Comparison: Yokogawa FLXA 21 vs Mettler Toledo M400 2-wire

In process automation, you have to find the best device for your process requirements. So you spend a lot of time scouring the internet for information and translating all the technical datasheets. But it’s not easy to compare two devices, much less more than two. To help solve this problem, we offer our product reviews and comparisons.

We strive to provide all the information you need to figure out if a device can fit your application, both in specifications and features. Some users consider features merely frills, but most features exist to reduce unscheduled downtime, intermittent problems, complex diagnostics, and more. Of course, performance is the most relevant point to decide the best device for your application.

Speaking of points

Today, we have a new principle here on the comparison! We’re reviewing analytical transmitters, a new process variable on Visaya. Although sensors must provide accurate readings, what good will those do if no one can read them? So the transmitters play an important role in this process; they read and translate sensor data into standard output signals or digital protocols.

And now, our contenders! The crew from Yokogawa have brought the FLXA21 to battle the M400 2-wire from the gang at Mettler Toledo! Let’s see how these devices match up and which will win the bout for you. Ready?

Then grab some chips and dip, take a seat, have a read, and draw your own conclusions!

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


Here we go, opening the box of the FLXA21 from Yokogawa. The developers kept the design simple, no fancy details, so we don’t have much to say about its appearance. We can say the touchscreen display could use better resolution, but at least we have a touchscreen.

M400 vs FLXA21
Courtesy of Mettler Toledo and Yokogawa

On Mettler Toledo’s side, the design looks similar to Yokogawa’s, only in a different color. Frankly, you could say that of most of these devices. Beyond that, the display of the M400 2-wire looks brighter and better than the FLXA21, but it lacks the graphics that the FLXA21 has. It also has a keypad rather than a touchscreen. Meh.


Both companies tout the versatility of these devices, so let’s have a look. Yokogawa claims that the FLXA21 can read up to two sensors at the same time. Therefore you can read contacting conductivity, dissolved oxygen, inductive conductivity, or pH/oxidation-reduction potential (ORP) in both channels, right?

However, if you read the fine print you’ll see notes limiting the two channels. For example, you can only have two sensors measuring the same process variable. And if you have an inductivity sensor or digital SENCOM sensor, then you can only use one sensor. Not very versatile.

M400 2-wire vs FLXA21
Courtesy of Mettler Toledo and Yokogawa

The M400 2-wire operates on a similar level of expectation. This “multi-parameter” device can measure pH/ORP, oxygen, or conductivity. Notice I said “or” rather than “and.” It only has a single channel, but that makes it almost as good as the FLXA21!

Beyond that, the M400 supports analog or digital sensors. This option is interesting because if you’ve never used a digital sensor, then you can try one with the same transmitter that you used for your analog sensor. Not bad at all.

Materials and approvals

Some of y’all have your devices exposed to harsh environments, so they need to have the right materials for that. And for hazardous areas, you need devices with the right approvals.

The FLXA21 has a short list of housing materials and nothing out of the ordinary – plastic, stainless steel, or stainless steel with a corrosion-resistant coating. It offers IP66, though, so that’s good. And the modular design means you can easily replace a module if you have a problem, saving time in maintenance. As for approvals, the FLXA21 has a bunch, so you’ll likely find the one you need.

Materials M400 2-wire vs FLXA21
Courtesy of Mettler Toledo and Yokogawa

The technical documentation for the M400 2-wire has die-cast aluminum for its housing material. Nothing else. Next! This device is certified for hazardous areas up to zone 1, among others. You’ll probably find the approval you need for your process requirements. It too has IP66 for its enclosure.

Power and protocols

Both devices use loop power, which can bring benefits to a new application, but otherwise is more of the same. The FLXA21 has HART, Foundation Fieldbus (FF), and PROFIBUS PA for its protocols. For the FF and PROFIBUS PA, the device requires a minimum of 9 volts or maximum of 32 to work properly in the field network. However, it doesn’t have an input device or more features for communication.

On the M400 2-wire, you also have the basic digital protocols of HART, FF, and PROFIBUS PA. However, you can have more than one analog output, analog input, and digital input/output. This flexibility can help you connect a pressure compensator directly to the transmitter, for example. I like that.

Fancy features

The FLXA21 has trend graphics to give you your measurement history, even though you can do that analysis remotely. It also has self-diagnostics and can measure sensor impedance, slope, and other things to find damage, contamination, burnout, and such.

The M400 2-wire has its own tricks! The Intelligent Sensor Management (ISM) helps you do predictive maintenance, check online diagnostics, and use a remote platform to check all data coming from the field. Not super, but still nice to have.

Information and documentation

Yokogawa’s site is pretty responsive, even on your phone or tablet. The documentation could use some updating, though; it’s difficult to find information. And the home page of the site doesn’t have much, making your search even harder.

Websites M400 2-wire vs FLXA21
Courtesy of Mettler Toledo and Yokogawa

Mettler Toledo also has a responsive and mobile-friendly website. Sadly, they have the same issue as Yokogawa. Too much summarizing, not enough explaining, and the documentation doesn’t make much of anything clear.


Both devices will deliver the same type of data. The FLXA21 has two channels and trend graphics, but good luck getting full use of those channels. The M400 2-wire has just one channel without the graphics but  different input options, giving you more possibilities. Which will work best for you? Start scaling out, people!

Table of comparison


Table Comparison M400 2-wire vs FLXA21
Courtesy of Mettler Toledo and Yokogawa


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