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AUTOMATED CALIBRATION AND BUMP TESTING

Unfortunately, there is no industry consensus on what comprises a successful bump test. Also, with docking stations there is often a trade-off between completing the bump test quickly, and assessing performance to a higher level of accuracy. The emphasis is frequently on getting in and out of the test as quickly as possible and using as little gas as possible per test.

During a bump test the calibration of the instrument is assessed by exposing the sensors to known concentration test gas, and verifying that the readings are accurate. The response of the sensors is not adjusted during a bump test. A bump test is simply a yes/no verification of response. Of course, during a full calibration the sensor outputs are adjusted to match the concentrations of the applied gas. Because the sensors have to stabilize completely before they can be adjusted, a full calibration generally takes much more time and gas than a bump test.

To save time and gas many docking stations provide only a qualitative assessment of performance during a “bump test.” In many cases the docking station only flows gas until the instrument alarms are activated. You know that the sensors respond to gas and the alarms function, but you don’t really know if the readings are accurate. To make it worse, customers often use less expensive “bump gas” rather than “calibration gas” when performing functional bump tests. The “bump gas” is often packaged in cylinders (or aerosol cans) with poor stability and shelf life. The reactive components in the “bump gas” (like hydrogen sulfide) generally deteriorate more rapidly than when the mixture is packaged as “calibration gas” in a more expensive fully passivated cylinder.

At least some docking stations allow the user to define the level of accuracy desired. While the default settings of the docking station may emphasize speed and minimize gas use, the user can optionally specify more stringent pass fail criteria. One way to do this is to wait longer after the docking station begins to flow gas to the sensors before deciding whether or not the instrument is in calibration. However, the higher the level of accuracy specified, the longer the bump test takes.

In the case of the GfG DS-400 Docking Station users have three setup choices when specifying the amount of time the docking station waits before verifying the response of the sensors. The default setting for the bump test duration is “no time” specified. In this case the docking station determines from the shape of the sensor response curve when it has enough information to assess whether or not the instrument is in calibration, and whether or not the alarms are properly activated when exposed to gas. Typically with this setting a bump test takes about 15 seconds (which is enough time for all sensors to reach their high alarm setting, and for the docking station to have enough information to extrapolate their final T100 reading). On an optional basis users can select “T50” rather than the default setting. In this case the docking station only waits until the readings from all sensors have reached 50% of the concentration of the gas applied before verifying the calibration of the sensors, (any alarms that are set higher than 50% of the value of the gas applied will not be tested). Choosing “T50” speeds up the test a little, and takes about 12 seconds to complete. The third choice is “T90.” In this case the docking station waits until the readings from all sensors have reached 90% of the concentration of the gas applied (“T90”) before verifying the calibration of the sensors. A “T90” bump test takes about 25 seconds to complete.  Although it takes a little longer, it provides the most stringent evaluation of the calibration state of the sensors.

Of course, docking stations can just as easily be used to calibrate as bump test instruments. Calibration is a two-step process.  In the first the readings are “fresh air” adjusted in fresh air that contains no measurable contaminants.  In the second step the sensors are “span” adjusted while exposed to known concentration calibration gas. Because the readings must be allowed to stabilize completely (i.e. reach “T100”) before being span adjusted, the complete calibration process takes substantially longer than performing a “bump test.” In the case of the GfG DSA-400 Docking Station, a complete two-step calibration (including fresh air, span calibration and a purging interval) takes about 2.5 minutes. Some users prefer to calibrate their instruments on a daily basis rather than to perform a daily bump test. This is a completely valid approach, and provides the highest level of accuracy possible.

A WORD ON GAS MONITOR MANUFACTURERS

From time to time the name of a specific manufacturer may appear in this Blog. Any such mention is for illustrative purposes only and is not an endorsement, nor is it an indictment of the particular item mentioned. The purpose of this Blog is to address the technical and safety issues of Gas Detection Instrumentation.

WHAT IS MY O2 SENSOR TELLING ME?

