Application and Description of a Portable Calibration System

A newly introduced calibration system for air pollution monitors and its field use will be discussed. The system is based upon the Environmental Protection Agency's developed permeation tube technique and is fully portable, allowing intercalibration of monitors at different locations.

Two major requirements are necessary for accurate use of permeation tubes in the generation of primary standard quality calibration gas mixtures. The temperature of the permeation tube must be held within 0.1°C of the temperature at which it was calibrated to ensure accuracy within 1 percent. Also the flow rate of the gas across the permeation tube must be readable to within 1 percent accuracy.

The portable calibration system described controls the permeation tube at 35°C to well within 0.1°C. This temperature is fixed and is read out on the expanded scalemeter that is 0.4°C full scale to ensure that the permeation tube is within 0.1°C of the fixed 35°C.

The calibration system draws in ambient air through a particulate filter by an internal dual headed pump. One head provides a flow of 10 ml/min through a restriction, a charcoal scrubber, the permeation chamber, and a flow directional valve. The other head provides a diluent flow, settleable on the front panel, from 300 to 1500 ml/min. This flow is read out on a special dual ball rotameter, individually calibrated, with settleability to within 1 percent. This diluent flow can provide a fivefold change in concentration available for calibration, permitting calibration at a number of points. In addition the flow direction valve has a high concentration position in which all of the permeation effluent is mixed with the diluent air, a low concentration position in which only 20 percent of the permeation effluent is mixed with the diluent air, and a “zero” air position in which no permeation effluent is used. This last position provides only air that has passed the charcoal scrubber for the analyzer to be calibrated. The high/low concentration positions provide a 5 to 1 concentration range and when coupled with the diluent air flow range provide a 25 to 1 concentration range.

This calibration system can operate for at least 8 h from internal batteries, indefinitely from 110 V lines, while the batteries are recharging, or from an auto cigarette lighter adapter supplied as standard.

The present emphasis and concern on data correlation among the various locations and agencies involved in the air pollution effort requires the instrumentation used to be calibrated properly. A portable calibration system provides the necessary common demoninator between a number of remote monitoring systems. This common standardization is the first crucial step before data can be compared.