You may gain insight into the performance of your process equipment by taking independent temperature measurements. Though the controller on the oven wall tells you that temperatures are at standard levels, you may find that a temperature sensor has drifted or failed. If the sensor fails by opening up electrically, your controller will alert your production team quickly. But if it drifts, becomes covered with debris, or moves into contact with some warmer or cooler item in the oven, the controller may try to regulate the temperature with the sensor in that new condition. This will change the temperature in the working zone of your equipment. Without a measurement from a separate temperature instrument, you may notice indirectly that the heat supplied to other zones has changed, or your finished product may show some effect. But, it is usually less costly to detect a drift in process temperature through an independent measurement.
If you shut down your equipment on nights or weekends, some part of your valuable time is occupied by the warmup of your process equipment. Logging the process can help you find the minimum time necessary to ensure that conditions are stable enough to get consistent product quality from the process. Waiting only until the controllers are at setpoint may not be enough.
Benchmarking your process involves establishing a test procedure and equipment that allow you to test your process equipment, then go back and repeat the test later to see what has changed. This can be helpful when you are preparing to replicate your production process at another production line. You may find that other equipment cannot duplicate the conditions in your current line, and you can find out how they differ. If you need to substitute or upgrade a piece of equipment in your current line, you can see how the temperature history changes. When you need to increase production rates by increasing product flow, or when you change raw materials, having a before-and-after temperature history comparison can help you confirm that the process is meeting specifications. If there are problems, benchmarking your process while it is operating well can help you localize them. Temperature sensor errors can occur suddenly and often can be pinpointed on a temperature history comparison. Process feed materials can change characteristics such as moisture content, and that may be reflected by changes in temperature. Air distribution can be changed through loosening of fans or movement of dampers and other deflectors, and through operator adjustment. Comparison of an imperfect temperature history to a known-good condition is vital to troubleshooting these defects.
Standard Tools for Temperature TrackingTraditionally, an operator keeps an eye on the control readings, and any problems are brought to the attention of maintenance personnel. A standard temperature instrument and probe can be used to verify readings on the controls and correlate them to product temperatures, and a clipboard might be used to record readings for a time to see if the system cycles or if it responds poorly to loading. This commits a person to observing the process, which may have a side benefit if some unexpected condition is discovered at the same time. Unscheduled opening of inspection ports or outside doors have been known to disturb tricky heating processes, and problems of that sort are easier to track down if someone notices the relationship as it happens. The paper on the clipboard is able to collect any information worth keeping as a free-form note in the margins.
Dedicated logging instruments can be used for unattended monitoring. Graphical chart recorders are the old standard. These can be useful for revealing trends and cycling, but the data is recorded in a paper format that must be digitized by "picking off" points if more analysis is desired. Digital versions of chart recorders can export data to a computer, and that enables further examination. Most of these instruments are powered from an AC line. Digital temperature instruments with logging capabilities can be battery powered or line powered with an AC adapter. Some log to paper tape while others store data for export later. These instruments can record temperature at the interval necessary to see details of interest. Instruments are available with one or several channels.
Computer-based temperature logging tools have been available for several years, and the software has become easier to use. These systems offer many channels at moderate cost but tie up a computer for the testing. Additionally, providing AC power to a location on a manufacturing line can be an issue when using a desktop machine as a testing platform, and laptops typically offer less than two hours of operation before they need to be connected to the AC line.
Sometimes in-process temperature profiling is the best solution. This class of loggers includes a protective enclosure that is compatible with the process environment -- at least for a time sufficient to allow the equipment to enter and be removed from the process. The instrument typically is connected to a computer for programming of sampling interval and duration, run through the process with product while collecting temperature information, and then reconnected to the host computer to retrieve the temperature history. If your process is conveyorized, it may be the best choice. Likewise, if it is difficult for temperature sensors to reach from a safe external location to the working zone of your process, in-process profiling may best meet your needs.
A New ToolMiniaturization of electronic components has enabled the development of the personal digital assistant (PDA). These simplified computers come with a choice of several operating systems. Devices running the Palm operating system (Palm OS) and Windows Pocket PC dominate the U.S. market. The Palm OS is designed to do a reduced set of tasks efficiently while the Pocket PC devices have more features and, therefore, require more system resources like faster processors and more memory. These extra features are not particularly important for data collection applications. Both systems have loyalists, and both can be used as platforms for data collection and temperature logging. Battery life is better on devices running Palm OS, and many users find that to be the most important factor in selecting a platform. Rechargeable batteries in either Palm OS or Pocket PC devices reduce battery cost, and units using standard AAA alkaline cells are always ready to go as long as spares are handy.
A central feature of these handheld systems is the ability to easily transfer data between the PDA and a desktop computer. They all provide a system to create duplicate data sets. Palm OS devices use the term "HotSync" to denote this process. In general, a small program runs in the background on the host computer, and a cradle or cable is connected to its serial or USB port. After connecting the handheld to the host computer, a single pushbutton or screen click starts a comparison of data between the devices. Any new information on the handheld such as test data is copied into a folder on the host computer.
