A temperature value can be stable and still be wrong. Infrared sensors are affected by emissivity, wavelength, viewing angle, reflected radiation, transparent sheet behavior, dirty optics, and the size of the measured spot. Contact sensors can be affected by placement, attachment, thermal lag, and wiring.
Different colors, finishes, thicknesses, coatings, and polymers may require a different measurement setup. A pyrometer calibrated on black ABS cannot automatically be trusted on clear polycarbonate or reflective masking.
Verification
The measurement method should be checked against an independent reference suitable for the material and temperature range. Sensor position must follow the sheet rather than viewing heaters or frame edges. Optics, air purge, shielding, and calibration should be included in maintenance.
Recipes should state what is being measured, where, and with which settings. A number without that context cannot be compared reliably between machines or materials.