Technical Overview

•   Transmittance
•   Opacity
•   Optical Density
•   USEPA PS-1
•   USEPA PS-11
•   Opal Technology

What is a Transmissometer?

A transmissometer is a continuous emissions monitoring instrument for the determination of smoke and particulate concentrations in industrial flue gas emissions. Based on the principle of transmitted light measurement the transmissometer can provide continuous measurement of opacity (visible emissions) and optical density (quantitative dust concentrations).

The transmissometer transmits and measures the intensity of a light beam (transmittance) after it has passed through an industrial stack, or duct system. The beam intensity will be reduced by either absorption or deflection by the presence of particulate concentrations in the flue gas stream.

Transmittance expressed as a percentage, is the ratio of measured light (I_M) against the measured value of the same light beam when it has passed through the same path under clean stack conditions (I_O), using the following formula:

From the measured value of transmittance, we can then calculate the following:

Opacity, expressed in a scale of 0 – 100%, is the measurement of "visible emissions" as seen by the human eye (100% representing total obscuration).

Optical Density, is the calculated measure of the rate at which light energy is lost as it passes through the flue gas stream. This value is directly proportional to the mass concentration of particulate and can be calibrated for the quantitative measurement of flue gas emissions.

OPAL 100 and 200 Series Opacity and Dust monitoring systems provide continuous measurement signal outputs for both opacity and optical density for use by industrial sites for improved process control and EPA site license requirements.

What standards of Performance are applicable for OPAL Transmissometers?

OPAL 200 series opacity and dust monitoring systems are designed, manufactured and test certified to the ASTM D 6216–03 design and test standard for opacity monitor manufacturers. This is the prerequisite for compliance to the below USEPA performance standards and guidelines, which are the most widely accepted international standards.

USEPA Performance Specification #1 is the specification for users of opacity monitors and can be simplified down to three basic requirements for compliance:

1. Purchase a transmissometer that has been designed and tested in accordance with the American Society for Testing and Materials standard ASTM D 6216–03.
2. Install the system in accordance with stated guidelines to ensure the most suitable sampling position has been achieved.
3. Conduct and record a 168 hour (7 day) performance test to demonstrate that the system, and associated recording equipment, will be suitable and reliable for the specific application.

USEPA Performance Specification #11 is a newly promulgated specification / guideline for end users required to continuously measure quantitative dust concentrations. Unlike PS-1, there is no specified instrument technology for the purpose of quantitative measurement of particulate, recognising that there are both suitable and unsuitable applications for each instrument technology. There are four basic requirements for end user compliance:

1. Select a suitable instrument (technology) for the application. This should be done in consultation with various preferred suppliers to avoid misguided selection and ensure suitability. There are some basic technical requirements that the instrument must have, including the ability to be able to undertake automatic self testing, and be capable of external audit test validation. The OPAL 200 series systems fully comply all of these conditions.
2. Install the system in accordance with stated guidelines to ensure the most suitable sampling position has been achieved.
3. Conduct and record a 168 hour (7 day) performance test to demonstrate that the system, and associated recording equipment, will be suitable and reliable for the specific application.
4. Carry out system response correlation against the results of a series of not less than 15 separate gravimetric sample tests. When completed the instrument must demonstrate a "confidence factor" of not less than 85% against all of the stack tests. Failure to comply may require that the instrument be replaced, or the correlation tests be repeated.

OPAL Technology – Did You Know?

The OPAL 100 and 200 Series product range designs have many unique and innovative features that competitor systems do not have (yet!). Here are just some of them:

•  Push button digital calibration – All OPAL transmissometer systems require only a clean stack condition (0% opacity) and a totally blocked stack condition (100% opacity) for system calibration. If the calibration is carried out in–situ, the total blockage condition is best achieved by simply swinging open the transceiver and directing the light beam in any non reflective direction. (Incidentally, the OPAL systems are 100% immune to all stray light). The OPAL system calibration procedure requires no special calibration filters, no tools, no adjustments, takes less than 2 minutes to complete, and offers non drift calibration that is typically better than +/- 1.0% opacity.

•  Ball / socket alignment flange – If you are familiar with general transmissometer system designs, you will have seen many wierd and wonderful alignment flange arrangements. Most competitor systems have dual alignment flange plates that are bolted together with space separation using stacked curved washers or springs. (Tightening the alignment bolts will compress the spacers and change the beam alignment.) Once completed a large rubber sealing band is fitted to prevent flue gas escape or water ingress between the flanges. The OPAL 100 and 200 systems have a very unique alignment flange design based around two mating flange plates that couple together with a ball/socket arrangement. There are four adjustment bolts offering north/south/east/west adjustment over a very wide arc of +/- 4 degrees from the centre line. The key benefits of the OPAL design is rigid positioning, no effect from vibration, zero flue gas or water leakage under all conditions of pressure and temperature, and no inconvenient rubber seals or consumable spacer parts required.

•  Local control and test – The OPAL 200 transceiver has a local keypad and display for one–man service and commissioning convenience. The keypad has real time display of the transceiver and the remote located main control unit outputs, scaled in % and actual mA values. the keypad also allows local control of manual span and zero tests, as well as ability to initiate the autocalibration test cycle. During manual audit filter tests, the transceiver can be switched to MANUAL TEST to disable any path length correction factors and allow direct display of actual test filter values, thus avoiding the inconvenience of external filter calculations at each instrument. Other key features of the local keypad / display are:

  Carry out calibration cycle	Display diagnostic values and faults
  Set up system operating requirements	Service adjustments of autocal servodrives

For more information on this subject, contact our office at sales@opalenvironmental.com.au