Calibration of laser power meters for ASME code welding

Abstract

This paper describes the calibration technique that was used to provide National Institute of Standards and Technology (NIST) traceable calibration for six identical 1 kW power meters. Power control to ± 2% is required for American Society of Mechanical Engineers Boiler and Pressure Vessel (ASME B&PV), welding in a nuclear steam generator. Laser beam power measurements are required as close to the work piece as possible and to accomplish this the welding tool is inserted into a power measurement box. A power meter and enclosure was designed to be durable enough to permit general laborer handling under dusty, radioactively contaminated conditions. The measurement box includes an alignment device and provides a Class I laser safety enclosure at the steam generator working platform. Up to three welding work stations are operated simultaneously from the same Nd:YAG laser source, using a 700’ (214M) length fiber distributed delivery system and identical calibrated power meters are required at each station.At the time of the first field application, in early 1992, there was no direct NIST traceable calibration available at the 300 watt level so that an indirect, electrical energy technique was used. The water cooled thermopile power meter incorporates an internal resistance heater corrected by the supplier to YAG laser light absorption at the face plate. Using the electrical calibration technique, six power measurement heads, paired to three dedicated display meters, were calibrated to within ± 2 watts at the maximum capability of the resistor, which was 200 watts. As a second step in the calibration, the six power heads were inserted into a stable 330 watt YAG power beam in a short time period and their responses were compared. The group average of this comparison was then used as a target for adjusting all six of the power heads to as close as possible to the average. Over 600 ASME Code welds were made using power meters calibrated in this fashion with 100% acceptance based on full volumetric ultrasonic inspection.This paper describes the calibration technique that was used to provide National Institute of Standards and Technology (NIST) traceable calibration for six identical 1 kW power meters. Power control to ± 2% is required for American Society of Mechanical Engineers Boiler and Pressure Vessel (ASME B&PV), welding in a nuclear steam generator. Laser beam power measurements are required as close to the work piece as possible and to accomplish this the welding tool is inserted into a power measurement box. A power meter and enclosure was designed to be durable enough to permit general laborer handling under dusty, radioactively contaminated conditions. The measurement box includes an alignment device and provides a Class I laser safety enclosure at the steam generator working platform. Up to three welding work stations are operated simultaneously from the same Nd:YAG laser source, using a 700’ (214M) length fiber distributed delivery system and identical calibrated power meters are required at each station.At t...