When to Replace a CEMS (Probably Never!)


When to Replace a CEMS (Probably Never!)

If you own a Continuous Emissions Monitoring System (CEMS), it’s probably crossed your mind that you may need to replace it at a certain point. After all, one thing that you can count on with any piece of equipment is an eventual failure.

But should you ever need to replace the entire CEMS… cabinet and all?

The answer is probably no.

Almost 95% of CEMS were built using a “modular” approach in the last few decades. That is, the CEMS is made up of many individual components that are fitted together to create the entire system. The advantage of this approach is that a CEMS should never need wholesale replacement. There are rare exceptions.

You can replace each component as it fails or becomes obsolete. In doing so, this allows a CEMS owner to have a system that is up-to-date using the latest component technology.

To understand why a CEMS never needs replacing, we must understand how it’s constructed and operates. We are talking about Full-Extractive CEMS. There are always a few exceptions (like in-situ CEMS), but for the most part, nearly every CEMS manufacturer builds a similar modular system.  

These systems start at the stack with a probe. The probe is typically heated and has a filter, and the sample is drawn through it into the umbilical.

The umbilical carries the sample down the stack to a CEMS shelter or cabinet. It transports calibration gas up to the probe and carries purge air to the probe to clear the lines after.

The sample arrives at the sample conditioner, typically found in the CEMS cabinet/rack or inside the CEMS shelter. The sample conditioner removes the moisture from the sample gas and reduces the temperature to somewhere around ambient, so it’s safe to introduce into the analyzer(s).

The analyzer(s) measures the sample gas for the specific types of gases. Sometimes an analyzer might measure more than one gas. Typically CEMS analyzers use different measurement technologies for each gas (i.e. NOx is measured with Chemiluminescence technology, Oxygen is measured with Zirconia or Paramagnetic technology, CO, and CO2 with Non-Dispersive Infrared technology). After measurement, the gas is exhausted into the atmosphere.

Along the way, the gas passes through a sample/calibration gas control panel made up of solenoids and rotameters. The calibration gas control panel controls the sample gas and calibration gas flow to the analyzers and probe. Calibration gas bottles are plumbed through the sample/cal control panel to allow for the required calibrations of the CEMS.

The last component of the CEMS is the Data Acquisition System (DAS). It is sometimes abbreviated to DAHS, which stands for Data Acquisition and Handling System.

The DAS is made up of a PLC (can also be a Datalogger) and a PC. It collects and stores all the information, performs the emissions calculations, tracks emissions, highlights exceedances, has an alarm system, and more.

There is no single manufacturer for all these parts. For instance, there are three major manufacturers of probes and sample conditioners. There are two major Umbilical manufacturers, a plethora of analyzer manufacturers, and several DAS manufacturers (often, the CEMS manufacturer makes their own DAS). Moreover, a probe manufacturer does not make analyzers, and no umbilical manufacturer makes a DAS, etc.

CEMS manufacturers are called “integrators”. They are given that name because they purchase the components and integrate them into a full CEMS.

All the components off the stack are often put together in a standard 19” instrument rack. They are wired and plumbed together as a complete CEMS. If you routinely replace individual components, your CEMS could always be up to date.

In rare cases, a rack could suffer damage or be so badly rusted that it is more cost-effective to replace everything. But even then, the probe and umbilical could be re-used. In some cases, a shelter could be in such poor shape it needs replacing, but often everything inside of it can be relocated to a new shelter negating the expense of wholesale replacement.

Let’s look a little closer at how easy and cost-effective it is to replace components over the lifetime of a CEMS:

CEMS system components


In a clean burning process like a natural gas boiler, probes can last upwards of 15-20 years before failing. Because the heater or heater electronics are directly exposed to the unfiltered and hot-wet stack gas, they may fail earlier from corrosion–especially in an excessively corrosive process.

A typical extraction probe cost range is $5,000-$7,500.  

Replacement is as simple as disconnecting the umbilical lines and unbolting the probe from the stack port flange. Replacement should take less than four hours.


An umbilical should last 10-15 years and even beyond. If it’s secured well and doesn’t bounce in the wind, or stretch, or get hit with a fork truck or crane, it shouldn’t need much attention over its lifetime. In dirty processes, it may need cleaning from time to time, but that should not affect its lifespan.

The most common failure mechanism after mechanical damage is heater failure. The umbilical will get brittle over time from the sun’s rays. But again, if it’s secured and doesn’t move, it shouldn’t crack or split.

A replacement umbilical costs around $55-$70 per foot. Pricing varies depending on the configuration. Factors that impact the cost of an umbilical include the number of lines, materials used, how many wires, length, etc.

Taking an old umbilical out and putting a new one in can be a big job, depending on how high the stack is and how the umbilical is run. Typically, the existing cable tray is re-used making installation easier the second time.

In most cases, you need to use a crane or lift. The umbilical needs to be handled carefully (lift with support every 10 feet, don’t stretch it, don’t overtighten clamps, etc.). Your CEMS service provider is in the best position to help re-terminate the ends at the probe and at the CEMS cabinet or shelter.

