Measuring Total Soluble Solids with Refractometers

Inline, process refractometer for beverage production
Inline, process refractometer for beverage production.
Just as weight is expressed in pounds, the level of soluble solids in a solution is measured in degrees Brix (symbol °Bx).  The Brix scale is based on a solution of pure sucrose diluted with water. Adolf Brix first developed the Brix scale in the 1800s. For example, a 100 gram solution with a Brix 50 reading contains 50 grams of sugar (and other dissolved solids) and 50 grams of water.

Fruit juices, wine, nectars, and other beverages all contain soluble solids. Total Soluble Solids (TSS) refers to the total amount of soluble constituents of the juice, wine or other beverage. These are mainly sugars, with smaller amounts of amino acids, pectin, and organic acids. For example, approximately 85% of the total soluble solids of citrus fruit are sugars. Because sugar is the most abundant soluble solid, the Brix scale is used by the beverage industry in determining the sucrose equivalent of soluble solids in their products. The term "Brix" or "degrees Brix" is used interchangeably with % sucrose or % soluble solids by weight.

Refractometers are instruments that determine soluble solid concentration by evaluating the solution's refractive index. Changes in direction of a light beam passing through the solution correlate to the amount of dissolved solids in the solution. Basically, the higher the level of soluble solids in the solution, the greater the bending of the light beam. In large scale beverage plants, inline process refractometers are used to control quality and consistency by continuous monitoring of the soluble solid concentration.

For more information about measuring TSS and/or Brix in a commercial beverage production facility, contact Electron Machine by visiting or calling 352-669-3101.

Safe Firing of Black Liquor in Black Liquor Recovery Boilers: Refractometer Black Liquor Solids Measurement System

recovery boiler
Recovery Boiler
(Courtesy of Wikipedia)
The following is reprinted from Chapter 4 of the Black Liquor Recovery Boiler Advisory Board (BLRBAC) Recommended Good Practice document titled "Safe Firing of Black Liquor in Black Liquor Recovery Boilers" (April 2016).

Information on the BLRBAC can be found here. The full document, as well as other important information, can be found here.

Refractometer Black Liquor Solids Measurement System

4.1 General

The heart of the system for the safe firing of black liquor is the ability to correctly, accurately and reliably measure the solids in the black liquor stream immediately prior to the black liquor guns.  To accomplish this solids measurement, refractometers have proven to be effective for black liquor recovery boiler service. As new techniques in measuring solids are developed and proven, they can be considered. For the solids measurements, two refractometers in series must be used. When both refractometers are in service, the requirement for an automatic black liquor diversion can be satisfied by either of the following options:
  1. If either refractometer reads dissolved solids content 58% or below (62% or below if firing >70% solids per guidelines in 6.4 of this document), an automatic black liquor diversion must take place.  
  2. When both refractometers read dissolved solids content 58% or below (62% or below if firing >70% solids per guidelines in 6.4 of this document), an automatic black liquor diversion must take place.  
Either option is satisfactory.

If the instrument readings disagree on the percent solids by 2% absolute value, an audible and visual alarm must be given.

If one refractometer fails, or is removed from service, black liquor diversion must then be controlled by the remaining in-service instrument; and if this remaining instrument reads 58% or below solids, an automatic black liquor diversion must take place (62% or below solids if firing >70% solids per guidelines in 6.4 of this document). Black liquor shall not be fired if neither refractometer is in service.  The refractometers should be part of a specifically integrated system adapted to the black liquor service, and include a system to monitor their operation and indicate trouble or failure of the individual refractometer.  Refractometers used without such a monitoring system can fail unsafe and can give improper and unsafe dissolved solids readings under certain conditions.

4.2 Refractometer Control System Functions

The refractometer control system shall be capable of performing the following functions:

1. Monitor the positive (+) and negative (-) supply voltage of each refractometer independently. The refractometer's supply voltage shall be maintained within the predetermined minimum and maximum limits for safe operation.

