Process Control & Instrumentation: NDIR as a Tool for Higher Quality Atmosphere Heat Treating
Traditional high-reliability atmosphere carburizing has many tools in the box to produce high-quality parts. This includes oxygen sensors, dewpointers, shim stock analysis and NDIR (Non-dispersive Infrared) systems.
Oxygen SensorThe conventional oxygen sensor measures the oxygen in a furnace, and that value is converted to "carbon potential." The normal process for conversion from oxygen concentration to carbon potential requires numerous assumptions to be made about the carburizing atmosphere. In practice, many of these assumptions (such as CO2 concentration) may not be true. In order to offset the errors introduced by these assumptions, daily compensation with a dewpointer, shim stock analysis or NDIR analyzer is often employed. Probe correction is implemented in the control instrument by means of a calibration factor often termed the "process factor."
Shim StockThe best means to determine carbon concentration is shim stock analysis, as shim stock is a direct measurement that does not assume equilibrium within the furnace. With precise analytical procedures in place, this method yields the best means to determine carbon potential, especially as load sizes and parts processed varies in many facilities. This technique, however, is fraught with potential for errors and bad data resulting from simple human contact.
DewpointDewpointers are used to measure the dewpoint of the furnace gas and thus correct the oxygen probe based upon dew-point/carbon charts. Use of traditional dewpointers is highly subjective and requires training and consistency in personnel taking the readings. Capacitance-type dewpointers are more reliable for readings and measurements. They are, however, susceptible to noise (parasitics) and have poor long-term stability. While both methods are quick and relatively accurate, they require trained users roaming the floor of the facility.
Non-dispersive Infrared (NDIR)NDIR operates on the fundamental principle that all gases absorb light. The absorption occurs at specific wavelengths that are determined by the molecular structure of the gas, and this absorption is directly proportional to the amount of gas present in the sample. For probe compensation, NDIR measures CO, CO2 and CH4 and calculates percent carbon, which is then compared to the probe calculation, and a correction can be made if necessary. Many factors affect absorption, but only gas concentration is desired. The other factors are changes in temperature, atmospheric pressure, other gas concentrations, optical path clarity and detector drift. The key to reliable NDIR is Signal to Noise Ratio (SNR), and this is the toughest part of NDIR. The higher the ratio of absorption caused only by the gas of interest to the absorption from all other sources and electronic noise, the higher the quality of the result.
The type of detector employed in the NDIR analyzers is most typically single-beam; dual-beam, single-path; micro-flow; or dual-beam, dual-path. Robustness and cost typically dictates which type of detector is employed in the analyzer.
Single-beam benches (Fig. 1) are a cost effective means for gas analysis. These rely on frequent "zero" and "span" and add "fudge factors" to correct for temperature and pressure changes. Most single-beam analyzers do not have temperature and pressure compensation, so they require frequent calibration for accurate readings. While an effective bench for portable systems, these analyzers have high drift, with some manufacturers' specifications stating 1-2% drift per 20 minutes.
Single-beam analyzers resolve gases well enough, but complete resolution for CO and CO2 is tenuous with this type of bench, as the overlap is significant. Some manufacturers' specifications state that the cross-sensitivity between CO and CO2 is so significant that a 1% increase in the amount of CO2 present causes a 1.2% greater reading for CO.
Dual-Beam, Single-Path NDIR
The Dual-Beam, single-path bench (Fig. 2) is an improvement in both reliability and resolution over the single-beam bench. For this bench, a "reference" wavelength that is not absorbed by the gas of interest is subtracted from the target gas signal. This provides "common mode" noise rejection and allows for improved temperature and pressure compensation as well as some compensation for electronics drift. Because of the "common mode" noise rejection, this bench has lower drift (1-2% per day). However, daily calibrations (zero/span) are critical for accuracy.
The Micro-flow bench (Fig. 3) is a significant improvement over either the single-beam or the dual-beam, single-path analyzers. A single beam is passed through a sample cell where absorption by the sample of interest occurs, and that beam is then passed through a two-chamber micro-flow detector. The detector contains the gas of interest, and some energy of the IR beam is absorbed, causing pressure increases in both chambers. That pressure differential causes gas flow between the chambers. This flow is detected by a mass-flow sensor and converted to the AC signal. The dual-chamber design allows for sharp filtering at the target wavelength and provides very good resolution between CO and CO2.
