Calibrating Air Sampling Pumps
Air Flow Calibrations for Personal Monitoring Pumps
While most detector tube applications are health and safety related, many others are process related. Detector tubes are often used to monitor for combustion products in flue gases, contaminants in natural gas pipelines, ethylene levels in greenhouses or other measurements associated with process control. One of the challenges connected with process measurements is to obtain an accurate detector tube reading from a closed system that is operating at a high pressure. Detector tube systems are calibrated at atmospheric pressure, and connecting a detector tube to a pressurized system directly can increase the flow rate through the tube dramatically over that of the factory calibration conditions. This can cause the tube to overstate or read high, due to the reduced contact time between the target gas and the reaction layer. Some percentage of their action layer is missed, and the target gas travels farther into the tube before being consumed. This produces a stain that is longer, but paler in color. Since we only read the length of the stain and not the intensity of the color, the tube overstates.
The solution to the problem is to collect the sample in such a way that the pump is working against atmospheric pressure and not some elevated pressure. This way the system is matching the factory calibration as closely as possible. Two methods are commonly applied, and both work quite well.
The sampling bag method utilizes a gas sampling bag to collect the sample from the pressurized system. The bag is then removed from the system, and the detector tube sample is drawn from the bag. Such sampling bags are made of inert materials such as Tedlar, Mylar or Teflon in order to be compatible with the target gas. They generally include a valve to open and close the bag entry tube, and it is good field practice to fill and exhaust the bag prior to the actual sample in order to purge the bag of any entrapped air. This method works well for samples that maybe toxic, since the bag can be relocated outdoors or to a fume hood.
The flow-through container method routes flowing gas through a chamber prior to venting to atmosphere. The routing of the gas inside the chamber causes the gas to collect or accumulate en route, and therefore the device is self-purging. The tube inlet is placed inside the chamber through the exit vent such that the sample is drawn from the center of the chamber. Such sampling devices are typically made from relatively inert materials like polyethylene, polystyrene or stainless steel. The flow rate should be maintained at one to two liters per minute, in order to stay ahead of the sampling pump. This method works well outdoors, when the gas being sampled is not particularly toxic.
The key to sampling pressurized systems with detector tubes is to introduce the sample at atmospheric pressure and to keep outside air from entering the sample. This promotes sample integrity and allows the detector tube system to operate at its optimum accuracy.