Measuring Temperatures in Furnaces with Oxidizing Atmospheres

Measuring temperatures inside a furnace presents several challenges: high temperatures, temperature cycling, and hostile atmospheres exceeding the limits of many measurement devices while others have greatly reduced lifetimes and poor accuracy.

This White Paper from OMEGA Engineering explores temperature measurement in furnaces. Particular emphasis is placed on the challenges associated with types of oxidizing and reducing atmospheres in furnaces used in microelectronics fabrication. After studying this paper, the reader will have a deeper appreciation for the challenges and the options available.

Contents:

  • Furnaces Overview
  • High Temperature Measurement Options
  • Thermocouples
  • Infrared Pyrometry
  • High Temperature Thermocouples
  • Sheaths for Thermocouples
  • Thermocouple Insulation

FURNACES OVERVIEW

The need to heat is common to many manufacturing processes. Rubbers and adhesives are cured, metals are annealed to modify their metallurgy and properties, coatings are dried, metals are melted, and ceramics are red or vitri ed.

Many of these processes are carried out in ovens, heated either by electricity or gas. An oven that can heat to above 1000°C (1832°F) is termed a furnace. A kiln is a particular type of furnace used in ceramics.

At high temperatures many materials start to react with the surrounding atmosphere. If that atmosphere is very short of oxygen, it may pull oxygen from the material being heated. Such an atmosphere is termed “reducing”. Gas heating usually results in an oxygen-de cient atmosphere. If the atmosphere is oxygen-rich, the material being heated will capture a proportion, forming an oxide layer. Such an atmosphere is termed “oxidizing.” This is the process employed in diffusion furnaces used in microelectronics fabrication to produce SiO2. Electrical heating is more likely to produce an oxidizing atmosphere.

Control of the atmosphere can be achieved in several ways. Gas may be piped into the chamber, which might be done to create an inert atmosphere. Alternatively, a vacuum furnace could be used.

HIGH TEMPERATURE MEASUREMENT OPTIONS

The upper limit for thermistor devices is around 100°C (212°F) and RTDs are limited to around 750°C (1382°F). That leaves thermocouples and infrared pyrometers or imagers as the most suitable devices for measuring temperatures above 1000°C(1832°F).

THERMOCOUPLES

Thermocouples utilize the Seebeck effect (the difference in EMF between dissimilar metals) to produce a signal proportional to temperature. Nickel-chromium and nickel- alumel are the metal pairs most commonly used in what is called the “Type K” thermocouple.

The Type K is inexpensive and can be used across a temperature range from -200 to 1250°C (-328 to 2282°F). However, metallurgical changes at temperatures above 1000°C (1832°F) reduce accuracy, and cycling through this temperature induces hysteresis effects, further reducing accuracy. Type K thermocouples are also vulnerable to corrosion in an oxidizing atmosphere.

Thermocouples can be damaged or fail in-service, requiring replacement. If this entails shutting down and cooling a continuous furnace it can be a dif cult and expensive undertaking. For this reason it’s common to include redundant thermocouples throughout the heating chamber.

IR PYROMETRY

Infrared (IR) pyrometry presents a convenient contactless method of measuring high temperatures. This technology takes advantage of Planks Law, whereby the wavelength and intensity of the IR radiation emitted from a surface is proportional to its temperature. A pyrometer or thermal imager detects this radiation, converting the signal to a temperature.

IR pyrometry works well when the surface of the hot material is exposed, as with molten metal in a ladle. Using it to measure temperatures inside a furnace is more dif cult, as it needs to be viewed through a window. This window must transmit IR radiation of the wavelength corresponding to both the sensitivity of the detector and the temperature being measured.

Regular glass is opaque to some IR wavelengths, particularly between six and seven microns. Chalcogenide glass is manufactured especially for IR transmission applications but is limited to temperatures below around 370°C (698°F). Sapphire is an alternative window material that transmits wavelengths up to four microns but is relatively soft and easily damaged. When a sapphire IR window is used as a viewing port it should be designed without any projections that would make it vulnerable to damage. Sapphire also has a temperature limit of around 450°C (842°F), making it unsuitable for furnace applications.

Emissivity is always an issue with pryometry: different materials at the same temperature radiate different intensities of IR radiation and the sensor must be calibrated for this. The window will have an in uence on the radiation transmitted.

