Honeywell Midas Installation Manual

Protocol for Testing Toxic Gas

1998-0219 Rev 5 August 2012

Technical Note

Introduction

This protocol provides a method to evaluate the performance of toxic gas detectors. A
worksheet is included; this is a useful guide in recording the performance of gas detection
sensors. It is also useful as part of a maintenance log for complete gas detection systems.

To understand the merits of specific gas detection equipment, several parameters must be
tested. These factors include response time, environmental conditions, affects of temperature,
accuracy and sensitivity to potential interfering substances, recovery time, failure indication,
stability (drift) and repeatability over time. Test conditions must simulate the real world;
therefore the test conditions must simulate the working environment (temperature and
humidity). Supplies and materials must be selected accordingly.

The gases utilized may be very toxic. It is therefore essential that a thoroughly trained
safety engineer or industrial hygienist be responsible for the generation of these gases
and that the gas is generated in a well-ventilated area and exhausted safely.

Equipment and Test Gas

1. Zero Air for zero calibration

In applications where the ambient air is normally containing a low level of target gas,
some sensors may require zero calibration with “clean air”.

a. Compressed air

(Filtered through activated charcoal to remove most of unwanted gases and water
vapor)

b. Zero Air in Lecture bottle

2. Span Gas for Bump Test and Calibration

To achieve the best accuracy, a mixture of the target gas balanced in the background
environmental air is the best calibration gas. However, it usually requires that the
operators be more skilled, with precise equipment and reference standard method to
analyze the gas concentration. The following are recommended methods of gas setup
for bump and calibration.

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a. Disposable Calibration Gas Bottle

(Low pressure, premixed with Air or Nitrogen)
This method with fixed flow or demand flow regulator is easiest and practical way to

bump test EC sensors (both extractive system and passive sensor wit h calibration
cap or flow housing).

For extractive sampling systems when the concentration of gas lecture bottle is
higher than the detection range, test gas can be diluted with fixed flow rate regulator
and T fitting in sample line. Use fixed flow regulator having lower flow rate than
extracting sample flow rate and attached clean air bag at T fitting. (ex. Using 0.25
L/min regulator with clean air at T fitting, the sample test gas concentration for a
MIDAS with ~0.5 L/min flow is approximately half of the concentration of bottle.)

A zero air lecture bottle with fixed flow regulator can be used for dilution (and use
another T fitting to vent overflow with extractive systems). This method would also
work with passive detection systems.

The lecture bottle dilution method is suitable only for bump testing as the gas mixture
accuracy depends on the flow rate precision.

The type and concentration of calibration gas, sample tubing, flow regulators and
calibration adapters are key links in the calibration chain. The instrument can only be
as accurate as the gas used to calibrate it.

As concentration stability and shelf life is dependent on the mixture of gas and bottle
type, do not use uncertified and/or expired gas cylinders. Most highly reactive
chemicals are mixed with nitrogen.

Check that all gas wet materials including regulator and tubing are preconditioned
with the gas sample before applying gas to sensor.

Some sensors may need moisture to get a proper reading. A humidifier, such as a
“Nafion®” humidifier, can be added in sample line. Check the compatibility of the
humidifier with the target gas before use.

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b. Sampling Bag (Tedlar or Teflon)

This method is good for both extractive systems and non-reactive gases fill i ng fr om a
gas cylinder, diluted gas, or permeation device. Some gas bag materials are
permeable and have gas adsorption characteristics.

c. Permeation/Diffusion Device

A permeation device has some advantages over a calibration standard cylinder;
providing precise concentrations and a wide range of concentrations easily generated
by varying the dilution flow rate and or the set point of oven temperature.

At a known rate of permeation and a given temperature, a constant flow rate of air
mixed with the permeated chemicals forms a constant stream of calibration gas. A
calibrator with constant temperature and flow regulation is needed. Portable
permeation devices are commercially available.

Prior to use, permeation devices should be conditioned at the calibration temperature
and carrier flow to bring the rate to its equilibrium value. Most devices require 30
minutes to 3 hours to reach equilibrium. Heavy wall tubes, low vapor pressure
compounds, and halogenated compounds typically take longer. The best procedure is
to set up the calibration system the day before it is needed, allowing the system to
equilibrate overnight. Conduct repeated tests over a period of time to ensure that
equilibrium has been achieved. Test gas can be filled in a sample gas bag, or feed
sample gas to passive sensor, or extract test gas directly connecting at T fitting in
Span with Overflow (Vent) output mode.

There may be an activated charcoal scrubber for carrier/dilution air before the
permeation chamber in commercially available portable permeation gas generator; the
generated test gas will be dryer than ambient air, requiring additional humidification
(such as Nafion® humidifier) for some gases and sensors.

d. High Pressure Gas Cylinder

High-Pressure Cylinders are designed for hazardous chemicals and for high
concentration mixture which is much stable than above mentioned methods.

d-1. High Pressure Gas Cylinder - Single Stage Dilution Method

This method is good for both passive or extractive system, and Gas Bag filling for
remote test. Schematics of the recommended gas generation and dilution system
are provided in Figures 1 and 2.

Figure 1 shows a typical single stage dilution gas generation rig. This is best
utilized when diluting c once nt rat e d cyli nd er s at rati os bel ow 10 00-to-1.

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d-2. High Pressure Gas Cylinder - Double Stage Dilution Method

This method is good for both passive or extractive system, and Gas Bag
filling for remote test.

Figure 2 shows a typical double stage dilution rig. This is utilized for diluting
cylinders at ratios greater than 1000-to-1. Double stage dilution is the best
method when generating low level concentrations is required.

The concentration of calibration gas can be calculated as follows:
Concentration (ppm) =

Cylinder Flow (cc/min) * Cylinder Concentration (ppm)

Total Flow [Cylinder +Dilution F low] (cc/min)

Note:

Multiply gas concentration in ppm units by 1000 to obtain concentration in ppb units.

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