Does it Impact Calibration and the Alarm Set Points?

CETCI’s available fixed system gas detection devices include the FCS and QCC Controllers, the DCC and SCC Self-Contained Controllers, the ART, LPT family and AST-IS series of analog and digital transmitters. Each system is configured by the factory prior to shipping. Specifically, both the controller and the associated transmitters arrive at their destination fully calibrated and pre-programmed with the channel number(s), name(s), sensor range(s), Modbus ID(s) (if applicable) and the factory default Cal gas levels and alarm set points.

Depending on the application, 6 to 12 months after installation, the sensors will require calibration. Part of the calibration procedure is selecting an appropriate Cal gas concentration.

For electrochemical sensors (CO, NO2, H2S, etc.), CETCI suggests using a gas concentration that is 50% of the range of the sensor. This keeps the sensitivity of the sensor balanced on both the high and the low side. Ideally, a CO sensor with a range of 0 – 200 ppm, would be calibrated with 100 ppm Cal gas concentration. The suggested Cal gas concentration for Ammonia with a sensor range of 0 – 500 ppm, is 300 ppm. For combustibles such as hydrogen, methane and propane with a sensor range of 0 – 100% LEL a concentration of 20% LEL or 50% LEL is preferable.

NOTE: If you have calibration gas in your tool kit that is not the suggested concentration, the calibration can still be performed. But understand that the sensitivity of the gas readings may be skewed because the measurement values for comparison are more heavily weighted in one direction, away from the center of the range.

Once you know the concentration of calibration gas you will use, you need to set the Cal gas level in the device you are calibrating. This is especially important if you are not using the same Cal gas concentration that was used previously to calibrate the device. It doesn’t matter if you are using a cylinder of gas or a gas generator, if the gas detector isn’t set to match the actual concentration of gas being used, the calculations that the gas detector goes through to produce accurate readings will be completely out of whack and can pose a serious health risk. It is always good practice to check the settings before proceeding with calibration.

Here are a few examples to illustrate the importance of setting the Cal gas level to match the calibration gas concentration being used to calibrate the sensor. For a CO sensor, if the Cal gas level was set at 200 ppm and 100 ppm calibration gas was used to calibrate the gas detector, this would result in the sensor being too sensitive. The low alarm set point of 25 ppm would be triggered at 12.5 ppm. If the Cal gas level was set at 50 ppm and 100 ppm calibration gas was used, this would result in the sensor being too insensitive. The low alarm set point of 25 ppm would be triggered at 50 ppm.

Common calibration gas concentrations used to calibrate Ammonia sensors are 100 ppm and 300 ppm. Let’s say the last time the gas detector was calibrated, the Cal gas level was set to 100 ppm and 100 ppm calibration gas was used. The next time it was calibrated, the technician failed to change the Cal gas level setting and used 300 ppm to calibrate the device. This would result in the sensor being too insensitive and the low alarm set point of 25 ppm would not trigger until the device detected 75 ppm.

Another common practice is to use a Chlorine Gas Generator to calibrate Chlorine and Ozone sensors, using a concentration of 1 ppm for Ozone and 3 ppm for Chlorine. Let’s say after calibrating an Ozone sensor, the technician forgot to switch the gas concentration for Chlorine and used 1 ppm to calibrate a Chlorine sensor with the Cal gas level set at 3 ppm, the sensor would be too sensitive and would be in alarm before the low alarm set point of 0.1 ppm.

The relationship between the Cal gas concentration setting and the alarm set points is inconsequential. If the alarm set points were left unpopulated, the system wouldn’t know when to trigger the alarms. However, if the alarm set points are set, it doesn’t matter if the value is 3 ppm or 25 ppm or 50 ppm, etc., whatever the value, it has no impact on the calibration procedure. The alarm set points are used to tell the device when to trigger the alarm based on how much gas is present. How much gas the sensor detects is directly related to the Cal gas setting and the actual concentration of calibration gas used during calibration. The alarm set points can be changed at any time and do not require a calibration before, during or after changing them.

NOTE: Calibration is done to the transmitter (or at the self-contained controller with an internal sensor), not at the controller. The controller (without internal sensors) does not need to be told what calibration gas concentration you are using.

For digital transmitters such as the LPT-P, LPT-M and LPT-B entering the calibration gas concentration is as easy as selecting the SPAN calibration and the calibration gas value will be shown. If required, you can change the value before starting the calibration procedure.

For analog devices, the procedure differs slightly depending on the design of the device and whether it has a display.

For the LPT-TCO or LPT-END you have a total of three fixed calibration gas concentrations to choose from:

You must choose the jumper position that matches the calibration gas concentration you are using and then carry on with the calibration procedure.

With a DCC or LPT-A, both of which are analog devices with displays, to set the calibration gas level, you need to move a jumper to the CAL GAS position and turn the encoder to select the gas concentration that matches the concentration you are using to do the calibration. (In the DCC you will need to use the second jumper indicate which channel number you are setting the Cal gas for.)

The SCC is another analog device but it does not have a display and thus requires the use of a volt meter (unless you are using a CDA Calibration Display Adapter). The voltage reading is correlated to the calibration gas concentration setting by doing the following calculation.

Test Point Voltage Calculation Equation:

[Span Gas Concentration / Sensor Full Scale Range] x [4.0 Volts] = Test Point Voltage

For example, you have 100 ppm concentration of Cal gas for a CO sensor that has a range of 0 – 200 ppm. The corresponding voltage reading is:

[100 ppm / 200 ppm] x [4.0 volts] = 2 volts

With the volt meter attached to the test points on the SCC, to set the Cal gas level, you need to move one jumper to the channel number you are setting and another jumper to the SET CAL GAS position and turn the encoder until the volt meter reads 2 volts.

Factory Set Cal Gas Levels

REMEMBER: If you decide to use a different calibration gas concentration you must reset the Cal gas value in the device you are calibrating.

NOTE: Full details on how to complete the calibration procedure for each device can be found in their individual Operation Manual.

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