HIGH TEMPERATURE THERMOCOUPLES

Two families of thermocouples are available, those using tungsten-rhenium junctions and those of platinum-rhodium. Thetungsten-rheniumthermocouples,(TypesG,CandD) operate at temperatures as high as 2320°C (4208°F) but will not survive an oxidizing atmosphere.

For oxidizing atmospheres platinum-rhodium thermocouples, sometimes referred to as “noble metal thermocouples,” should be selected. These are available as Type R, [maximum of 1460°C (2660°F)] S, [maximum of 1450°C (2642°F)] or B, [maximum of 1700°C (3092°F)]. They are more expensive than base metal thermocouples.

SHEATHS FOR THERMOCOUPLES

Depending on the installation, it’s common to protect thermocouple wires by placing them inside a protective tube or sheath. Stainless steel is widely used as it is inexpensive and resists corrosion. However, it has a melting point of around 1400°C (2552°F), limiting service temperature to under 1100°C (2012°F) and reacts with oxidizing atmospheres.

For highest temperature capabilities, consider using either tantalum or molybdenum sheaths. These will go up to 2315°C (4199°F) and 2200°C (3992°F) respectively, although both are sensitive to oxidation, so should not be used in oxidizing atmospheres. The alternatives are ceramic sheaths, which will withstand up to 1960°C (3560°F), platinum-rhodium alloy sheaths, which will withstand 1650°C (3002°F), or Inconel® 600, which goes up to 1150°C (2102°F). All of these can handle oxidizing atmospheres.

THERMOCOUPLE INSULATION

Insulation is incorporated into a thermocouple sheath to keep the wires from contacting the sides. This insulation must have a temperature rating appropriate to the environment. Common materials for furnace temperatures are alumina, magnesia, and hafnium oxide. Alumina has a maximum temperature rating of 1540°C (2804°F) while magnesia and hafnium oxide will go to 1650°C (3002°F) and 2500°C (4532°F) respectively.

TAKEAWAYS

Thermocouples are a good option for measuring temperatures inside furnaces. While the widely-used “Type K” thermocouples will handle furnace temperatures, better performance is offered by Types G, C and D and R, S and B. At furnace temperatures, the type of atmosphere employed becomes an important consideration. In particular, an oxidizing atmosphere, as used in microelectronics fabrication, will cause a reaction with both Types G, C and D and the stainless steel sheaths often employed.

IR pyrometry is an alternative for measuring high temperatures, but requires a viewport or window to measure inside a furnace. For this reason it is generally preferred when there is an uninterrupted line-of-sight.

Factory Air Quality : Monitoring as the First and Last Step in Risk Management

Ambient air, that you breathe every day, contains life supporting oxygen, nitrogen, carbon dioxide, and chemicals like formaldehyde or carbon monoxide that can harm you.

Characteristics of that air, like temperature, humidity and barometric pressure, determine the comfort to humans at work in that space.

Now, start with that air and add the chemicals used in manufacturing, and then concentrate them in a con ned space like a factory. The resulting air can become even more uncomfortable and toxic fairly quickly unless properly managed.

The health effects of ingesting chemicals include:

  1. Irritation—mucous membranes (eyes, nose, throat)
  2. Strained breathing—coughs, wheezing, chest tightness
  3. Existing health problems become worse
  4. Increasing risk of heart disease, cancer and kidney and liver diseases

Healthy adults in the workplace can tolerate lower levels of carbon monoxide and formaldehyde, for example. But, they can bring home these chemicals, attached to their clothing, to infants and children.

Toxicity levels are relative to the mass of the person ingesting the toxin. Even if the adult tolerates the mass breathed, their child may not tolerate that mass when the child is fteen percent of their parent’s mass. Further, formaldehyde is more dense than ambient air, it sinks to a child’s breathing level. For example: formaldehyde off-gassing from ooring is a health and safety concern, especially for children.

RISK MANAGEMENT

Risk management surveys begin with de ning the current situation. Factory air risk management begins by monitoring the air and de ning the current situation. The air can be tested to satisfy OSHA, health studies for workers’ compensation, HVAC temperature monitors as well as safety concerns for extreme conditions.

The rst step is monitoring. The data is collected, analyzed, and interpreted. Issues discovered, and solutions implemented. The nal step is to monitor on an ongoing basis to assure ef cacy of the solution.

Proper monitoring should result in solutions that assure health and increase safety.

IMMEDIATELY DANGEROUS TO LIFE AND HEALTH (IDLH)

Baseline monitoring includes IDLH substances. Although you may think you do not have these issues, unfortunately, chemicals mix in the air. Do you have bleach and ammonia fumes? Several explosives and poisons can be formed.

