ISO/TR 10064-5:2005 pdf download – Cylindrical gears — Code of inspection practice — Part 5: Recommendations relative to evaluation of gear measuring instruments.
4.1.2 Practical guidelines The following are practical guidelines for gear measurement. However, compliance with these guidelines does not guarantee measurements to a specific accuracy. Artifact temperature. Tooling, artifacts and other test pieces should be left for an adequate period to stabilize to ambient temperature. Artifact temperature ideally should be the temperature at which it was calibrated.  Mean temperature variation. The instrument manufacturer’s temperature variation guidelines for the desired accuracy should be consulted. If this information is not available, it is recommended that the mean temperature should not change more than 1 °C per hour, with a maximum change of 3.5 degrees per day.  Temperature cycles. The temperature may cycle ± 2 °C, centred on the mean temperature, every 5 min or faster. The thermal inertia of most mechanical systems will allow for rapid cyclic temperature undulations within these guidelines for the stated accuracy. If a temperature cycle of the instrument approaches 1 °C in 15 min, serious effects on the measuring system accuracy may occur. Many people use an air conditioner in an attempt to achieve thermal control. The temperature sensors in these units may be very slow to respond to temperature changes. If the response is slower than 5 min, serious effects on measurement accuracy may be noted.  Temperature gradient. The temperature should be within 0,5 °C over the entire area of the instrument surface. The best way to do this is with a high air flow. Air flow must be uniform throughout the room to prevent dead spots and gradients. To accomplish this, diffuse the air coming in to the room and, if possible, design multiple air returns to further diffuse the air uniformly in the room. The goal is to have all air moving uniformly in the room and at the same temperature. Moving air must remove heat from electronic controls, computers, motors, hydraulics, people, lights, etc., to prevent gradients.