Sample Conditioner

A sample conditioner is another component that should last 10-15 years. Of course, this is predicated on good maintenance practices, including proper Preventative Maintenance (PM).  Some parts of the sample conditioner, like the sample pump and peristaltic pumps, will be renewed periodically through routine PM.

When sample conditioners fail, it’s usually the electronics of the thermo-electric chiller. Even these parts can be replaced though. Some manufacturers support their units for upwards of 20+ years.

Sample conditioners start at $8,600 and can run upwards of $14,000 for large systems or ones that handle acid gases.

Replacing a sample conditioner is straightforward. The main connections are the sample lines coming in and going out, the power cord, and a signal wire for alarming. They are often mounted in large u-shaped brackets that fit right into the 19” rack. Like a probe, a replacement can be about a four-hour job.


With good maintenance and standard repairs, an analyzer can last 10-15 years and even longer in some cases. Unlike the components above, though, analyzers are something that CEMS owners are forced into replacing due to obsolescence and end-of-life support. Typically, the manufacturer will announce obsolescence when a new model is introduced. Most manufacturers offer seven years of support, parts, and repairs beyond the date of the obsolescence announcement.

Most analyzer companies introduce a new model every 10-12 years. If you bought your analyzer eight years into its active lifespan, you might only have 11 years total before it’s no longer supported. If you end up on the wrong end of the life-cycle timing, CEMS companies (not the analyzer manufacturer) still support and repair analyzers past the seven years as long as they can still get parts. Thankfully, parts are often interchangeable across old and new models.

Of course, there are plenty of other ways analyzers can fail. But if the necessary parts are still available, they can be repaired and rebuilt. Sometimes it can be more cost-effective to replace an analyzer rather than repair it.

Single gas analyzers vary in cost depending on the gas. The price range is typically somewhere between $12,000 & $22,000 for the most common gases (NOx, CO, CO2, THC & SO2).

Replacing an analyzer is a simple job. There are connections on the back for the sample in, sample out, power, and signal wire (4-20mA). The units come with rack mount kits and slides to be placed into the 19” CEMS rack. You can replace a typical analyzer in less than an hour.

Sample/Calibration Control Panel

Sample/ Calibration panels come in two typical varieties. First, you have a 19” rack mounted “case” with all the components inside it. The more common option is a front plate with the rotameters and gauges connected to a slide-out shelf with the solenoids and other components mounted behind.

The first type typically has more electronics built into it and can fail to the point of needing replacement. It’s unlikely that you’ll need to replace the second type. You can replace rotameters, solenoids, and gauges intermittently over the life of the CEMS as needed.  

Data Acquisition System

The DAS is a commonly replaced component of a CEMS. There are various reasons why you might replace a DAS system. Some customers will even switch a DAS due to preference or frustration with the manufacturer.

The DAS is most commonly made up of two components–the PLC (or a Data Logger) and the PC (or server). PLC’s tend to last a long time with very few issues. The need to upgrade or replace a PLC is usually due to obsolescence and the inability to get components. Often just upgrading the main CPU/Processor module is all that’s needed. This is a simple process and eliminates the need for re-wiring the entire system. When customers switch DAS manufacturers, the existing PLC is often utilized to make the upgrade path simpler and more economical.

The other issue with old PLC’s is the DAS manufacturer’s ability to program them. Eventually, they get too old, and the programming language knowledge slowly disappears in the industry.

The PC’s are another story. Microsoft makes sure they need to be replaced more than any other component in the CEMS. Operating systems are constantly evolving. A typical PC is probably good for 3-5 years. As the DAS is part of the compliance system, it’s best to replace it before it crashes and affects your data collection capability. And, as Microsoft moves forward, support for past operating systems goes away, leaving DAS manufacturers forced to keep up, and in turn, forcing their users to keep upgrading. 

A replacement PC frequently costs upwards of $5,000. That cost covers the programming, setup, and data transfer from the old machine to the new one for seamless reporting. A replacement DAS using an existing PLC might run to $35K-$40K depending on configuration and the extensiveness of the data collection and reporting requirements. A complete new system with a new PLC can run upwards of $50k-$75K.

Replacing a DAS is complicated, and a user can’t do this on their own. The actual installation is quick with an existing PLC. You just replace the CPU module and connect the new computer. A lot of programming and configuration work goes into making sure everything works with the CEMS and all the other components.


One important thing to keep in mind with any CEMS upgrade is the potential requirement for a new certification test. A CEMS is certified as a combination of all its components. When a component is changed out, the entire CEMS must be re-certified. That applies to probes, umbilical, sample conditioners, and analyzers. 

One exception is the DAS. In most cases, a DAS replacement works a little differently. A regulatory agency is given a heads-up beforehand and then after installation some formula verifications are run along with a set of reports. That is often acceptable in lieu of a RATA. But of course, all states are different, and it’s best to check with your regulatory agency to see what they expect.


The great thing about owning a modularly built CEMS is that the large initial cost outlay should only happen once. With proper planning through the years, you can maintain a state-of-the-art CEMS with “current” components for a lifetime. Yes, those components still cost a pretty penny, but they can be budgeted. You spend a little every year versus one big major cost. And most of those costs shouldn’t start until 10-years into the life of your CEMS.

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