O2 SENSOR LOW ALARM FOR COMMON TOXIC GASES-    IDLH
	CHLORINE (CL2)	AMMONIA (NH3)	HYDROGEN CYANIDE (HCN)	CARBON MONOXIDE (CO)
IDLH PRESENCE (ppm)	10	300	50	1200
O2 PRESENCE (ppm)	208998	208937	208990	208749
LOW ALARM (195000ppm)	NO	NO	NO	NO
O2 SENSOR LOW ALARM FOR COMMON TOXIC GASES-    IDLH
	HYDROGEN SULPHIDE (H2S)	METHANE (CH4)(10%LEL)	PROPANE (C3-H8)(10%LEL)	TOXIC TWINS (HCN+CO)	CONFINED SPACE MIX (CO+H2S+CH4)
IDLH PRESENCE (ppm)	100	5000	2100	1250	6300
O2 PRESENCE (ppm)	208979	207955	208561	208739	207683
LOW ALARM (195000ppm)	NO	NO	NO	NO	NO
O2 SENSOR LOW ALARM FOR COMMON TOXIC GASES-    LEL
	CHLORINE (CL2)	AMMONIA (NH3)	HYDROGEN CYANIDE (HCN)	CARBON MONOXIDE (CO)
LEL PRESENCE (ppm)	NA*	150000	56000	120000
O2 PRESENCE (ppm)		177650	197296	183920
LOW ALARM (195000ppm)		YES	NO	YES
Most combustibles will burn in chlorine as they do in oxygen.
Chlorine can support combustion and is a serious fire risk.
O2 SENSOR LOW ALARM FOR COMMON TOXIC GASES-    LEL
	HYDROGEN SULPHIDE (H2S)	METHANE (CH4)	PROPANE (C3-H8)	TOXIC TWINS (HCN+CO)	CONFINED SPACE MIX (CO+H2S+CH4)
LEL PRESENCE (ppm)	40000	50000	21000	176000	210000
O2 PRESENCE (ppm)	200640	198550	204611	172216	165110
LOW ALARM (195000ppm)	NO	NO	NO	YES	YES

O2 Sensors read 20.9% of the environment. That means everything is MINIMIZED by about 5 times. Take a look at the IDLH of the common toxics as related to O2 alarms. A monitor set up for LEL-O2-CO-H2S will NOT alarm for a IDLH presence of Chlorine, Ammonia or Hydrogen Cyanide. Small changes in O2 percentage are BIG CHANGES in safety levels.

FIRE SERVICE- MONITORING OVERHAUL

I believe that due to the nature of modern construction practices and products Overhaul is a neglected aspect of Fire Department Operations that requires a gas monitoring protocol. The byproducts of combustion such as Hydrogen Cyanide, Carbon Monoxide, Acrylonitrile and other toxic gases can remain at the fire scene long after the flames are out. The act of walking through the ashes is enough to release pockets of trapped gases and stir up dust clouds that may be coated with these chemicals.

Monitoring for the presence of these chemicals during overhaul requires at the very least, CO and HCN capability. The addition of a Photo-Ionization Detector (PID) for broad range volatile Organic Compounds (VOC’s) is desirable.

While levels of toxic gases may be very low in Overhaul, the Time Weighted Average (TWA) becomes critical. Prolonged exposure at low levels is the same as short exposure at high levels.

When in doubt, wear your SCBA!!

For more information on the toxic effects of smoke
visit FireSmoke at: http://www.firesmoke.org/

FIRST RESPONDER- GAS MONITOR SELECTION

GAS MONITOR SELECTION

DIRECT READING, DIGITAL, REAL TIME MONITORS

FIRST RESPONDER OPERATIONS

USE:

5 MAJOR CATAGORIES ARE IDENTIFIED:

     STRUCTURAL FIREFIGHTING

     USAR

     HAZMAT

     CO CALLS

     GAS CALLS

OCCURANCE:

FREQUENCY OF GAS PRESENCE BY USE CATEGORY.

