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Comparison Between Thermal Imager and RF Moisture Profiler

Sensortech Systems, Inc., founded in 1983, has been the World leader in gypsum board moisture measurement for more than three decades. Based on a patented “Resonant Frequency” measurement principle, Sensortech gauges originally offered a single-point measurement in various sensor lengths up to 48-inch. Displayed moisture represented an average over the sensor length with high accuracy and reliability.

A competitor, anticipating a need for more than just an overall average moisture, announced a thermal imaging system at the 2000 Global Gypsum Conference in San Francisco. Claiming to offer a moisture profile, the thermal imaging system was an instant competitor to traditional RF techniques.

In response, Sensortech immediately began to develop an Instant Moisture Profiling System. The IMPS system consisted an array of small (50mm wide) RF sensors spanning the entire board line and located between dryer unloader (cascade) and dry-end transfer. This location allows cross board moisture profiling of every board from every deck.

Products of this level of complexity and sophistication are not developed overnight. During the 2-year IMPS-4000 development time, the competitor took advantage installing many thermal imaging systems in board plants throughout the world.

Measurement Comparison

Radio Frequency

Radio Frequency moisture measurement depends on the fact that water has a very high relative dielectric compared to the solids in the gypsum board. The fringe-field RF transmitted energy penetrates up to 10cm of gypsum, more than the thickness of any board (see figure 1).

Fringe Field Board Penetration

Moisture content is directly proportional to the board dielectric and, combined with the deep penetration, the RF measurement effectively counts the water molecules in the RF field.

The IMPS system, located after the dryer unloader, sees every board on every dryer deck making it an excellent tool to assist in dryer balancing. Figure 2 illustrates one of the many screens available in the IMPS software suite and depicts the moisture condition on each deck of a 16-deck x 2-wide dryer.

Multiple Deck Moisture Profile

The influence of incremental damper adjustments in any zone will be revealed in this graphic. In addition to multiple available moisture visualizations, every deck is recorded in a history file with deck I.D., date and time, average moisture and maximum moisture.

Finally, after optimizing kiln balance, the IMPS-4400 continues to be useful both as a real-time QC indicator and to facilitate a moisture control scheme using an output signal based on overall deck averaging.

Thermal Imaging

Thermal imaging claims to be representative of moisture content and, to some degree, it is. However, the thermal image is that of the board surface and very near sub-surface.

Figure 3 is a depiction of moisture distribution through the board cross-section. Exiting a hot dryer, any free moisture will not be at the board surface but will be in the center core of the board.

As the board exits the dryer, it passes slowly through an open section, sometimes called the accumulator section prior to being released to the take-off. Board is often halted here for varying times waiting for the unloading gate to open. While waiting, the board is subject to ambient temperatures. A temperature sensor may be used to partially compensate for seasonal and day/night ambient changes. It will not compensate, however, for cross currents of air due to open windows or doors which may have a dramatic influence on the board surface temperature and may cause real temperature profile changes unrelated to moisture content.

Moisture Distribution and Evaportion

The color bars shown in figure 3 denote moisture level through the board, red being low moisture. The surface board temperature will not accurately represent the core moisture shown above in blue.

An additional problem for thermal imaging is it being influenced by board color. Surface color effects emissivity such that dark paper makes the board appear hotter. A trim control is used to adjust the color/temperature scale but requires human intervention.

One significant advantage of the thermal image is its ability to view sub-surface conditions like an x-ray image and flaws such as blisters are readily visible. The thermal imager may be used to help optimize final zone dryer balance but it is unlikely that earlier dryer zone temperatures will influence the board surface temperature as it exits the dryer.

Once dryer adjustments have been made the thermal image is not suited to any automatic control mechanism and is only used as a relative indicator.

Comparison Summary:

Penetration The RF field penetrates the entire board thickness indicating core moisture The thermal image indicates only surface and near-surface conditions
Product Color Influence No influence Strongly influenced by surface paper color
Ambient Conditions No influence Influenced by ambient temperature in general and particularly to cross currents of air due to drafts
Kiln Balance A useful tool to assist in damper adjustment in all kiln zones Useful to adjust final zone air flows
QC Indicator Measures and records with time/date stamp, every board produced Has very limited historical record capability
Real-time Control Tool Software provides output representing overall dryer output moisture level facilitating final zone heat control. In conjunction with Sensortech In-kiln sensors provides comprehensive kiln control possibilities No control capability
Flaw Indication Limited ability to show serious defects Clearly indicates surface and sub-surface flaws such as blisters but only useful while an operator is watching the screen