McNab Model RS Particle Contamination Monitor (Forward Scatter)

Features

• Signal ratio measurement of suspended solids in process fluids.
• Full range digital meter with engineering units such as PPM or percent indicating.
• A light scattering turbidity measuring instrument, self correcting for variations in bulb illumination, color and fouling of optical surfaces.

Technical Papers:

• Size Distribution and its Effect on Particle Measurement in the Beer Line
• Payout Economics on Process Stream Analyzers
• Optical Measurement of In-Process Fluids

Principles of Turbidity Measurement

Turbidity is the effect suspended particles have on a light shone through a liquid. The degree of turbidity is expressed in such units as PPM (parts per million), FTU, JTU, D.E., Helm, Wedge and others.

In a turbidimeter, a collimated light shines through the liquid. A direct-reading turbidimeter (such as the McNab Model HSA) measures the light which passes straight through the liquid. Nephelometric turbidimeters (McNab Model HSB) measure light which is scattered by reflection from the particles in the liquid.

Light, of course, is scattered in all directions by the suspended particles (the Tyndall Effect.) The amount of scatter in any particular direction may vary with various characteristics of the particles— shape, size, color, reflectivity, absorption and so on.

How the Electronic Ratio Technique Works

The forward scattered light (off the particle) and the direct beam light (transmitted through the pipe) are measured in the flow cell assembly, and because of its very low level, is amplified to appropriate voltage levels and is sent by shielded cabling to the chassis-mounted electronics. The scattered light voltage is then divided by the direct beam light voltage. The resultant signal is indicated in various units, e.g., PPM, JTU, and concentration (%) on the digital meter.

The Model RS consists of a flow-cell assembly and an indicator/control unit. (See "Ratio Measuring Technique" diagram).

As can be seen in the accompanying illustration, the flow cell, which is mounted directly in the process pipe, includes:

1. The light source and the associated collimating optics.

2. A combination detector which incorporates:

A reference detector which measures direct light from the bulb. Output from this detector is used to compensate for variations in input light intensity due to such factors as supply voltage variations, fouling of windows, bulb aging, color variations and others. It also provides bulb failure alarm.

A forward scatter detector, placed behind the reference detector and a light blocking disc which prevents direct light from interfering with the scattered light. Forward scatter light is collected in a non-imaging optical chamber (patent pending) and directed to the photocell.

The indicator/control unit contains the modular electronic circuitry to process the photosensor signals, provide and display readings in appropriate units, input desired set-points, indicate alarm levels, operate relays and alarms, provide continuous process documentation signals and perform self-diagnostics.

Forward Scatter Instrument

McNab Clarity Analyzers measure suspended solids in process liquids with:

• Bright self-lit digital meter for clear, easy reading
• Modular optical cells — newly introduced to forward scatter instruments
• 10-step range switch for best resolution
• Electronic preamplifier at flow cell assembly — up to 2,000 foot cable run

Modular Optical Cells

The RS flow cell assembly is small and compact; about one-half the size of older designs. This newly developed sensor/detector includes the modular optical cell and is light-weight and mechanically balanced—a feature exclusive to McNab instruments.

This combination has been developed to overcome glass-cleaning and gasket-replacement problems commonly associated with earlier, non-modular designs. Previous designs did not allow for servicing at the wetted face and required the entire flow cell assembly to be removed from the pipe for servicing (which is why flanges on spool pieces were required). In some cases, previous flow cell assembly designs had to be returned to the manufacturer just for window servicing.

Inside of McNab's exclusive modular optical cell are all of the elements that may be attacked or severely coated by the process; e.g. windows and gaskets. The Model RS modular optical cell can be removed and replaced in less than 5 minutes, preventing long down-time and costly delays.

Noisy Factory Environment Is Overcome By Flow Cell Preamplifier

Photodiodes (used to convert light to electricity) arc fast and stable, however their signal output is very low. A preamplifier and filter arc required to maintain the quality of the signal. A properly designed photodiode preamplifier circuit allows the instrument to function in a hostile factory RFI (radio frequency interference) and EMI (electro-magnetic interference) environment. This feature is exclusive to the McNab Model RS. The electronic preamplifier is custom-packaged to ensure a noise-resistant, high-gain, high-speed and temperature-stable preamplifier detector assembly, allowing sensors to be mounted up to 2,000 feet from the panel.

Built-in Zero Standard

The unique built-in zero standard can be used at any time to verify standardization, even when the process is operating. Simply press the test button, and the system verifies that actual standardization has been maintained; no drift has occurred. No down-time or removal is needed.

The RS achieves measurement stability of 0.2% per year, allowing maintenance-free operation. This is made possible by matched signal verifying modules (SVM's) with 100-megohm input impedance. These SVM's eliminate drift, so there is no need for constant restandardization. SVM's also correct for any variation in sensor cable length, so nearby electric interference and cable noise does not affect operation, even at cable distances of 2000 feet (600m).

Optical lamp life has been extended to an average of five years by use of optional improved lamp control circuitry. This makes a truly maintenance-free system.

McNab, Incorporated, in business for more than 90 years, designs and manufactures in-pipe analyzers for multi-parameter measurements.

We built our first fluid analyzer back in 1930. Today our product line has grown to include not only conductivity, and pH, but also absorption, color, infrared and scatter turbidity measurements.

Our products range in complexity from small electronics, such as industrial timers, to multi-point monitoring systems for the newest nuclear power plants.

Whether simple or complex, McNab instrumentation can be relied on to have well engineered solid-state circuitry, sound construction, reliable transducers and the McNab commitment to quality products, on-time shipments, reliable field service and reasonable prices.

Most McNab instrumentation is manufactured to our standardized designs with pre-existing software, but we maintain the engineering capability to create innovative designs to solve our customers' special instrumentation problems.

Specifications

A91-86