You must use one of two techniques to calibrate torque: the lever-arm dead-weight or the transfer technique. You may use other techniques if you show they are equally accurate. The NIST "true value" torque is defined as the torque calculated by taking the product of an NIST traceable weight or force and a sufficiently accurate horizontal distance along a lever arm, corrected for the lever arm's hanging torque.

(a) The lever-arm dead-weight technique involves placing known weights at a known horizontal distance from the torque-measuring device's center of rotation. You need two types of equipment:

(1) Calibration weights. This technique requires at least six calibration weights for each range of torque-measuring device used. Equally space the weights and make sure each one is traceable to NIST weights. You also may use weights certified by a U.S. state government's bureau of weights and measures. If your laboratory is outside the U.S., see §1065.305 for information about using non-NIST standards. You may account for effects of changes in gravitational constant at the test site.

(2) Lever arm. This technique also requires a lever arm at least 20 inches long. Make sure the horizontal distance from the torque-measurement device's centerline to the point where you apply the weight is accurate to within ±0.10 inches. You must balance the arm or know its hanging torque to within ±0.1 ft-lbs.

(b) The transfer technique involves calibrating a master load cell (dynamometer case load cell). You may calibrate the master load cell with known calibration weights at known horizontal distances. Or you may use a hydraulically actuated, precalibrated, master load cell and then transfer this calibration to the device that measures the flywheel torque. The transfer technique involves three main steps:

(1) Precalibrate a master load cell or calibrate it following paragraph (a)(1) of this section. Use known weights traceable to NIST with the lever arms specified in paragraph (b)(2) of this section. Run or vibrate the dynamometer during this calibration to reduce static hysteresis.

(2) Use lever arms at least 20 inches long. The horizontal distances from the master load cell's centerline to the dynamometer's centerline and to the point where you apply weight or force must be accurate to within ±0.10 inches. Balance the arms or know their net hanging torque to within ±0.1 ft-lbs.

(3) Transfer calibration from the case or master load cell to the torque-measuring device with the dynamometer operating at a constant speed. Calibrate the torque-measurement device's readout to the master load cell's torque readout at a minimum of six loads spaced about equally across the full useful ranges of both measurement devices. (Good engineering practice requires that both devices have about the same useful ranges of torque measurement.) Transfer the calibration so it meets the accuracy requirements in §1065.105(a)(2) for readouts from the torque-measurement device.