Questions occasionally come up about problems with gas proportional detectors. Some problems are eliminated or reduced with simple preventative maintenance. Some can be corrected with minor changes in how the detectors are set up for use. The following information should assist you in preventing or resolving common problems with your gas detectors. As always, if you need more information, please contact us.
Maintenance
Dust Caps
Plastic dust caps should always be in place on detector gas connectors when gas lines are disconnected. Dust caps prevent dust and other materials from interfering with the function of check valves and should be in place when the detector is operating on a static charge.
Gas Quality
Ensure that the gas used is appropriate for the application and that it is supplied by a reputable vendor. Low-quality gas or gas containing contaminants or moisture can cause arcing and spurious counts. As an example of this, a gas detector that counted 5,000 - 10,000 counts per minute for background was returned to the factory, fully charged with P-10 gas, for examination. At the factory, charged with the "old" gas, the count rate was excessively high, as reported by the customer. However, the detector was purged and charged with P-10 used at the factory. The count rate dropped to the normal 300 - 400 counts per minute.
In addition, the mixture of the counting gas will affect the operating voltage of the detector. LMI recommends the use of high-quality P-10 counting gas.
Detector Response To Changes In Pressure
Factory Setting
The operating voltage is determined at the factory by "running a plateau" with the detector. It should be noted that the detector performance is sensitive to atmospheric pressure (and thus elevation above sea level of the calibration facility) because of gas compression and expansion.
The most common way to compensate for this effect is to apply a correction factor to the detector operating voltage. No correction factors are applied to detectors that are shipped to locations having elevations other than the 660 m (2,160 ft) above sea level at Sweetwater, Texas. When a gas detector is used at elevations differing from Sweetwater's elevation by 305 m (1000 ft) or more, LMI recommends reevaluating the operating voltage. (Instructions for determining the operating voltage are available from LMI.)
Detectors Used with Alpha-Beta Counters
Changes in pressure are more critical for detectors used with alpha-beta counters due to the beta window. Thus, it is even more important that the operating voltage be reevaluated for elevations other than 660 m (2,160 ft) above sea level. In this case, LMI recommends reestablishing the operating voltage when the user's elevation deviates 152 m (500 ft) or more from Sweetwater's elevation.
Detector Setup
Purging
As a general rule, gas detectors should be flushed for 15 - 25 minutes at a flow rate of 100 - 150 cc/minute before use. For a flow rate specific to the detector being used, consult the detector instruction manual. Note: Most detectors use double-ended quick connects. Both the male and female quick connects have to be connected to allow counting gas to flow through the detector.
Fast Flush
A faster flush time can be realized if the output gas line (only) is removed at the output flow meter and the flow rate increased to maximum flow rate. Flush time may be reduced considerably. For a fast flush time specific to the detector being used, consult the detector instruction manual. Note: The quick-disconnect fitting should not be removed. When the fitting is in place, gas flows out the connection. With the fitting removed, a check valve is actuated that prevents output flow.
Fitting and Regulator
Ensure that the gas bottle valve matches its fitting to the regulator. The industry standard for a P-10 gas fitting is CGA 350. A good-quality 2-stage regulator, such as the one offered by LMI, should be used, along with a gauge that has a proper scale (0 - 30 psi is preferred). The scale on the gauge needs to be readable at the pressure used. For example, the recommended pressure for normal operation is less than or equal to 5 psi (2 - 3 psi is better). If the gauge scale range is 0 - 100 psi, it may be difficult to regulate the flow accurately at pressures lower than 5 psi.
Once the gas bottle valve is opened and the system is at the desired pressure, use either the valve between the regulator and the tubing or the flow meter valve to regulate the gas flow.
Please note that several parts are needed to assemble the system. Tubing is obviously required for the gas flow from the gas bottle to the detector. An extra fitting is required to fit the tubing and nylon tubing nut onto the regulator valve (Swagelok B-400-7-4). In addition, a brass insert is helpful when placed inside the tubing as a bearing surface to keep the tubing from collapsing when the tubing nut is tightened. These parts (tubing, fitting, nut, and insert) are available as a kit (PN 4085-621) for convenience to the LMI customers. The individual parts may also be ordered separately.
Troubleshooting High Background Counts or Low Efficiency - Possible Causes and Fixes
Operating Voltage
If the operating voltage is too high, the background count rate will be too high. If the operating voltage is too low, low efficiency will result. In either case, re-plateau the detector and determine the proper operating voltage.
Probe Contamination
Clean or replace the screen and Mylar window. The screen may be cleaned with a special wash or soap and water.
Low-Quality Gas
Replace gas bottle. If the problem persists, notify the gas supplier.
Light Leak
Test the detector under low-light conditions. If the background or source count rate changes significantly, there is likely a light leak. For older detectors, the standoffs may be one source of the leak. While new detectors have black or internal standoffs that do not allow light to penetrate, older detectors have light-colored standoffs that may act as "conduits" for light. The situation may be corrected by covering the standoffs with black tape or painting them with quick-dry black paint.
Another possible light leak source for detectors with a Mylar window is a small puncture or tear in the window. Such a hole, while not visible upon examination, may allow light to penetrate the window and cause spurious counts. If the window has been punctured, it requires replacement.
Threshold Setting
If the threshold on the counting instrument has been set too low, a high background count rate can result; if too high, low efficiency can result. This is more or less a last-resort check for problems with count rates or low efficiency, as the threshold is set at the factory. It should be accurate unless it has been changed or the instrument is malfunctioning. Access to a Ludlum Model 500 Pulser or equivalent is required to check the threshold setting. Contact LMI or refer to the counting instrument manual for instructions on checking and setting the threshold.