Moisture Measurement in Gypsum Board

Process Sensors will be attending the 9th Global Gypsum Conference for the global trading, marketing, production and processing technology of gypsum.? An application sheet explaining the importance of moisture measurement in gypsum board follows:

The Importance of Moisture Measurement in Gypsum Board

The durability, strength and hardness of finished board is heavily dependent on moisture content. Measuring moisture content not only improve quality, but ensures ongoing product consistency, resulting in fewer reject boards and saving on rework costs.? RFM-1000, Process Sensors? on-line RF sensor, measures moisture, enabling optimization of the production process through feedback and/or feed forward control of the temperature set point within the different kiln zones. Process optimization maximizes quality product throughput, saving on energy & raw materials and also indirectly on labor and plant costs.

Gypsum Board Production Process

Gypsum (CaSO4.2H2O) is obtained either naturally from gypsum deposits in the ground, or synthetically as a byproduct of another process. Most synthetic gypsum is derived from Flue Gas Desulphurization (FGD) of fossil burning fuel power plants. Board can be manufactured from natural gypsum, synthetic gypsum or a combination of the two. Regardless of the derivation of the gypsum, it is crushed, dried, ground to a fine ?flour,? then calcined to drive off all the chemically combined water as steam. Water and other ingredients, e.g., sand and fire retardants, are added, and the wet mix is pumped to a conveyor whereupon it is sandwiched between 2 layers of paper. The board sets as it?s conveyed towards the kiln, is cut into drying lengths and loaded onto a deck within the kiln. The dryer comprises 3 or 4 temperature zones. Upon exiting the kiln, the board is cut to final length, trimmed then packaged.

Sensor Details & Installation

The RF sensor comprises a 30, 60 or 90cm aluminium antenna with teflon spacers, cabling and operator interface. Temperatures exceeding 50? C necessitate the use of aluminium/ceramic antennae, and 260? C; stainless steel/ceramic antennae. Coax cabling is required for all applications where ambient temperature exceeds 60? C. Sensor mountings are constructed from stainless steel. Owing to the differential expansion coefficient of aluminium and steel with temperature, the aluminium sensor is only fixed at one end within the kiln. The other end is free to expand horizontally eliminating the potential for dryer blockages owing to the sensor bowing upwards. The sensor is located 6mm below the bottom face of the board, and requires good clearance beneath, to allow debris to fall through the open frame enclosure.

RF Moisture Measurement in Gypsum Board

Within the dryer, the sensor is normally located on a representative deck, usually deck 4 or 5 in an 8 deck dryer. Sometimes the sensor is located on a wetter deck to most quickly eliminate the source of reject boards. However, this could cause burning on the drier decks unless the kiln is balanced.

Measurement

The RFM-1000 generates a RF fringe field of approximately 2 MHz into the board. The board conductance changes with water born dielectric variation, and this produces a raw dielectric value that is directly proportional to the moisture content of the product. Moisture can be measured in the range 0-80%, but will require different calibrations for low, medium and high moisture levels. Radio frequency measurements are not influenced by changes in PH and dissolved salts, but are affected by shifts in distance between product and sensor, density changes and appreciable temperature changes. Pass height variation is not of concern when measuring on the lower edge of the board. Compensation inputs are available for density and temperature changes if required, but all that is typically needed is a change of calibration when switching between products of different thicknesses. The sensor features high speed measurement capability, up to 200 measurements per second, and together with the automatic product loss and return measurement hold signal, this enables the output of individual board moisture averages.

Speak Your Mind

*