The temperature and force sensor systems in Dynisco/Theller heatsealers both measure electrical resistance. The automatic calibration method depends on the fact that resistance of the sensor is directly related to the variable being measured - either temperature or force. The corollary to this fact is that the resistance of a calibration resistor is directly equivalent to some specific value of temperature or force.
Conditions for automatic calibration: First, the quantitative relationship between sensor resistance and the variable to be measured is determined through the primary calibration procedure (i.e., determining sensor resistance at known values of temperature or force). This information is stored in calibration files in the controller. Then, calibration resistors of known resistance, equivalent to specific values of temperature or force, are momentarily substituted for the sensor into the measuring circuit by the computer. While the substitution is in effect, the computer then adjusts (calibrates) the circuit to read the values of temperature or force equivalent to the calibration resistances.
Automatic temperature calibration. The temperature sensors in Dynisco/Theller machines are platinum RTDs (resistance temperature detectors) - not thermocouples. RTDs are capable of measuring temperature with an uncertainty of less than 0.001°C. In Dynisco/Theller machines, high-precision calibration resistors mounted on the machine circuit board, equivalent to known values of temperature, are periodically switched into the circuit in place of the sensors. This generates a temperature reading that the computer compares with the calibration value. If the reading is off, the computer makes a software adjustment to bring the reading to the correct value.
The temperature of the upper and lower dies are measured and controlled separately by their own circuitry. Each circuit carries two calibration resistors, one equivalent to a temperature at the low end of the machine's range, and the other equivalent to a temperature in the upper part of the range. The computer
switches these resistors sequentially into the circuits and adjusts the software offset to get the proper reading. This has the effect of adjusting the primary curves of temperature vs resistance for both offset and slope.
The controller automatically carries out the calibration of both temperature circuits every 3 minutes. The process requires only a few milliseconds, and the operator is unaware when it is happening, but the net result is that the system is maintained in proper temperature calibration at all times. The Dynisco/Theller machine is the only heatsealer that has this capability.
Automatic load-cell calibration: Dynisco/Theller Hot Tack machines employ two strain-gage load cells: one for measuring the sealing force on the sample (which the computer translates into sealing pressure), and one that measures force required to peel the seal. (The PC2 is equipped with a sealing-pressure load cell only.) The controller calibrates each load cell in much the same manner described above for the temperature sensors. Each circuit has a calibration resistor of resistance equivalent to about 70% of the force capacity of its load cell. The controller momentarily substitutes the calibration resistance for one of the load-cell resistances. This generates a force reading that the controller compares with the calibration value, and if necessary, makes a software adjustment to the correct value.
The sealing-pressure load cell is automatically calibrated at the beginning of each test cycle -- in the first few milliseconds after the operator presses the switch to start the test.
In the Hot Tack Heatsealer, the peel load cell is automatically calibrated prior to each cycle, immediately previous to the test strip being clamped in place.
