In the field of industrial instrumentation and process control, the word analyzer generally refers to an instrument tasked with measuring the concentration of some substance, usually mixed with other substances of little or no interest to the controlled process. Unlike the other “bulk” measurement devices for sensing such general variables as pressure, level, temperature, or flow, an analytical device must discriminately select one material over all others present in the sample. This single problem accounts for much of the complexity of analytical instrumentation: how do we measure the quantity of just one substance when thoroughly mixed with other substances?
Analytical instruments generally achieve selectivity by measuring some property of the substance of interest unique to that substance alone, or at least unique to it among the possible substances likely to be found in the process sample. For example, an optically based analyzer might achieve selectivity by measuring the intensities of only those wavelengths of light absorbed by the compound of interest and absorbed by none of the other wavelengths. A “paramagnetic” oxygen gas analyzer achieves selectivity by exploiting the paramagnetic properties of oxygen gas since no other industrial gas is as paramagnetic as oxygen. A pH analyzer achieves hydrogen ion selectivity by using a specially prepared glass membrane intended to pass only hydrogen ions.
Problems are sure to arise if the measured property of the substance of interest is not as unique as originally thought. This may occur due to oversight on the part of the person originally choosing the analyzer technology, or it may occur because of changes made to the process chemistry, whether by intentional modification of the process equipment or by abnormal operating conditions.
For example, a gas that happens to absorb some (or all!) of the same light wavelengths as the gas of interest will cause false measurements if not properly compensated for in the analyzer. Nitric oxide (NO) gas is one of the few gases also exhibiting significant paramagnetic, and as such will cause measurement errors if introduced into the sample inlet of a paramagnetic oxygen analyzer. A pH analyzer immersed in a liquid solution containing an abundance of sodium ions may fall victim to measurement errors because sodium ions also happen to interact with the glass membrane of a pH electrode to generate a voltage.
Modern process control requires current information on the state (composition, temperature, pressure, flow rate, etc.) of the material being produced. Modern distributed control systems linked to various control elements (valves, pumps, etc.) require data much more often than the manual control systems of years gone by. While grab samples taken to a modern laboratory will be the gold standard for quality control purposes, the effort and time it takes to get a sample, transport it to the lab, and wait for the result can add significantly to the costs of the process in process output, energy, etc. For many industries, the solution is to move the analysis directly to the process.
This need for more and better analyzers does not mean that their application is, or ever will be, a simple, routine task. For an analyzer system to fulfill its expectations, careful planning and evaluation must precede its purchase, and the users must realize that if an expensive analyzer is worth purchasing, it must also be worth calibrating and maintaining. The most important aspect of an analyzer application is a clear understanding of why the analyzer is needed, what information is needed, and what will be done with the information. Put another way, the analyzer problem must be clearly defined and understood. Too many analyzer applications have failed because the needed measurement was not the one installed in the process.
In general, the instrument engineer (analyzer engineer, process analytical chemist, etc.) is asked by a process control engineer to install an analyzer in a process. It is the analyzer engineer’s duty to his company to push back and ensure that the application is needed and clearly understood. Merely doing what is asked, in many cases, will lead to unnecessary or even failed installations.
The Selection Process: The process to follow in selecting the appropriate analyzer for an application involves eight steps:
1. Problem definition
2. Information gathering (to understand the problem fully)
3. Analyzer selection (making sure to consider and balance these criteria)
- Specificity
- Accuracy and precision
- Calibration
- Analysis frequency
- Kind of analyzer
4. Sampling
5. Analyzer location
6. Handling of the data
7. Maintenance issues
8. Total cost: hardware, installation, maintenance, etc.
It is not uncommon to cycle back from one step to another, as more information is needed to assure that the proper decisions are made. Installing the wrong kind of analyzer or putting it in the wrong place will not solve the problem, and instead of saving money or improving quality, money and effort will be wasted.
Instrumentation Basics 