2. Monitor the lamp voltage or lamp output of each refractometer independently. The refractometers’ lamp voltage must be within the predetermined minimum and maximum limits for safe operation.

3. Monitor the signal amplitude (if chopper circuit devices are used) of each refractometer independently. Each refractometer's signal amplitude must be maintained within the predetermined minimum and maximum limits for safe operation.

4. Monitor the liquor temperature at each refractometer’s sensing head independently assuring that each refractometer's liquor temperature is within the predetermined minimum and maximum limits for safe operation.

5. Monitor the automatic prism cleaning timer system of each refractometer. The sensor output circuit, prior to the hold circuit, should go negative or adequately decrease during the purge cycle.

6. Monitor the automatic prism cleaning timer system to assure that the purge occurs within the predetermined time.

7. Monitor the cooling water to each refractometer sensing head to assure that cooling water is not lost to a sensing head.

If any of these malfunctions (Items 1 through 7) occur, the following action shall be initiated:

a) An alarm shall be activated, identifying the refractometer and circuit at fault.

b) The refractometer shall be electrically removed from the refractometer control system.

c) The remaining “good” refractometer shall remain in service.

8. Compare the refractometer meter outputs. If a difference of 2% (absolute value) solids or greater exists between refractometer readings, an alarm shall be activated.

9. Performs a black liquor diversion, if one refractometer is removed from service or fails in prism wash, and the remaining refractometer fails or reads a solids of 58% or less.

10. Monitor all cables from the refractometer and the components of the control system. If any cable is cut or removed, an alarm shall be activated.

11. Provide primary alarm or diversion functions by a means other than the refractometer indicating meter’s contacts.

12. Have the capability to allow the manual removal of either refractometer from service retaining the remaining refractometer in full service for diversion purposes.

13. Require a manual reset following a black liquor diversion or malfunction of the refractometer control system.

14. Monitor the position of the sensing head isolation valves. A partially closed or closed valve shall activate an alarm and remove the refractometer from service.

15. Initiate a low solids alarm signal from each refractometer at 60% solids or at 70% solids if firing >70% solids per guidelines in 6.4 of this document.

16. Prohibit the simultaneous washing of the individual refractometers.

17. Require manual restoration of a refractometer which has been removed, either automatically or manually, from service.

18. Have provisions for manual prism washing.

19. Require an automatic switch to single refractometer diversion (for systems set to require both refractometers read low solids to divert – dual refractometer diversion) when one refractometer is in a prism wash cycle. Automatic return to the chosen dual refractometer diversion will occur after completion of the prism wash cycle.

All of the above functions may not apply to all refractometer control systems since some refractometers:

a) Do not utilize cooling water,

b) Have sensing heads that are not affected by liquor temperature, etc.,

c) May have differences in electronic circuitry.

4.3 Refractometer Control System - Controls & Indicators 

The refractometer system shall be equipped with the following controls and indicators:

1. Reset switch.
2. Switch or other means to manually remove either refractometer from service.
3. Visual solids display for each refractometer.
4. Status lights indicating “in service”. “inoperative” and/or “malfunction” for the individual refractometer and status of diversion valve.

4.4 Refractometer Control System - Alarms and Indicators  

The recommended alarms and indicators of the refractometer control system are:

4.5 Installation Requirements

1. The refractometers shall be installed in series.

2. The refractometer sensing heads shall be installed in such a manner that the individual sensing heads can be taken out of service or removed without having to valve off the liquor piping or open bypass valves.

3. All cabinets, wiring, etc., shall be suitable for the atmosphere and service conditions normal to a recovery boiler installation.

4. The refractometer sensing heads shall be installed so that the y are accessible and readily serviceable.

5. The refractometer sensing heads may be installed in any position on a vertical pipe run. On a horizontal run of pipe, the sensing heads must be installed on sides of the pipe. The reason for this is to ensure that the prisms are always covered with liquor.  

