The Three Major Causes of Refractometer Trouble in Black Liquor Recovery Boilers

Pulp and paper mill
Pulp and paper mill.
The three major causes of refractometer trouble or failure in black liquor recovery boilers are:
  1. Loss of cooling water and its effect on the sensing head.
  2. Lack of reliability of the prism wash.
  3. Condensation in the sensing head.
These may not apply to all refractometers due to differences in construction and circuitry.

Cooling Water Loss

It is of vital importance that the loss of cooling water be detected. This may be done through a temperature sensing element or flow monitor which shuts down the refractometer involved.

Damage to the sensing element of a refractometer does not occur instantaneously, but it is essential that the system detect abnormal temperatures due to cooling water loss, flow blockage, etc., and that the cooling water be promptly restored.

The individual refractometer manufacturer’s instruction and maintenance manuals shall be consulted with reference to: potential damage to the sensing element; identification of a damaged element; how and when to replace a damaged element.

Prism Wash

The time interval between prism washes may vary with the black liquor composition. It is recommended that the minimum wash period be 7-10 seconds of wash every 20 minutes. Short duration washes at more frequent intervals are more effective than long washes at long intervals. Ideally, steam pressure for prism washing should be 35 psig above the black liquor pressure, plus the pressure required to open the protective check valve.

Awareness must be maintained of the effect of changes to the prism wash programming variables. Various refractometer systems have the capability to adjust: condensate drain time, steam on time, recovery time and interval between wash time. It may be possible to configure the system to have the total time that both refractometers are in their wash cycle represent a significant percentage of operating time. If one refractometer is out of service for repairs and the remaining refractometer is in prism wash, black liquor solids are not being monitored. Prism wash should be minimized to that needed to maintain the system.

If high pressure steam is used, it may abrade the prism. If only high pressure steam is available, a reducing valve shall be used.

The refractometer prism must have a clear polished optical surface, and if it becomes abraded, it must be replaced.

If the prism wash system has not operated properly and the prism becomes coated, it must be removed and properly cleaned.

Condensation in Sensing Head

Condensate may build up in the refractometer sensing head and if this occurs, the instrument operation will be erratic.

The procedure for determining this condition and for the elimination of excessive moisture in the sensing head is not the same for all refractometers. The manufacturer’s instruction and maintenance manuals shall be consulted and followed carefully.

Reprinted from "Recommended Good Practice: Safe Firing of Black Liquor in Black Liquor Recovery Boilers" courtesy of the Black Liquor Recovery Boiler Advisory Committee.

Image by AlexiusHoratius [CC BY-SA 3.0  (], from Wikimedia Commons

Electron Machine Corporation's C.A. Vossberg Receives Distinguished Partner Manufacturing Service Award

2018 FLATE Award Winners
2018 FLATE Award Winners
The Florida Advanced Technological Education Center (FLATE) and the Florida Association for Career and Technical Education (FACTE) awarded Electron Machine's President, C.A. Vossberg, their 2018 Distinguished Partner Manufacturing Service Award.

The Awards represent FLATE’s efforts to
recognize leaders who have been at the forefront of manufacturing workforce education and training.

This is the eleventh year of our FLATE Awards program and is one of FLATE’s many efforts to showcase and recognize the contributions of educators and industries in advancing technician education and training on a regional and statewide level.

Distinguished Partner Manufacturing Service Award
C.A. Vossberg (middle)
Electron Machine, headquartered in Umatilla, Florida, is a premier manufacturer of industrial
refractometers and process control equipment.

For more information, contact Electron Machine by visiting or by calling 352-669-3101.

The Basics of Process Refractometers

Light refractionA refractometer is a process instrument capable of determining a solution's refractive index. Light bends and changes velocity as it travels from one media into another through the media interface. When light traveling through air enters liquid, the light rays change direction by an amount determined by the liquid's density.

Angle of refraction refers to the magnitude the light bends as it exits one media and enters the interface of another. With the angle of refraction defined by their densities, different liquids display different amounts of refraction - for example, a higher density juice such as orange juice will have significantly different refraction than cranberry juice, because of its higher density.

Light refraction
Light bends when traveling through
different media. That's why this pencil
appears to be "broken" when it enters
the water.
A liquid's refractive index relates to the amount of light bending that liquid displays. The greater the bending, the greater the refractive index. The lower the bending, the lower the refractive index.

Standard tables are available that correlate refractive index to a variety of materials. These same tables also correlate refractive index to varying concentrations of particular liquid media at a particular temperature. Take corn syrup for example. Different refractive indexes are observed for different corn syrup samples of different concentrations. Therefore, by using a process refractometer to observe the refractive index of a particular corn syrup sample, a determination of the concentration of that particular sample can be made. By referring to the table or scale that correlates the refractive index to concentration at a particular fixed temperature, liquid concentration can be determined.

The refraction index of the liquid medium readings will vary at different temperatures, and therefore, the sample's temperature must be measured and compensated for in order for refractive index readings to be accurate and repeatable.

