How to Choose an Industrial Opacity Monitor

Industrial Equipment Selection Guide

The right opacity monitor is determined by more than stack diameter or price. Start with the purpose of the measurement and the facility’s permit, then evaluate optical path, process conditions, outputs, alarms, calibration and long-term service requirements.

1. Define the Monitoring Requirement

First determine why opacity is being measured. Common objectives include:

• Meeting a continuous-opacity requirement in an air permit
• Providing operators with continuous process information
• Activating an alarm or relay at a selected visible-smoke level
• Monitoring combustion, baghouse or duct performance
• Replacing an obsolete smoke or opacity instrument

A monitor intended for a process alarm should not automatically be treated as a regulatory continuous emissions monitoring system. Obtain the applicable permit language and confirm the governing federal, state or local requirements before choosing equipment.

2. Identify the Required Standard

If the application requires U.S. EPA 40 CFR Part 60, Appendix B, Performance Specification 1, the instrument and installation must be selected around that requirement. Both the EMS WatchDog PS-1 and the standard EMS WatchDog meet or exceed Performance Specification 1 and ASTM D6216 and are supported by startup, certification and audit services. Choose between the touchscreen PS-1 configuration and the standard WatchDog’s traditional panel controller and 3–50 ft path capability.

If the permit does not require PS-1, a single-pass process monitor or smoke detector may provide the needed information more economically. Never assume that a product is suitable for compliance solely because it displays opacity.

3. Measure the Optical Path and Review the Installation

Confirm the inside distance the light will travel, mounting-port size, flange or pipe arrangement, available access and clearance for alignment and maintenance. The measurement location should provide a representative path and allow safe access to the optical components.

EMS systems are available for different path-length and mounting requirements. The standard WatchDog supports physical paths from 3 to 50 ft, the WatchDog 800 supports 3–8 ft paths, and the Guardian supports 2–8 ft paths. All PS-1 configurations should be reviewed against the stack, exit correlation and site-specific compliance requirements.

4. Evaluate Process and Environmental Conditions

Temperature, vibration, moisture, ambient light, dust loading and outdoor exposure can affect instrument selection and installation. Review process-gas temperature at the mounting location and the ambient temperature around the sensor, detector, reflector and controller.

Optical windows also need protection from contamination. Proper purge air, alignment and access for cleaning help maintain reliable light transmission. Enclosure ratings should match the installation environment rather than being treated as a general specification.

5. Choose the Needed Outputs and Alarms

List every device that must receive opacity information. Common requirements include 4–20 mA analog signals, relay alarms, digital communication, local displays and trend information.

• WatchDog PS-1: 4–20 mA output, RS-485 stack-to-controller communication, touchscreen trends, diagnostics and configurable warnings.
• Standard WatchDog: two individually configurable 4–20 mA outputs, six programmable relays and MODBUS over RS-485.
• WatchDog 800: one 4–20 mA output and a 2 A alarm relay for supported process applications.
• Guardian: two relay contacts for response at a selectable opacity set point.

Confirm the input type, scaling, cable distance and control logic of the receiving equipment before installation.

6. Plan for Calibration, Certification and Maintenance

Opacity monitors require correct alignment, clean optics and periodic functional checks. Regulated applications may also require initial certification and recurring audits. Consider who will perform the work, how quickly service is available and whether certified neutral-density filters are needed.

EMS provides startup, certification, quarterly audits, field service and factory service. EMS also supplies and certifies audit neutral-density filters.

7. Compare Total Ownership Requirements

Purchase price is only one part of the decision. Compare installation hardware, cabling, purge-air requirements, control-system integration, routine cleaning, replacement parts, certification, audit support and expected equipment life. A monitor that closely matches the application is usually less expensive to own than a system that requires workarounds or cannot support the permit.

EMS Opacity Monitor Comparison

Information to Gather Before Requesting a Quote

• Facility permit language and required monitoring standard
• Process and application description
• Inside stack or duct dimensions and desired optical path
• Existing port, flange or pipe-mount dimensions
• Process-gas and ambient temperature ranges
• Power available at the stack and controller location
• Required analog, relay or digital outputs
• Required alarms, display and data-recording functions
• Photos, drawings and details of equipment being replaced
• Startup, certification, audit and service requirements

Frequently Asked Questions

What is the best opacity monitor for a crematorium?

It depends on the required function. The Guardian provides a selectable smoke-opacity alarm, while the WatchDog 800 provides continuous process measurement. If the permit specifically requires EPA PS-1, evaluate the WatchDog PS-1.

Can EMS replace an older opacity monitor?

Yes. EMS can review the existing mounting, ports, wiring, required outputs and compliance needs to recommend a replacement configuration or adapter arrangement.

Does EMS provide installation and certification?

EMS provides application review, startup, certification, quarterly audits, field service and factory service for supported systems.