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 The CGT-501


  Using the CGT-501 for
Home Inspections

What you should know about the detector
IMPORTANT: The CGT-501 was designed to be non-quantitative and non-selective. It is intended for the indication of the presence of combustible and/or toxic gases and vapors by relative magnitude only. Its primary purpose is to locate the source of a contaminant gas or vapor and/or compare concentrations of a gas or vapor relative to other locations.

About the CGT-501 detector

1. Batteries:
Two types of CGT-501 units are available.
The Alkaline Battery Model (CGT-501AK) which comes with 4 AA Alkaline Batteries installed. The 501AK is recommended when the detector is seldom used or when storing the detector for long periods of time. One of the main advantages of alkaline batteries is that they hold their charge for long periods of time when not in use. When the detector is needed, the alkaline batteries have plenty of stored power and are ready to operate.
The Nickel Cadmium Battery Model (CGT-501NK) which comes with a ni-cad battery pack that can be recharged over 500 times if properly maintained and recharged according to manufacturers specifications. The 501NK is recommended when the detector is used frequently (daily or several times a week). Being able to recharge these batteries keeps down the cost of battery replacement.

Both battery units are 6 volts DC power sources and allow the unit to perform best when fully charged.

2. Accessories
Several accessories are available for the CGT-501 which can help you in performing several of the more difficult tests and checks that most home inspections require. See Accessories Available.

3. Sensitivity Levels
The CGT-501 has three sensitivity levels adjustable by a three position sensitivity switch located inside the unit. Access to this switch is made by removing the three slotted screws on the back of the CGT-501 and locating the switch in the upper right hand side of the unit. The red dot on the switch indicates the most sensitive setting. The medium setting on the unit (middle position) is the most popular setting for the home inspection application. It allows enough sensitivity to locate the small ppm levels of Carbon Monoxide in a range that is of concern and also allows sensing of the Methane and Propane Combustible vapors in very low %LEL concentration.

4. The Sensor
The sensor is a solid state tin dioxide substrate deposited on a ceramic tube. Through its center is a coiled heater filament which keeps the sensor heated at the proper operational temperature which assures better accuracy during detection. The sensor responds well to most combustible and many toxic gases and vapors, although it cannot distinguish between the different gases or vapors. It can provide a quantitative indication when only one gas or vapor is present. The sensor is intended to be good for over 10,000 exposures. The sensor cannot be ruined by over exposure to high gas concentrations it detects. The flashback arrestor operates effectively to prevent flame propagation if ignition inside the sensor should occur, but the sensor is not intended for hazardous locations. IMPORTANT: the temperature of the gas or vapor sample being detected and the ambient wind conditions can significantly affect the accuracy and reliability of the output to the meter signal.
When the CGT-501 has not been used in the last five days
The start up sequence is very important in assuring the most accuracy and reliability out of the CGT-501. It is recommended that you turn the unit on 5-15 minutes before starting detection. The initial warm-up time is lengthened to allow the tin dioxide sensor to burn off all the oxidation which has accumulated on the sensor. Continuous zero adjustments will be needed in order to maintain a zero reading on the meter face.

Keep the sensor head at constant temperature
during detection and warm-up so erroneous readings will not be made due to large temperature swings delivered or diffused to the sensor head by the sample. Watch out for significant temperature changes when testing areas such as flues, stacks, registers, etc. and when entering a new room or area.
TIP: Capture samples of the gas or vapor being tested in a container (such as a plastic bag or plastic or glass bottle) and insert the CGT-501 sensor into the opening of that container. This will help eliminate significant temperature differences and air flow rate differences which have a large effect on the output to the meter. Re-adjustments of the zero knob may be required to re-zero the background reading observed.

Learn basic information about the gases or vapors you are trying to detect
This will greatly increase your success rate. Characteristics such as vapor density (heavier or lighter than air), toxicity levels and/or lower explosion levels(LEL), dispersion characteristic of a gas or vapor, can mean the difference between successful detection or erroneous readings.
For example if you know that:
Carbon Monoxide is slightly lighter than air and when heated will it will rise. Detection can be made at ceiling levels for this gas.
The LEL of Propane is 2.1% by volume or 21,000ppm. Detection of a small leak can be done in the 100-500 ppm range. This range is easily obtained in medium sensitivity on the CGT-501. Propane is 1.6 times heavier than air and should be detected closer to the ground or floor for the best results.

Approximate meter needle indications
The following chart gives approximate meter needle indications on a CGT-501 when concentrations of the listed gases or vapors are present. NOTE: Tests were conducted with detectors with fully charged batteries.

Carbon Monoxide From 3 to 20 From 10 to 75 From 30 to 200
Formaldehyde From 5 to 25    
Gasoline From 1 to 25 From 50 to 200 From 100 to 2000
Methane  From 10 to 75  From 50 to 500 From 200 to 2000


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