Effects of Atmospheric Conditions

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Atmospheric conditions can have multiple effects on Vantage measurements. Temperature gradients, air turbulence, or air pockets of different temperatures in the path of the laser beam will affect the direction of the laser beam resulting in a loss of accuracy in both the angular and radial distance measurements. Typically, gradients, turbulence, or air pockets are always changing, so this accuracy loss is not always the same magnitude or even the same direction. This effect creates poor repeatability in addition to the accuracy loss. Increasing the number of samples per reading can sometimes improve the repeatability of measurements in these conditions, but it does not necessarily improve the accuracy of each measured reading. Avoid these errors by not measuring near heating and air conditioning ducts, doors that frequently open, or any other source of these temperature effects. Airflow in general is not the issue as an environment with high airflow of a uniform temperature can result in very good measurements. It is the regions where two different air masses of different temperatures that causes the laser beam to bend. Another method to managed the effect of non-uniform temperature is to use a fan to mix the air so there is no pockets of different air temperatures in the measurement area.

As the temperature, barometric pressure, and relative humidity of air change, so does the index of refraction. Knowing the air’s index of refraction is critical for the accuracy of the radial distance measurements performed by the Vantage's ADM system because these systems need to convert the waves of light being measured into a distance and the speed of light changes based on environmental conditions. The index of refraction cannot be calculated correctly without accurate atmospheric condition values. The Vantage system is equipped with weather sensors that measure the temperature, pressure, and humidity of the ambient air every five seconds. A 1 part per million change in the index of refraction, and the resulting radial distance measurements, occurs for a 3 mmHg change in pressure, a 1°C change in temperature, or a 40% change in relative humidity at 40°C. Because the air temperature has a significant effect on the index of refraction, the Vantage's external air temperature sensor should be placed in air at the same temperature that the laser beam is traveling through and not inside a work cart, near heating or air conditioning ducts, or any other source of a thermal effect.