The micro-flow NDIR bench has less drift than other benches, typically 1-2% per week, but still requires daily zero/span for accurate readings. The design provides improved temperature and pressure compensation and makes this bench ideal for stationary NDIR systems.
Dual-Beam, Dual-Path NDIR
The dual-beam, dual-path NDIR bench (Fig. 4) is the most accurate design, but it is best suited for the laboratory. It is very costly and may not be as robust as other benches in the heat treatment environment. Light is passed through both a sample and a reference path. The internal reference is a known concentration of the target or sample gas. This internal reference allows for the best temperature and pressure compensation with very low drift (1-2% per week). While daily zeros are still required, the internal reference provides ongoing span.
Portable Versus Fixed SystemsThere are two types of NDIR analyzers employed in atmosphere carburizing: portable and fixed, on-line. Portable analyzers are relatively inexpensive and allow the user to measure multiple points intermittently. Fixed, on-line (stationary) NDIR analyzers are typically more robust and measure multiple carbon zones at fixed time intervals, providing a nearly real-time method for probe compensation.
Portable NDIR Analyzers
Portable analyzers are a cost-effective means to perform manual analyses on the furnace atmosphere and perform process factor corrections throughout a facility. Many units have data logging and wi-fi capabilities. The portable NDIR is reasonably accurate with regular span and frequent zeroing to compensate for the higher drift of the benches typically incorporated into these units. Because portables typically employ single-beam, single-path benches, they have poor pressure and temperature compensation, making regular zero and spanning a requirement for acceptable data. Many portable units require regular factory calibrations about every 12-18 months and cell replacement about every 2-5 years. Keeping a spare in a facility where a portable is the only means for probe compensation and atmosphere verification may be necessary. A drawback to portable units is soot accumulation in the sample filter. Some analyzers are outfitted with very small filters requiring frequent replacement, and thus a large capacity filter is recommended.
Fixed, On-line (Stationary) NDIR Analyzers/Systems
A stationary NDIR system automatically corrects multiple carbon zones within a facility. While more costly than portable units, these are typically more robust and configured to provide continuous feedback to the controllers, providing fully automated sample, measurement and correction capability. Due to the very nature of the stationary NDIR system, better quality benches are often used, providing better accuracy and less drift. While not all manufacturers include this feature, it is recommended that any stationary NDIR system include automatic zero and span capability to ensure accurate readings. Some units have automatic leak checking and self-diagnostics with alarms that keep operation very simple.
Because on-line NDIR analyzers are in continuous operation, maintenance requirements are higher than for the portable units. Continuous use means faster filter saturation and more frequent changes; on average every 30 days. Stationary systems have higher initial costs for the equipment and require ongoing maintenance, including the costs for calibration gas. As these systems correct for multiple carbon zones, the use of longer sample lines is often required, making it necessary to filter the sample better at the furnace as most clogs occur in the first few feet of tubing.
ConclusionsChoosing the most accurate means to determine carbon potential and compensate for oxygen probes can be a daunting and expensive task. Shim stock analysis is reliable, but time consuming and prone to personnel errors. Dewpointers are affordable, but have their problems.
NDIR has tremendous promise and widespread usage, but requires training and maintenance. In order to be satisfied with the performance of NDIR in atmosphere carburizing, the user must beware of slick sales jargon. As stated above, all NDIR benches require zero and spanning for accurate results. Calibration of the detector and the cell response are essential to the proper operation of any NDIR bench, as it is an analog measurement that can be converted to digital signal at the detector. For good results, understanding the drift specifications is critical. Drift is a function of temperature, pressure and time, and if the specification does not provide a "per unit" reference, the user is incapable of proper calibration and, all results are suspect. A user of NDIR is best advised to always know the manufacturer of the bench in the system - whether portable or fixed.
Even with the apparent drawbacks and difficulties, NDIR can provide a reliable alternative to shim stock analysis. All NDIR can provide reliable data and be used for probe correction provided the user is aware of the calibration (zero/span) requirements.
For more information: Yvonne H. Spooner is Vice President and Eric S. Boltz is President of Marathon Sensors, Inc., 3100 E. Kemper Rd., Cincinnati, Ohio 45240. ph: (513)-772-1000; fax: (513)-326-7090; e-mail: firstname.lastname@example.org
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: NDIR, oxygen sensor, shim stock, dewpoint, carburizing atmosphere, signal to noise ratio, carbon potential, oxygen concentration