THE PROBLEM
KEY CAUSES OF ELECTROCHEMICAL SENSOR VARIABILITY
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1. Sensor Ranges: Electrochemical Sensors have a limited exposure capacity within which they can preform reliably. Too great an exposure and the sensor becomes overloaded affecting its accuracy and responsiveness - often becoming saturated. Too low a concentration weakens the reaction signal making the reading susceptible to instrumental noise, and poor accuracy and resolution. (CTM-022 Sec. 2.3.2)
2. Cross-Interference: Electrochemical sensors typically rely on the generation of an electrical current as a result of common oxidation or reduction reactions. These processes are common to a number of gases resulting in a range of unwanted interferences. This issue has been successfully addressed with the utilization of Interference Rejection Filters (IRF) that either chemically react with or adsorb the cross-interfering gases, removing them before they can react with the sensor's electrode. (CTM-022 Sec. 4.5.1.)
3. NO Sensor's Zero Drift: The NO sensor zero drift is a function of the interaction between temperature (Arrhenius Law), and the NO sensor's previous exposure history (CTM-022 Sec.2.3.2). Since the drift is a function of the interaction of two variables, it cannot be solved by temperature compensation.
To address these issues, all Enerac 3000 units are equipped with special Dual Range switching, Auto Sensor and Filter Status Checks, Re-placeable Interference Rejection Filters, and Auto NO Sensor Cooling System.
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THE SOLUTION
PRECISION CONTROL MODULES (PCM)
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To increase flexibility, and avoid the causes of electrochemical sensor variability, each Enerac 3000 Series unit is equipped with Precision Control Modules (PCM). The NO, CO, and SO2 have two types of PCMs available; Low-Mid (L) and a Mid-High (M). Each PCM accommodates two ranges with both share a mid range capability.
Each PCM allows the SEM sensor to switch between two different monitoring ranges with a simple flip of a switch. Additional Modules can be interchanged to extend the sensor's operating ranges even further.
Each Precision Control Module (PCM) also incorporates field replaceable Interference Rejection Filters (IRF) to insure sensor specificity and data accuracy. |
DUAL RANGE SWITCHING
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All Model 3000E units are equipped with unique dual range switches for the CO and NO measurements. There is not longer a need for two sensors, two calibrations or two QA/QC performance checks When the SO2 option is ordered, it is also equipped with a dual range capability.
A. Mid-High (MH) PCMs are mounted on all sensors when the instrument leaves the factory.
The High range of the Standard NO PCM has approximately twice the exposure capacity of the conventional electrochemical sensor and is excellent for monitoring the emissions from Rich Burn engines.
B. Low-Mid (LM) PCMs are supplied for the CO and NO sensors. Special SO2 PCMs are also available as an option. The low range of the Special NO PCM has approximately five times the sensitivity of a conventional electrochemical sensor, and is excellent for the monitoring of Turbines, Low NOx , and Ultra Low NOx burners etc.
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STANDARD PCM RANGES
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| Sensor |
PCM type |
Low Range to |
High Range to |
| CO |
Low-Mid (L) |
500 PPM |
2000 PPM |
| CO |
Mid-High (H) |
2000 PPM |
20,000 PPM |
| NO |
Low-Mid (L) |
300 PPM |
1000 PPM |
| NO |
Mid-High (H) |
1000 PPM |
4000 PPM |
| SO2 |
Low-Mid (L) |
500 PPM |
2000 PPM |
| SO2 |
Mid-High (H) |
2000 PPM |
6000 PPM |
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OVER RANGE WARNINGS
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A warning system is included to alert the operator, when any of the toxic gas sensors is about to over range. All the LEDs on the face plate will begin to blink and the following message will appear on the display every 15 seconds:
"XXX SENSOR OVER-RANGE"
If this message appears, simply toggle the appropriate dual range switch to the upper position or remove the probe to avoid sensor saturation.
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PRINTOUT IDENTIFICATION
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The carbon monoxide, nitric oxide and sulfur dioxide printouts automatically include either an "L" , and an "M" in parentheses to indicate the PCM used during testing, as well as a caret (^) to document the position of the range selector switch. |
INTERFERENCE REJECTION FILTERS & ELECTRODES
Replaceable
Early Warning of Failure
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The Carbon Monoxide, and Nitric Oxide sensors incorporate special proprietary inboard chemical filters in their PCMs. These filters function to remove interfering gases from reaching the sensors. These filters have a long but limited life. When the capacity of the filter has deteriorated, the appropriate PCM must be replaced with a fresh one. The NO and SO2 sensors incorporate special additional electrodes that are also used to remove cross interfering gases.
The ENERAC's Calibration Certification Protocol automatically provides a warning of impending filter exhaustion during instrumental calibration.
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CALIBRATION CERTIFICATION PROTOCOL
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Automatically prints out both auto zero and span calibration test results, sensor and filter performance status. This protocol also provides the user with an early warning of both sensor and/or filter deterioration or failure. (See CTM-022 Sec. 6.6) |
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