GAS HAZARD SELECTION:

5 MAJOR CATAGORIES ARE IDENTIFIED:

     PRODUCTS OF COMBUSTION

     HAZARDOUS CHEMICALS- FIRESCOPE CONFORMANCE

     CONFINED SPACE OPERATIONS

     CARBON MONOXIDE

     EXPLOSIVE GASES

RISK GROUPS:

5 MAJOR GROUPS ARE IDENTIFIED:

     FIRE RESPONDERS

     OTHER FIRST RESPONDERS

     FIRE COMMAND

     OPERATIONS COMMAND

     PUBLIC

DETECTION TYPES:

2 MAJOR CATAGORIES ARE IDENTIFIED:

     TOXIC SPECIFIC SENSORS

     NON-SPECIFIC SENSORS

SUMMARY OF APPLICATION DATA:

BY CHEMICAL HAZARD (RANKED BY OCCURANCE):

     METHANE & OTHER EXPLOSIVE GASES (NON-SPECIFIC)

     OTHER TOXIC GASES (NON-SPECIFIC)

     OTHER VOLITILE ORGANIC COMPOUNDS (NON-SPECIFIC)

     HYDROGEN SULPHIDE

BY TOXIC SPECIFIC SENSOR (RANKED BY OCCURANCE):

     CARBON MONOXIDE

     HYDROGEN SULPHIDE

BY NON-SPECIFIC SENSOR (RANKED BY OCCURANCE)

     METHANE & OTHER EXPLOSIVE GASES (NON-SPECIFIC)

     OTHER TOXIC GASES (NON-SPECIFIC)

     OTHER VOLITILE ORGANIC COMPOUNDS (NON-SPECIFIC)

     HYDROGEN SULPHIDE

IDENTIFICATION OF OPERATIONAL CRITERIA:

CATEGORY A:

            SINGLE SENSING CAPABILITY

            MULTIPLE SENSING CAPABILITY

            SPECIFIC SENSING CAPABILITY

            NON-SPECIFIC SENSING CAPABILITY

FREQUENCY OF OCCURANCE

CATEGORY B:

EASE OF USE

COST

ANALYSIS & CONCLUSIONS:

SENSOR SELECTION- ALL SENSORS (CATEGORY A):

            NON SPECIFIC SENSORS LEL

            NON-SPECIFIC SENSORS PID

            NON-SPECIFIC SENSOR O2 (CONFINED SPACE)

            HYDROGEN SULPHIDE

            CARBON MONOXIDE

SENSOR SELECTION- TOXIC SPECIFIC SENSORS (CATEGORY A):

            HYDROGEN SULPHIDE

            CARBON MONOXIDE

            HYDROGEN CYANIDE

SENSOR SELECTION- NON-SPECIFIC SENSORS (CATEGORY A):

            NON SPECIFIC SENSORS LEL

            NON-SPECIFIC SENSORS PID

            NON-SPECIFIC SENSOR O2 (CONFINED SPACE)

EASE OF USE (CATEGORY B) SIMPLE TO COMPLEX:

            TOXIC SPECIFIC SINGLE GAS INSTRUMENT

            NON-SPECIFIC SINGLE GAS INSTRUMENT

            TOXIC SPECIFIC MULTI-GAS INSTRUMENT

            NON-SPECIFIC MULTI-GAS INSTRUMENT (LEL-O2-PID IN COMBINATION)

            SPECIFIC/NON-SPECIFIC MULTI-GAS INSTRUMENT

COST (CATEGORY B) LOW TO HIGH:

            O2 MONITOR

            TOXIC SPECIFIC SINGLE GAS INSTRUMENT

            NON-SPECIFIC SINGLE GAS INSTRUMENT

            TOXIC SPECIFIC MULTI-GAS INSTRUMENT

            NON-SPECIFIC MULTI GAS INSTRUMENT (LEL-O2-PID IN COMBINATION)

            SPECIFIC/NON-SPECIFIC MULTI-GAS INSTRUMENT

CONCLUSIONS:

            HAZMAT TO BE EQUIPED WITH SPECIFIC/NON-SPECIFIC MULTI-GAS

INSTRUMENTS WITH A MINIMUM CAPABILITY OF:

                        LEL-O2-CO-H2S-PID

                        ADDITION OF A 3^rd TOXIC SENSOR FOR HCN IS
                        RECOMMENDED.

            TRUCKS AND/OR ENGINES TO BE EQUIPED WITH TOXIC SPECIFIC
            SINGLE GAS INSTRUMENTS FOR:

CARBON MONOXIDE

METHANE & OTHER EXPLOSIVE GASES

2 GAS INSTRUMENTS (CO-CH4) MAY BE CONSIDERED AS

ALTERNATIVES.