The platinum RTDs that sense the temperature of each sealing jaw are located in the precise center of the jaw sealing face, 2.5 mm below its surface. Each sensor has been calibrated against ASTM mercury thermometers in a heavily-insulated copper block, maintained at 160° C with a precision of ± 0.1°.
Calibration is accomplished by inserting the sensor into a hole in the block, within 3 mm of two calibrated thermometers that have 0.1 ° C resolution. During the procedure, the sensor is connected to a hot-tack machine through a wiring harness, so the monitor displays the apparent temperature of the copper block -- as it normally would the temperature of the die. A computer calibration routine permits the operator to adjust the monitor reading to be equal to the temperature reading of the two mercury thermometers.
The amount of adjustment required is measured and automatically recorded by the computer in a reference file as the calibration offset for that sensor. At this point, the system is in correct adjustment. It reads in agreement with the calibrated mercury thermometers.
A calibration resistor is then plugged into the machine in place of the sensor, and the temperature equivalent of its resistance is read from the monitor. This value is recorded as the calibration value for that sensor. Thereafter, to check that the machine circuit has the proper offset adjustment for the specific sensor in use, it is necessary only to plug the calibration resistor into the machine in place of the sensor and adjust the machine offset to read the sensor calibration value on the monitor.