Identify and quantify contaminants through a professionally designed and executed monitoring program. Airborne contaminants present a major pathway for disease. Proper monitoring will point to proper:

  1. Selection of personal protection wear and ltering systems
  2. Delineated mandatory protection areas
  3. Risk to health from exposures
  4. Medical monitoring
  5. Decontamination requirements at the day’s end

FORMALDEHYDE

Formaldehyde is a carcinogenic category 2, which means a non-lethal, but irreversible effect after a single exposure. The irreversible effects include:

  1. Damage to the central nervous system
  2. Kidney necrosis
  3. Liver lesions
  4. Anemia
  5. Paralysis

In 2015, The Formaldehyde Act under the Toxic Substances Control Act (TSCA), extended regulations to reduce emissions of formaldehyde from composite wood products, like ooring products used in the course of scienti c research and other professions where tissue is preserved in some manner.

A Formaldehyde Monitor and Data Logger is an ideal instrument for indoor air quality (IAQ) diagnosis and HVAC system performance veri cation.

CARBON MONOXIDE

When workers breathe carbon monoxide, it moves to the bloodstream as oxygen does; however, carbon monoxide binds to hemoglobin to prevent oxygen from doing so. Less oxygen in the blood leads to less oxygen in the brain, heart and other vital organs.

Symptoms include fatigue and confusion in healthy employees. These symptoms render employees unsafe, unable to respond rapidly or make clear decisions. Vision, mental alertness and productivity suffer.

Carbon monoxide is particularly cruel to employees who already suffer heart disease, experiencing more chest pains or angina.

There are a variety of Carbon Monoxide Monitors and Data Loggers available to detect indoor air quality (IAQ) diagnosis and HVAC system performance veri cation.

PARTICULATE MATTER

Dust and particulate carry other pathogens and irritants too. Keeping solids out of the air supply requires sophisticated capture and containment. Proper monitoring to determine the ltering requirements is the first step.

In any manufacturing, but particularly high technology, particulate management is a quality assurance—quality control issue. Use a portable instrument to measure and report air contamination and download data to a personal computer using the USB interface cable.

COMFORT CONTROL

Comfort creates productivity. Too hot or cold, low oxygen, high humidity environments are just distracting, or worse, sleep inducing.

Modern buildings attempt to “positive pressure” the indoor environment to prevent mold spores from entering the building through leaks in the envelope.

Increased pressure actually raises the ambient temperature as well. A balance between temperature control and barometric pressure control must be designed into the system. Balance is key. A wide variety of instruments are available for indoor air quality (IAQ) diagnosis and HVAC system performance veri cation. Measure O2 concentration, pressure, air temperature and relative humidity.

HVAC design requires sophisticated engineering since the components of air and the dynamics of air ow can be incompatible. Humidity removal affects temperature and pressure. Positive pressure on the building maintains control of outdoor pathogens from entering.

Cooler air holds less water, but that raises relative humidity if the air was not saturated before it cools. Oxygen levels should be high while carbon dioxide release should be low and carbon monoxide emission eliminated or directed outside.

HVAC design and balance is critical, and begins and ends with monitoring.

OCCUPATIONAL SAFETY and HEALTH ADMINISTRATION (OSHA)

The OSHA mission began with indoor working conditions. Embrace the mission and use their training tools that are available to the public through the OSHA website.

One of their most useful services is Spanish language materials, especially those concerning technical data like tting personal protection gear, or the necessity to wear it. Be sure everyone understands the system from gearing up to decontamination.

RESOLUTION

Remember: Outsource your monitoring needs to the professionals at OMEGA Engineering.

Founded fty years ago, OMEGA Engineering Inc. began manufacturing thermocouples and has grown to be your one- stop source for process measurement and control.

Now a global technology leader with more than 100,000 innovative products for measuring and controlling temperature, humidity, pressure, strain, force, ow, level, pH, and conductivity.

OMEGA offers a complete line of data acquisition, automation, electric heating, and custom engineered products for use in manufacturing, test and research environments.

Trust OMEGA as hundreds of major manufacturing companies do.

Temperature Monitoring During Transportation, Storage and Processing of Perishable Products

In transporting perishable, product it’s becoming ever more important to provide records of the temperatures, and in some cases, humidity, to which cargo has been exposed. This White Paper provides some background to temperature and humidity logging in logistics and discusses equipment options. After nishing this paper the reader should have an appreciation of the reasons for capturing temperature and humidity data and an understanding of the equipment available and how it can be used.