6. The electrical power supply to the refractometer control system shall be from a dependable (stable) source.  

7. A dependable supply of cooling water of satisfactory capacity must be provided for refractometers requiring sensing head cooling water.  

8. Dry oil-free instrument air shall be provided to the refractometer sensing heads to prevent and control condensation in the heads.  

9. A steam supply source of sufficient capacity shall be provided to meet flow, and minimum and maximum pressures requirements.  All installation requirements may not apply to all refractometers and refractometer systems.  

4.6 Refractometer Problems 

The three major causes of refractometer trouble or failure 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.  

4.7 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. 

4.8 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. 

4.9 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. 

4.10 Refractometer Calibration Standardization (Zero Offset) to Off-Line Test  

A Refractometer Standardization (“zero shifting” or “bias adjustment”) is an adjustment of the refractometer calibration curve to an off-line test to account for un-dissolved solids and/or changes in the black liquor chemistry.  This is normally performed while the instrument is actively measuring black liquor solids.

All refractometers shall be verified against a reliable periodic off-line test. (See Chapter 6 – Off-Line Black Liquor Solids Measurement)

The refractometers shall be standardized:

1.  On initial start-up of the recovery boiler.

2.  At any time it is felt or known that one of the refractometers may be deviating from the known black liquor solids content.

3. Any time there is a 2% difference between refractometers. 

The reading of the refractometers shall be checked against the moisture analyzer or microwave analyzer at two hour intervals (8 hour intervals if firing above 70% solids), and the moisture analyzer or microwave analyzer shall be checked by the TAPPI Standard Method, T650-om-05, weekly.

All refractometer standardization changes shall be entered in the recovery boiler “log book.”   

4.11 Refractometer Calibration  

A Refractometer Calibration involves placing two or more “samples” onto the sensor to generate a refractive index vs. dissolved solids curve.  This is typically performed utilizing calibration oils or electronically (depending on supplier) in a controlled environment, while the sensing head is off of the process line.

Calibration procedures shall be done in a manner that does not affect the system’s ability to automatically perform a black liquor diversion utilizing the remaining (active) in-service refractometer.  Improper procedures, or those that defeat the monitoring system described in Chapter 4, can result in the system failing in an unsafe condition.  Refer to the manufacturer’s appropriate procedures.

If the continuous solids monitor refractometer differs from the off-line test field measurement by more than 2% on an absolute basis, the off-line test results must be confirmed and then if required the continuous monitor refractometer should be standardized and/or recalibrated according to the manufacturer’s recommended procedures.  Repeated errors may indicate a failure of a refractometer component.  Refer to the manufacturer’s recommendations for repair or replacement.

Get to Know Electron Machine Corporation

Electron Machine Corporation, headquartered Umatilla, FL, manufactures industrial, inline, process refractometers. As a vertically-integrated manufacturer, we have complete control over the time it takes to manufacture our instruments providing the highest levels of service and support to our customers. Superb quality control is attained by adapting modern technology and practices to existing designs. These include in-house microprocessor and DSP software design, surface-mount PC card design and assembly, 3D CAD/CAM designing, CNC machining, and MIG/TIG welding. Additionally, our founder's innovative nature is still with us as we continue to research and develop new products.

Learn more about Electron Machine at or by calling 352-669-3101.

Technical Sales Representatives: The Often Underutilized Asset

Work with your technical sales rep
Work with your technical sales rep.
It will pay off in ways you haven't imagined.
Process refractometers are sold with the support of sales engineers working for the local distributor or representative. By realizing what these specialists have to contribute, and taking advantage of their knowledge and talent, you will save time and money and experience a better project outcome.

Consider these contributions:

Product Knowledge:
Sales engineers, by the nature of their job, are current on new products, their capabilities and their proper application. Unlike information available on the Web, sales engineers get advanced notice of product obsolescence and replacement. Also, because they are exposed to so many different types of applications and situations, sales engineers are a wealth of tacit knowledge that they readily share with their customers.

