Buyer guide: Yokogawa FLXA21 and Mettler Toledo M400 2-wire

If you need an analytical transmitter, you’ve come to the right place! Yokogawa FLXA21 and Mettler Toledo M400 2-wire both fit the bill. But which will fit your process? Let’s dig around and find out.

Yokogawa FLXA21

If you’ve hung around here long enough, you’ve met Yokogawa before. The FLXA21, a two-wire device for online analysis, has up to two input channels and that’s about it for the features. But it has a little bit more than its brother, the FLXA202. So let’s dive into the bliss of Yokogawa!


The Yokogawa FLXA21 comes in a compact size, 144 by 144 by 151 millimeters. Yokogawa wants us to get excited about its modules and multi-channels, but read the fine print before you get out the party hats.

The device reads a variety of sensors and shows the data on a touchscreen display. It can measure pH/ORP (oxidation-reduction potential), inductive conductivity, contacting conductivity, or dissolved oxygen.

Image of Yokogawa FLXA21
Courtesy of Yokogawa

Big Y wants you to know that you can connect more than one sensor to this device, which sounds great! But for the most part, you can only connect the same type of sensor. Not so great.

On the other hand, the module part? That’s actually kinda good. If you have a problem with one module, then swap it out for another. Bing bang boom, and you’re done.


It comes with some useful certifications, all listed in the manual, if you want to know more. As a loop-powered device, it communicates and powers up with the same cable. For integration, it offers HART, Foundation Fieldbus (FF), and PROFIBUS PA, getting you into most applications.

You can get it in plastic, stainless steel, or corrosion-resistant stainless steel. It’ll can talk to you (not literally – yet) in 12 languages, so you can probably find one you can use.


 Firstly, you can set up the Yokogawa FLXA21 using the black-and-white touchscreen. Sure, it’s not a color display, but it’ll do the job.

Secondly, it can measure continuous sensor impedance, slope, asymmetric potential, and more. This data can help you diagnose damage, burnout, and other issues to reduce unscheduled downtime.

And it has a quick setup guide for the configuration, which saves time and can apply to similar devices, if you’re new to analytical transmitters. It can also save your last five calibrations and estimate future maintenance and calibration time.

Graphs FLXA21
Courtesy of Yokogawa

Finally, it has a feature called trend graphics that can display up to 41 averages of your measurements. These averages can map the behavior of your process and your sensor. The FLXA21 pulls data every second, so you can see the maximum, minimum, and average of your numbers.

Mettler Toledo M400 2-wire

Up next, we have the Mettler Toledo M400 2-wire. So what are you waiting for? Check out the deets on the M400!


You’ve heard me sing this song before. The M400 looks like everybody else. You might like the familiarity, or you might hate the lack of originality. Or you might not care. Anyway, it has a local setup but no touchscreen, just push buttons. Yes, some touchscreens have us favoring the push buttons, but they’re getting better!

At 144 by 144 by 116 millimeters and 1.50 kilograms, it feels sturdy yet compact and can install on a wall or pipe. And Mettler Toledo calls it multi-parameter, capable of measuring pH/ORP, oxygen, and conductivity.

image of Mettler Toledo M400 2-wire
Courtesy of directindustry

You can use it with analog or digital sensors, giving traditionalists a shot at trying digital without too much commitment. Too bad it only has a single channel to connect the sensors. But other models offer three channels or more, if you want to poke around in MT’s portfolio.


More loop power saves you money! And HART, FF, and PROFIBUS PA cover most applications.

The M400 has its certifications and approvals on its datasheet. And it comes in die-cast aluminum. Yes, just that. But it also has eight languages to choose from for your tasks.


The M400 has Intelligent Sensor Management (ISM), with a dynamic lifetime indicator, adaptive calibration time, and more. It monitors the health of your transmitter and sensors too. You can see most diagnostics on the screen and can do pressure compensation if you want.

image of Mettler Toledo M400 2-wire
Courtesy of Mettler Toledo


However, it doesn’t come with trend graphics. Sad. And if you need to know more about its accuracy and resolution, you’ll have to find your application and sensor on the datasheet to find out how it specs out for you.

Table of comparison



Category Yokogawa FLXA21 Mettler Toledo M400 2-wire Verdict
1 Unboxing simple design; touchscreen with good resolution; good graphics simple design; vivid display but lacks graphics; keypad instead of touchscreen FLXA21
2 Support and protocol loop power; HART, FF, and PROFIBUS PA; FF and PA require between 9-32 volts loop power; HART, FF, and PROFIBUS PA; more than one analog output, analog input, and digital input/output M400
3 Measurement up to two sensors, although choices are limited; contacting conductivity, dissolved oxygen, inductive conductivity, or pH/ORP multi-parameter but single channel; pH/ORP, oxygen, or conductivity; analog or digital sensors tie
4 Materials and approvals plastic, stainless steel, or stainless steel with corrosion-resistant coating; hazardous protection certifications; IP66


die-cast aluminum; hazardous protection certifications up to zone 1; IP66


5 Fancy features trend graphics; self-diagnostics that can measure sensor impedance, slope, and more Intelligent Sensor Management (ISM) offers predictive maintenance,  online diagnostics, and remote access FLXA21
6 General information and documentation responsive website and mobile version; clunky data searches


user-friendly website and mobile; home page could explain more tie


Both devices deliver the same data. The FLXA21 has two channels and trend graphics but limits itself to only one process variable. The M400 has one channel and no graphics but different inputs. You’ll need to decide which features you want to choose and which you can afford to lose. Good luck!

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