Individual sections address:

  • Background to temperature monitoring
  • The cold chain
  • Cost and liability issues
  • Recording temperature and humidity
  • Portable electronic temperature and humidity data loggers
  • EudraLex/FDA compliant temperature loggers
  • Loggers carried by OMEGA

BACKGROUND TO TEMPERATURE MONITORING

Many products deteriorate quickly at room temperature, and faster still as temperatures rise higher. Food items such as fruit, meat and sh cannot be shipped long distances without refrigeration. In some instances chilling to 13°C (55°F) is suf cient for produce to be sold fresh (or that does not freeze readily) such as bananas. Eggs are another example, where humidity control is also required. Other products need to be frozen to prevent deterioration. The same is true for many pharmaceutical and medical products. The Healthcare Distribution Management Association estimates that some 10% of drugs are temperature sensitive. Vaccines and blood products are other examples of medical products needing strict temperature control, although these are sometimes subjected to freeze-drying through the lyophilization process.

The need for temperature control extends beyond products for human consumption. Some specialist paints or coating materials will be damaged by exposure to low temperatures. Other chemicals must be kept within strict temperature limits during transportation.

Another area is the shipping of antiquities and works of art. For these humidity can be a bigger concern than temperature as it can lead to mold, especially if the two are combined.

THE COLD CHAIN

Perishable products must be kept at a controlled temperature, from point of origin to delivery to retailer or pharmacy. The logistics industry refers to this as the “cold chain” and it encompasses both “reefers” (refrigerated containers) as well as warehouses, distribution centers and the nal storage or holding areas.

Throughout this chain the risk of failure is ever-present, meaning there is always a possibility of cargo exceeding permissible or safe temperature levels, even if only brie y. For example, a truck might be stopped without power in desert heat, allowing temperatures in the reefer to rise. Then power is restored and the temperature in the container comes back down, but the product is damaged.

COST AND LIABILITY ISSUES

When cargo such as any of those items listed above are exposed to temperatures outside of prescribed limits it can be damaged. In some cases this is evident, such as with bananas, but in other situations, like the transport of vaccines, it may not be apparent that damage has occurred and the vaccine becomes ineffective. For some products, going over temperature, even only brie y, can reduce shelf life dramatically, incurring substantial costs when it cannot be sold.

Organizations contracting to ship perishable products often specify the permissible temperature range. However, even if it is possible to show that product was exposed to conditions outside of those contracted, proving where it happened, and thus responsibility, can be much harder. The answer is to create a time-based record of the temperatures experienced.

RECODING TEMPERATURE AND HUMIDITY 

The maximum-minimum thermometer was invented around 1780, and for a long time was the only means of recording the highest and lowest temperatures observed over a given period. Besides employing mercury as the expansion medium, its other weakness was the absence of any time record. As a result, while such a thermometer could be included in a shipment of fruit there would be no way to know when or for how long the peak temperature was experienced. Without this information it’s dif cult to determine responsibility for any spoilage.

The same applies to humidity indicators. Various types are available, most of which utilize some color-change effect to show either the peak humidity experienced or the current humidity. As with the max-min thermometer though, there is no time record to support claims for negligence.

PORTABLE ELECTRONIC TEMPERATURE AND HUMIDITY DATA LOGGERS

Advances in digital electronics led to the portable data recorder. Battery powered, these are small enough to be placed next to or among perishable cargo where they log the temperature or humidity. They can be set to create a record as frequently as every second or as infrequently as hourly. Some even allow an 18 hour interval. The only limitation is the number of data points that can be stored, so if set to record at a high rate they will run out of space sooner.

Temperature accuracy is typically ±0.5°C and humidity ±3.0% RH. They can be programmed to start after a delay period. Once recovered they can be plugged in to a PC and the data transferred into Excel® or any other package for analysis. Alarms can be set to illuminate LED indicators if maximum or minimum limits have been exceeded, making any potential problem immediately visible.

EUDRALEX/FDA COMPLIANT TEMPERATURE LOGGERS

Under CFR 21 the FDA requires that drugs be stored at appropriate temperatures. Loggers can be used to demonstrate compliance with this standard, and electronic records are admissible under 21 CFR Pt.11. (Within the EU, EudraLex Volume 4, Annex 11). The FDA also expects that temperature monitoring will be carried out during the lyophilization process when applied to materials of a biological origin.