Refractive index measurements have been used for process control in the food, juice and beverage industries for decades, with the most common applications being the measurement of sugars (Brix) and total dissolved solids (TDS). Large scale production and processing of fruit juices, jams, tomato products, wine, beer, coffee, and many other products rely on industrial refractometers for quality and consistency.

For more information contact Electron Machine Corporation by visiting or by calling 352-669-3101.

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What Are Critical Angle Refractometers?

Critical angle refractometersCritical angle refractometers use the principle of critical angle refractometry by which dissolved solids content is related to the refractive index of the solution. The basic refractometer unit unit consists of a sensing head and control console. The sensing head is mounted on the adapter installed in the process line and the process solution is usually viewed through a transparent window. The surface of the solution, which is in contact with the window, is optically scanned by an octagonal prism.

The refractive index is determined by a beam of light from the prism which repeatedly sweeps at a varying angle at the surface of the solution. At a point in the cycle, as the angle between the incident light beam and the surface decreases, the light beam instead of refracting into the solution is reflected back into the optical system. The point of change from refraction to reflection depends on the refractive index of the solution and is known as the critical angle. The prism scans the beam of light through the critical angle. A photodetector measures the alternating light and dark periods and its electrical output is used to provide a readout of refractive index in the required units (R. I., % concentration, degrees Brix., etc.).

Critical angle refractometers
Optical Path
Diagram - How They Work

Critical angle refractometers use an LED as a light source and utilizes state of the art CCD (charge coupled device) technology to accomplish scanning the reflected light returned from the prism.

Light radiated from the LED passes through the prism surface to be reflected off mirror 1 to the prism-to-process interface. The light reaching this interface intersects the same interface over a series of angles chosen to include critical angle for the process being measured. Light intersecting the interface at an angle greater than critical angle is refracted into the solution. Light intersecting the interface at less than critical angle is reflected up to mirror 2 and out of the prism up to the CCD linear array to be scanned.

A principal advantage of critical angle refractometry is that it measures the index of refraction at the surface of the process solution. Since the light beam does not penetrate into the solution, the instrument can be used for opaque as well as transparent materials.

Measuring Solids in Pulp & Paper Black Liquor

Pulp and paper plant
Pulp and paper plant.
Industrial paper manufacture involves a procedure known as the Kraft process, where wood is converted into wood pulp and then into paper. The process, however, produces a toxic byproduct referred to as black liquor. This primarily liquid mixture of pulping residues (such as lignin and hemicellulose) and inorganic chemicals from the Kraft process (such as sodium hydroxide and sodium sulfide) is toxic.

Until the invention of recovery boilers in the early 20th century, black liquor was often simply released into waterways. Black liquor recovery boilers allowed paper manufacturers to recover and reuse the inorganic chemicals and extract energy from the pulping residues.

Paper processingReliable, continuous measurement of black liquor solids content is a subject of considerable importance to the pulp industry. The solids content of liquor introduced into a recovery furnace can have a pronounced influence on firing behavior. Current trends toward better control of the recovery furnace for reasons of safety and reduced air emissions require a greater degree of control over incoming solids content and appropriate adjustment of operating conditions to handle variations in solids content. This, in turn, requires a reliable method for continuously monitoring solids content.

Instruments such as inline process refractometers successfully measuring black liquor solids concentration continuously. These instruments have the dependability, accuracy, and reliability to augment safe operation of recovery boilers. Refractive index-type instruments are more widely accepted and far more successful with less maintenance requirements than other types of instruments used for black liquor solids measurement, and have proven reliable and accurate for automatic monitoring of black liquor solids concentration.

Inline Process Refractometers for Fruit Juice Concentrate Production

Fruit Juice Concentrate
Just about every fruit harvested is processed to a concentrate. Fruit juice concentrate provides for easier transportation and longer storage life for both producers and consumers. Production technology for the juice concentration has become quite advanced, resulting in improved quality and consistency. Sweetness, color and solid components from the feedstock fruit juice carefully monitored and controlled.

Fruit juice concentrate production starts with dilute juice feedstock, the application of carefully controlled heat to evaporate off water, ultimately resulting in a uniform and consistent concentrated juice. The fruit juice stock is extracted from various fruits in a number of ways that are specifically adapted for the shape, size, and nature of the fruit. It is then purified and stored in primary holding tanks. Juice concentration will vary at this initial stage due to a number of natural factors and needs to be processed to desired quality standards.

One objective of the concentration process is to remove excess water in a consistent and uniform manner. Excess water removal is done through the use of specialized multi-stage evaporators that extract water without damaging the juice by applying improper amounts of heat. A closed-loop control system monitors a variety of process variables such as temperature, flow, and pressure from multiple process sensors. The readings from these sensors drive proportional outputs that modulate final control elements such as control valves.

Process refractometers are sensors used at strategic points to measure dissolved solids (sugar) concentration.  By monitoring and controlling percent solids and Brix, plant operators gain tighter control of product quality and more efficient use of equipment (possible energy savings).

For more information on the application of process refractometers in juice and juice concentrate, contact Electron Machine Corporation by visiting or calling 352-669-3101.