All test equipment and reference standards used in calibration laboratories are affected by environmental changes in temperature, relative humidity, barometric pressure, vibration and several other environmental conditions. The laboratory needs to monitor, control and record those environmental conditions that influence the measurement results. There should be a continuous monitoring system that records those environmental conditions at specified time intervals. The monitoring and control must also extend to encompass the storage and handling of equipment within the laboratory.
control of those environmental conditions in calibration laboratories is always
a tradeoff between the dimensional and
electronic equipment calibrations and the level of calibration accuracy
required and should be adequate for the calibrations being performed in them.
The publications NCSLI Recommended Practice RP-14 “Guide to Selecting Standards Laboratory Environments” and ISA-TR52.00.01 “Recommended Environments for Standards Laboratories” contain information and suggestions based on the types and levels of calibrations performed.
Calibration laboratories can work outside of the stated environmental conditions, however, they must compensate for the temperature coefficients and other environmental contributors to correct their measurement results and uncertainty values which is extremely time-consuming. Staying within the environmental requirements of the laboratory and equipment saves a lot of work.
Temperature is the most common controlled and measured variable in a calibration laboratory. Dimensional measurements are especially affected by temperature variations. Temperature fluctuations for electrical measurements are somewhat forgiving, however strict temperature ranges need to be maintained to perform measurements within the reported uncertainties.
All dimensional measurements are referenced to 68°F (20°C). Electrical measurements typically are referenced to 73°F (23°C). Most electrical specifications have a wider temperature range than dimensional specifications, so calibration laboratories that perform calibrations in both disciplines should center their laboratory temperature as close as possible to 68°F. Temperature specifications of ±4°F (±2°C) are standard for most calibration laboratories.
Dimensional temperature determinations are typically based on the coefficient of expansion of steel gage blocks, which is 6 uin per inch of gage block length per 1°F temperature difference (10uin/°C). If the calibration laboratory performs calibrations on only micrometers or other measuring instruments with accuracies no greater than ±0.0001 inch (100 uin), a ±4°F tolerance would be suitable. However, if the laboratory performs calibrations on pin gauges or like items with accuracies that approach ±0.00002 inch (20 uin) or better, the laboratory temperature specifications must be reduced to ±1°F or better to keep the coefficient of expansion value from influencing the measurements.
In most cases, dimensional measurements that require a high level of accuracy are performed in a dedicated ‘68°’ room. Standard cells and standard resistors that require stable temperature environments to obtain their precise accuracies are usually maintained in oil or air baths so that the temperature variations in the laboratory do not affect the performance of the items.
The stability of the temperature during the time it takes to make a measurement is an important aspect for calibration laboratories, and limits should be established and maintained.
Relative Humidity greater than 60% can lead to rust or corrosion effects in certain instruments such as Gage Blocks, where below 20%, electrostatic discharge issues can cause problems with electrical measurements or the handling of sensitive electrical components. Most calibration laboratories should maintain a Relative Humidity Environment of between 30% to 50% Relative Humidity to achieve the best balance.
Vibrations that are present in the laboratory should not compromise the validity of the measurement results or affect the life of standards and associated equipment. Specific standards such as analytical balances and surface plates should be isolated from vibration as much as possible by using isolated pads or special vibration tables. Those items should also be located away from laboratory foot traffic.
The barometric pressure can influence some mass and force measurements. Weather fluctuations make it difficult to control the barometric pressure in a laboratory environment. Accurate barometric pressure measurements are required to apply any necessary corrections to the measurement results.
Some electronic measurement equipment, especially used for RF and high resistance measurements, are susceptible to interference from electrical and magnetic fields. Sufficient shielding precautions, use of proper cables and filtering should be observed to keep the interference to a level that will not impact the measurement results.
Air Cleanliness and Airflow
A positive air-pressure differential between the inside of the laboratory and the outside areas is an important factor to reduce the influx of dust into the calibration laboratory. Air cleanliness limits can be critical for some calibration laboratories, especially within the medical field. Tack mats located within the entryway can also assist in reducing particles entering the laboratory.