LOGGERS CARRIED BY OMEGA

OMEGA offers a range of temperature and humidity loggers for various applications. The compact OM-90 series cover a range from minus 30 to 80°C (-22 to 176°F) and humidity from 0 to 100% RH and can log some 65,000 measurements.

Intended speci cally for monitoring the transport of eggs, the OM-CP-EGGTEMP series are egg-shaped for easy inclusion in shipping packages and operate over a temperature range of 0 to 60oC (32 to 140°F) and 0 to 95% RH. Over 32,000 readings can be held in memory.

The lyophilization process demands very low temperatures, so the OM-CP-LYOTEMP was designed with an operating range of minus 60 to 75°C (-76 to 167°F). This logger connects to a PC via USB docking station and the purpose-written software includes features such as mean kinetic temperature recording.

For situations demanding multiple data sets, such as when several freezers are used for storage, OMEGA offers the four channel OM-DVT4. Accepting inputs from three external probes plus the internal sensor, this has an LCD display showing all four temperatures along with alarm status, memory remaining, sample status, and other settings.

An alternative approach for cold chain temperature monitoring is the single use temperature recorder. OMEGA offers both the OM-CP-TRANSITEMP-EC and the OM-21 data loggers. Compact and inexpensive, these are intended to be loaded in with the cargo at the point of origin and retrieved on nal delivery. Their operating range is minus 20 to 70°C (-4 to 158°F) [minus 30oC (-22oF) for the OM-21] and as with the other loggers described, the data gathered may be easily transferred to a PC for review. (The OM-21 actually prepares the data in pdf format and behaves like a USB ash drive).

TAKEAWAYS

A wide range of products, from vaccines and chemicals to eggs and works of art, must be shipped under controlled environmental conditions. Portable temperature and humidity loggers provide a time-stamped record of the conditions encountered in transit and are an invaluable source of evidence should claims be made for loss or damage.

In the pharmaceutical eld FDA regulations insist that drugs be stored at correct temperatures and that lyophilization be performed under controlled conditions. Logging of temperature and humidity provides evidence of compliance, and if performed correctly, will meet FDA requirements for electronic record-keeping.

The latest generation of temperature and humidity loggers are compact and inexpensive, making them easy to include with shipping or transport packages. Accuracies are typically within 0.5°C and batteries are long-lasting. Such technology is enabling dramatic growth in temperature logging, helping ensure the safety of supply chains and products intended to maintain human health.

European Sales and Service

OMEGA offers international sales and distribution while servicing the expanding European marketplace from our UK office based in Manchester. We have multilingual sales staff well-versed in worldwide trade. International payment conveniences such as credit cards, bank transfers, and acceptance of local currencies make it easy for customers around the globe to work with OMEGA.

Precision Manufacturing

Our commitment to maintaining the leading edge through research development and state-of-the art manufacturing keeps OMEGA firmly at the forefront of technology. OMEGA’s Development and Engineering Center (ODEC), is home to OMEGA’s design and engineering laboratories. All product designs are tested and perfected here prior to marketing. This state-of-the-art facility houses OMEGA’s metrology lab and other quality control facilities. The testing that takes place here assures that you receive the best products for your applications. Once an OMEGA product design is perfected and tested, stock production takes place at our Manchester manufacturing facility. This site houses OMEGA’s advanced OMEGACLAD thermocouple wire production equipment along with a host of other computerised CNC milling machines, injection molding equipment, screw machines, braiders, extruders, punch presses and much more.

Custom Engineering

Omega Engineering proudly offers the most sophisticated and extensive Custom Engineering capabilities in the process measurement and control industry. Whether you need a simple modification of a standard product or complete customised system engineering, OMEGA can accommodate your special request.

Calibration

OMEGA is constantly striving to find new ways to increase the level of service available to our customers. To this end, OMEGA continues to expand the range of calibration services available, offering a broad selection of standards for use in calibrating temperature, infrared humidity, pressure, flow, and force products. By maintaining these standards in-house, we can ensure fast turnaround time on calibrations.

All calibrations are performed by our technicians. Contact OMEGA’s Customer Service Department (0161 777 2225) to discuss your specific calibration requirements.

  • Temperature, Infrared
  • Humidity
  • Pressure, Strain, Force
  • Flow
  • Five Different Calibration Levels
  • ISO9001 Corporate Quality
  • ISO10012-1, ANSI/NCSL Z540-1-1994
  • Global Calibration Services Available
  • Loaner Units Available

email sales@omega.co.uk

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