How to Use a Pyrometer to Verify Thermocouple Accuracy on Hot Runners?

A portable infrared pyrometer is a valuable tool for verifying thermocouple accuracy without shutting down the hot runner. Unlike contact thermocouples, a pyrometer measures the surface temperature from a distance, allowing quick spot checks of the manifold, nozzles, and heater bands. The first step is to select a pyrometer with the correct specifications. It should have a high temperature range (up to 500°C), an adjustable emissivity setting (critical for accurate readings on different metal surfaces), and a spot size that matches the measurement area. For hot runner applications, an emissivity setting of 0.8-0.95 is typical for oxidized stainless steel. The second step is to identify the measurement points. The most useful points are on the nozzle body near the thermocouple mounting hole, on the manifold surface between the heater zones, and on the heater band itself. Mark these points on the mold for consistent future measurements. The third step is to perform the measurement after the hot runner has stabilized at the setpoint. Aim the pyrometer at the target, ensuring the spot size is fully within the target area. Hold steady for a few seconds to allow the reading to stabilize. Record the reading. The fourth step is to compare the pyrometer reading with the controller's thermocouple reading. A difference of less than 2°C is acceptable; a difference of 3-5°C indicates the thermocouple may be drifting; a difference of more than 5°C warrants investigation. The fifth step is to account for the measurement offset. The pyrometer measures the surface temperature, which is typically 5-15°C lower than the metal temperature inside the nozzle, due to heat loss to the air. If you are using the pyrometer to calibrate the controller offset, you must add the appropriate offset. To determine this offset, do a calibration test at a stable temperature with a contact thermocouple placed on the same surface, and note the difference. The sixth step is to use the pyrometer for trend monitoring, not just absolute measurement. Even if the absolute reading has an offset, tracking the reading over time is valuable. If the pyrometer reading at a fixed point decreases while the controller's reading is stable, it indicates the nozzle surface is getting cooler, which may point to a thermocouple drift (the controller is overheating) or a heater issue. The seventh step is to use the pyrometer during troubleshooting. If a zone shows erratic behavior, measure its temperature at multiple points to see if there is a hot spot or cold spot that the thermocouple might not be seeing. The pyrometer is a quick and non-invasive diagnostic tool. However, it has limitations: it measures surface temperature, not melt temperature; it is affected by surface emissivity, dust, and viewing angle; and it cannot be used inside the hot runner. Therefore, it is a complement to, not a replacement for, proper thermocouple calibration. By using a pyrometer regularly, maintenance teams can quickly detect and correct thermocouple drift, maintaining high process stability.