Gas Detector FAQs

Frequently Asked Questions Regarding Gas Detectors

The design and location of the gas detector has much to do with the maximum area one detector can properly cover. A correctly positioned and mounted Brasch gas detector is designed to provide coverage over an area of 7000 to 9000 square feet when placed in a building with a ceiling height of 13 feet or less. This area should be reduced to 5000 to 7000 square feet when the detector operates in areas with ceiling heights greater than 13 feet. Detectors located in an area with restricted air flow, such as an enclosed room, should be considered as only protecting that area.

Brasch gas detectors are shipped to the customer fully calibrated. An existing sensor cannot be calibrated in the field because conditions that affect the calibration cannot be controlled. An otherwise functional detector requiring recalibration can be restored to “like new” condition by replacing the electronic board containing the sensor with a new, calibrated sensor board assembly. This assembly is shipped to the customer properly calibrated. Once installed, the detector can be considered accurate within design parameters.

Brasch electro-chemical gas detectors use carbon monoxide sensors with a nominal lifetime of two years. The nitrogen dioxide sensors have a stated nominal lifetime of two years. Many detectors operate properly for periods exceeding their published lifetimes. In the final analysis, the lifetimes of the sensors depend heavily upon the amount of exhaust gas to which the sensors are exposed. A sensor could be damaged beyond use in a matter of hours if continuously exposed to undiluted exhaust gasses.

Gas detectors designed to operate in automatic ventilating systems use three basic types of sensors. Detectors using the older solid state sensors can obtain an accuracy of +/- 10 percent of the full scale response. This type of sensor is rapidly being replaced by the electro-chemical type of sensor. This sensor can easily obtain an accuracy of +/- 5 percent of the full scale response. Some detectors use the “near infra-red” principle of measurement to monitor the target gas. This type of detector can obtain an accuracy of +/- 2 percent, and in some cases, reach a short term accuracy of +/- 1 percent. The “near infra-red” detectors are the most costly. Brasch gas detectors use the electro-chemical type of sensor to obtain better than required accuracy along with low cost.

Other than applying a known concentration of test gas to the gas detector, there is no tried and true method of determining the condition of the sensor. Brasch offers a test kit that can measure the response of GSE series carbon monoxide detectors to a selectable gas concentration. However, like all pieces of equipment, those who are familiar with the normal operation of the unit will come to expect a certain level of performance. When the detector appears not to react normally, it should be examined to determine a cause. At that time, you may get some indication that the sensor requires replacement.

Most of the detectors produced by Brasch have an output that can be connected to a building management or control system. This output can be configured to send a linear signal, proportional to the gas concentration, to the analog input of the system. Most Brasch gas detectors permit field selection of either a current or voltage signal.

Nearly all Brasch gas detectors are sold through distributors. These distributors are trained to answer most customer questions. Also, Brasch employs service technicians that have been trained to answer any question that may arise. Although the customer should include the distributor in any discussion, the customer is encouraged to contact the factory directly if necessary. Most questions can be answered over the telephone. In rare cases where factory repair is required, the detector can be returned to the customer within one or two weeks.

Brasch calibrates all of their gas detectors according to a procedure that has been developed to provide a final accuracy within +/- 5 percent of full scale response. This procedure requires at least six days. No detector is shipped from the factory without first successfully passing this procedure. All test gasses are analyzed to be within +/- 2 percent. Calibration procedures are performed in a building constructed and dedicated solely to that purpose.

In 1992 the U.S. Bureau of Mines produced a study correlating the concentrations of carbon dioxide and nitrogen dioxide gasses produced by a diesel engine. The study concluded that once tuned and characterized using direct measurements, the concentration of carbon dioxide could be used to indirectly determine the level of nitrogen dioxide. This correlation, however, was valid only for that specific engine in a closed system with fixed operating parameters. This assumption would not be valid in an open environment or with the presence of multiple engines. Brasch strongly recommends that if a nitrogen dioxide source is present, the concentration of this gas be measured with a nitrogen dioxide sensor in order to obtain proper ventilation control.

Instruction/Operation manuals for current and previous models are available in PDF format from this web site. Links to documents for current models can be found in the Literature section of the related detector page. Links to documents for superseded models can be found in the Obsolete Literature section of the Gas Detector Technical Information page.

The Brasch Onguard and GSE stand-alone detectors contain a feature which tests all detector functions except for the sensor’s response to the target gas. If the ALM OFF/SELF-TEST front panel switch is pressed within five minutes after power is applied, the detector will perform the self-test function. The ventilation fans can be actuated by this self-test feature if the fan is supplied with the proper operating voltage. For a complete description of the self-test feature, please read the Instruction/Operation manual.

Most specialty gas companies offer small containers of carbon monoxide mixed in air. These tanks, when supplied with the proper regulator valve, can be used to expose the detector’s sensor to a test level of gas. Due to numerous variables, the response to the test gas will only be approximate. Brasch offers a test kit that can measure the response of GSE series carbon monoxide detectors to a selectable gas concentration. Test gas for Brasch carbon monoxide detectors should have a concentration of between 50 and 100 PPM. Test gas for nitrogen dioxide is not available in the 1 to 4 PPM concentration required to test detectors with nitrogen dioxide sensors. The gas, when mixed at these levels, will not remain stable within the tank long enough to be of use.

Carbon monoxide has a molecular weight of 28 grams per mol. Air has a molecular weight of approximately 29 grams per mol. Carbon monoxide will homogeneously mix in air. Nitrogen dioxide has a molecular weight of approximately 46 grams per mol. This gas will tend to settle. However, it should be noted that hot exhaust gas from a diesel engine will initially rise, quickly cool and drift down to layer when mixed with room air.