While many of us are comfortable using PDAs as address books and memo pads, they are self-contained computers that are capable of running other software and supporting other hardware accessories. One of these is a temperature input module, which can be attached as a clip-on accessory or by a cable. Thermocouple types J, K, T and E are supported and units of oF and oC. Accuracy can be as tight as +/-1oC +0.1% of reading, and the temperature range possible includes -328 to 1,830oF (-200 to 999oC). Several models are available for compatability with different PDAs, offering from one to four channels.
The combination of handheld device and temperature monitoring accessories creates a powerful instrument. Software running on the handheld computer defines much of the way the device operates and looks, and the resources of the PDA are available to give added functionality at reasonable cost. The full graphics display of the PDA can show temperature data and program information. The operator can enter text by using a simple miniature keyboard on the PDA screen or a stylus and a slightly modified alphabet. This enables documenting conditions as you start a test when significant variables are clear in your mind. Adding text notes to the description of a data set helps you remember the test conditions later. Generally, handheld computers have ample memory to allow you to store many temperature measurements on the system before data is transferred to a desktop computer, and some programs support multiple tests.
One software program allows basic temperature display. Each time a button is pressed, the handheld device beeps and the current temperature is saved to memory with time and date stamping. Minimum, maximum and average temperatures for each channel are available on the Palm, and the complete dataset is available after synchronizing the device with the computer.
Another software program creates a programmable datalogger with selectable sampling interval. From one to four channels may be logged, depending on the hardware being used. Recording may be started by pushing a button after a time delay or after a rising or falling temperature threshold is passed. For example, connected to a temperature-tracking module, the Palm IIIe handheld with a set of fresh AAA cells will log temperatures every 15 sec for 60 hr. Software provides for a graceful shutoff if the batteries run out before a logging test ends. Of course, the test will end before the intended stop time, but the sampled data is retained in memory for several days.
The temperature logging software uses the PDA's synchronization process to transfer data files whenever the Palm OS device is connected to the computer. Once the data files are on the machine, they can be stored or examined in greater detail. The temperature-tracking products capitalize on the familiarity many users have with standard office software. Data can be imported to view in spreadsheet applications. Their graphing tools allow easy examination of data to observe trends and correlations. Also, using these tools allows easy insertion of graphics into word processing documents or e-mail reports. Database programs also can be used to manage temperature data.
Using a temperature module with a PDA offers advantages over other temperature-logging approaches. It may be lower in cost and more portable than other computer-based solutions and more intuitive and flexible than instruments without a graphical interface. The handheld computer is available for other duties when not being used as a temperature logger. In addition to the built-in address book and memo functions, other hardware devices are available such as calibration modules with models for temperature, pressure and other parameters.
Though not a factor for most applications, using temperature acquisition tools running on handheld computers also allows advanced users, writing their own custom software, to take advantage of wireless hardware adapters. Temperature information can be sent back to a company data network in real time if necessary.
The PDA's touchscreen and text and numeric input tools enable other information to be captured in addition to temperature. Desktop software allows building forms that can be used on the PDA for repeated inspections with pushbutton temperature input. It also allows an operator to note other conditions by checking appropriate boxes or selecting items from a list to describe the appearance of product exiting a finishing line. Other numeric information such as readings from process controllers and monitors can be entered by the operator using the keyboard or stylus. Each data field is date and time stamped when information is entered. The system becomes an electronic clipboard that can be programmed with the information on inspection forms already in use, plus direct input of temperature readings.
Sidebar:Ben Harmes, process engineer at Displaytech Inc., Longmont, Colo., manufacturers precision LCD displays. Temperature history affects product quality in very visible ways. He says, "The oven controller display tells you everything you need to know except what really goes on with your parts. Some of our controller programs are quite complex, and the temperature changes must occur at precise times to maintain the integrity of our assemblies. For that we find the [temperature tracking software and PDA] to be a valuable tool for setup and quality control. Engineers from other departments borrowed mine to monitor some of their testing and found it to be so versatile and accurate that they purchased their own for their departments.
A User Gives His Perspective
"It is easy to use, reliable, gives good battery life, remarkable accuracy and can accommodate many different types of thermocouples. We purchased one unit originally and it solved so many problems that we were having with some of our processes that I purchased one more for my department so I could adjust all of the air intake and exhaust dampers on the ovens in the final assembly area. This has given us consistent and repeatable results using the same programs on each oven."
Sidebar:The versatility of the temperature module combined with the handheld personal digital assistant has made it useful in applications as varied as:
Can a PDA Help Your Application?
- Testing temperature rise during adhesive cure.
- Testing substrate temperature for LCD screens during fabrication.
- Tracking temperature of auto racing tires to match the suspension adjustments to the track and conditions.