etc. “What are the consequences of the temperature measurement being off by three or four degrees?” asks Cupo. “Do I lose an entire batch? Do I no longer comply with an agency requirement? Are there safety issues?”

Specification & Installation Best Practices
The installation of modern-day temperature transmitters, while relatively simple, does require a mindful approach, according to Siemens’s Cushing. Two key characteristics Cushing recommends users consider are the physical size of the transmitter and the mounting type. He says users will on occasion purchase a transmitter without considering whether or not it will actually fit in the space in which they would like to install it. Meanwhile, regarding mounting, Cushing says users can typically choose between head-mount, rail-mount and field-mount devices. He says head-mount devices offer simplicity and low cost, while field-mount devices are typically a good fit for applications that involve high temperature and/or require a remote option.

In addition, Cushing says users should keep in mind the wiring requirements of their temperature measurement application. If the transmitter is a low-end device, for example, it may not have the proper RFI protection to prevent noise from interfering with the signal transmission.

Emerson’s Engelstad says users should also try to install their temperature transmitter as close to their sensors as possible to reduce the lead lengths of the sensor signal. In doing so, users can limit the potential for noise interference, as the low-level sensor signal will have a shorter distance to travel to the transmitter before it is converted into a high-level digital signal.

“If you can eliminate longer lead lengths from your sensors, that can make a big difference from a performance perspective,” says Engelstad.

Matt Migliore is the editor of Flow Control magazine. He can be reached at matt@grandviewmedia.com.