SMAR TT301 vs Rosemount 644

You know what a fancy device has that a simple device doesn’t? Diagnostics or a specific feature to avoid unscheduled downtime. All the bells and whistles boil down to pretty much that. However, some applications don’t need these kinds of features. Still, I believe you should use smart devices in most applications even you don’t have big risks to cover.

Story time again! A big metal company in Brazil had a problem with a thermocouple. The high temperature of the process was draining the life of the cable and threatening some crucial measurements.

To fix it, we cut off all the cables and set the measurement up with wireless solutions, and the customer loved the idea. But we were late to the party this time. A competitor did a good job before us and in the end, beat us to the purchase. Sales life, yo.

Anyway, on to the show! And yes, we’re back to the basics again. Today’s devices don’t have much in the way of fancy features, but they provide good measurements and information about your process. On the left, the SMAR bring the TT301 for the company’s first go-around on Visaya. And on the right, our friends from Emerson once again pop open their portfolio, this time to pull out the Rosemount 644. Let’s see how we can apply these devices.

Disclaimer: This comparison examines only features, not performance. If you’ve used either device, feel free to share your experience in the comments.


Can’t expect more than the standard features from these simple transmitters, right? The SMAR TT301 sports the same housing as most SMAR products. Guess the developers just swap a few internal components here and there to make a new model. It has a simple local LCD display that’ll show your process variables and diagnostics.

You can set up the device in the field, but not with push buttons or a touchscreen. The TT301 has two holes under the identification plate. You use a magnetic screwdriver, also called a magnetic pen, to set up the transmitter locally. I won’t lie; navigating the menu this way sucks, but it’s better than nothing, right?

Courtesy of SMAR and Rosemount

On the blue side of the comparison, the Rosemount 644 has many different faces! You have a local screen with two push buttons where you can set up the device, but its display has two different versions. The simple display has 11 digits in 2 lines, and the LCD with the local operator interface (LOI) has 14 digits in 2 lines. Your choice!

You can also choose the housing styles – field mount, rail mount, or head mount. The rail mount lacks the display option, so if you need a display, then you’ll have to get the field or head mount to work for you.

Measurement and input

We’re discussing temperature transmitters, so temperature is the flavor of the day, right? However, you can fit these devices with other sensors using the available inputs.

The SMAR TT301 supports not only thermocouples and resistance temperature detectors (RTDs) but also resistance and millivolt outputs. You can fit nearly any common temperature sensor in this device, but it can also read data from a sensor providing millivolt or resistance output using its dual input.

Courtesy of SMAR

The Rosemount 644 has similar inputs to support the same kinds of field sensors – thermocouples, RTDs, millivolt, and resistance signals. Pretty much identical apples here, so we’ll move on.

Field protocol and output

Integration! Seamless or no? If you decide to apply one of these devices in your process, then which kind of protocol do you need?

The SMAR TT301 only offers HART with the traditional analog output. But fear not, fair readers! The various model numbers indicate different protocols. Thus the TT302 provides FOUNDATION Fieldbus (FF), and the TT303 takes care of PROFIBUS PA (PB). These cover the most common protocols, so you’re good on that, as long as you choose the right model.

Rosemount 644
Courtesy of Emerson Automation Solutions

On the other side, the 644 has all the above protocols in the same model, so no need to choose. By the way, did you notice that a PROFIBUS option snuck into Emerson’s portfolio? Amazing! I love it!

Performance and approvals

Accuracy in temp measurement depends on many factors like time and range. We’ll read you the standard accuracy from the vendors and also give some examples using the data available in the manual.

The crew from SMAR says that the TT301 has an accuracy of +-0.02 percent! But what does that mean with a sensor? Well, for example, a Pt100 RTD has an accuracy of +0.2 degrees Celsius in a range from -200 to 850 degrees. Will that work for you? And the device has a few approvals for hazardous areas and harsh environments, but it lacks the housing materials and certifications needed for food plants.

Rosemount 644
Courtesy of Emerson Automation Solutions

The Rosemount 644 doesn’t show its standard accuracy information on the first page, oddly enough. You have to dig around in the sensor tables to figure it out. For instance, a type-K thermocouple has an accuracy of +-0.25 degrees Celsius in a measurement range from -180 to 1372 degrees. Better still, you have plenty of approvals and certifications to cover processes from chemical facilities to food and beverage plants.

Fancy features

On the SMAR side, the TT301 has optional proportional–integral–derivative (PID) control! You can have a control loop on the transmitter, then run the PID and control the final element through digital or analog communication.

Courtesy of SMAR

The Rosemount 644 has more frills here. The Hot Backup™ can switch between two installed sensors if you have a problem with one. And if you have a drift between your sensor measurements, then the device will alert you before switching. Furthermore, it’ll record the minimum and maximum temps in your process and monitor thermocouple degradation, then provide diagnostics through digital communication! Not bad, huh?

Information and documentation

The English version of the SMAR website could use a little sprucing up, but at least you can access it with phones and tablets. And when you find the device you want, you can easily download its documentation. The list of function blocks for FF is a nice example of documentation from SMAR.

SMAR vs Emerson Automation Solutions
Courtesy of SMAR and Emerson Automation Solutions

Everyone who has read our past reviews already knows my opinion on Emerson’s site. This company does a great job in digital marketing, with a sharp website, clear documentation, and even a blog! The older devices lack content and videos, but you can easily navigate the website with your laptop, phone, or tablet.


Both devices measure temperature. The TT301, as a simpler device than the Rosemount 644, has a lower price point. If you want diagnostics and continuous sensor monitoring to avoid unscheduled downtime, then plug the Rosemount 644 in. Either way, you’ll get a pretty solid product.

Table of comparison

SMAR TT301 vs Rosemount 644
Courtesy of SMAR and Emerson Automation Solutions
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