As a project engineer or leader, you may be treading on fresh ground with a refractometry requirement for your current assignment. You may not have a full grasp on how to handle a particular challenge presented by a project. If this is the case, call in the local technical sales representative - there can be real benefit in connecting to a source with past exposure to your current requirement.

Of course, sales engineers will be biased. Any solutions proposed are likely to be based upon the products sold by the representative. But the best sales people will share the virtues of their products openly and honestly, and even admit when they don’t have the right product. This is where the discussion, consideration and evaluation of several solutions become part of achieving the best project outcome.

Whatever your stake in an upcoming or ongoing project, it's highly recommended you develop a professional, mutually beneficial relationship with a technical sales expert, a problem solver. Look at a relationship with the local sales engineer as symbiotic. Their success, and your success, go hand-in-hand.

PID Control: The Basics

PID diagram
PID diagram (courtesy of Wikipedia)
PID is short for "proportional plus integral and derivative control", the three actions used in managing a control loop. Process loop controllers use one, two or all three of these to optimally control the process system. PID control is used in a wide variety of applications in industrial control and process system management.

Many types of PID controllers exist on the market and are used for controlling temperature, pressure, level and flow. PID control is also used in industrial, inline refractometers to control process variables such as Brix, Percent Solids, Dissolved Solids, Specific Gravity Units, and Refractive Index

Here is a brief explanation of the three actions that make up PID control.

Proportional Control Action (P): The controller output responds in proportion to error signal. The characteristic equation for this action is:
  • Where, Kp is called proportional gain, e is the error magnitude and B is the output from controller when there is no error. It is also called bias. 
  • In a proportional controller, the value of gain is set as required by the process and can be varied from 0 to ∞. 
Integral Control Action (I): The control system will respond if the error is present over a period of time. This type of control action is called Integral Control Action. The integral action is defined mathematically as:
  • Where, e= error, Ti= Time interval of integral action.
  • Purpose of integral action is to provide adequate control action on varying demands of process. In this type of action, output varies as per the time integral of error. This action does not exist independently and always associated with proportional control. 
Derivative Control Action (D): To achieve a stable process, wide proportional band and low integral action are set. Due to these settings, the control system can be too slow. If large system disturbances occur over a wide interval, PI controllers are inadequate. These large system disturbances can be managed if the controller output responds not only to the magnitude of deviation, but also to the rate of change of deviation. Derivative control action is that control action. 

Today's process controllers are much easier to set the PID, thanks to auto-tuning algorithms. What used to be a very time consuming and tedious job can now be done with the push of a button and allowing the controller to "learn" the process dynamics. PID controllers minimize error and optimize the accuracy of any process.

For more information on the use of closed loop control with industrial inline process refractometers. contact Electron Machine Corporation by visiting or calling 352-669-3101.

Understanding Error in Process Measurement

Instrumentation calibration is a procedure through which three general types of errors can be encountered. A typical signifier for a need of instrument recalibration is if the instrument is performing in an incorrect manner. This situation serves as a good way to showcase different types of error related to error analysis.

The three major category of errors regarding measurement are gross errors, systematic errors, and random errors. The first two categories of error, gross and systematic, are related to the two main elements of process control: controller and instrument. Gross errors are a product of the process controller or operator incorrectly evaluating the instrument value, with the best prevention of gross error being careful review of data both while recording and interpreting it.

Systematic errors impact every reading from a particular instrument, and are typically cause for instrument recalibration. Zero errors, where the instrument does not return to the predetermined zero value after each reading, are systematic errors because the same error in measurement is being displayed each time. Lastly, random errors impact instrumentation readings due to causes which are either unknown or simply unpredictable, meaning the error is both not able to be duplicated and is not a result of gross error. Random errors can be challenging to deduce due to both their singularity and their potential lack of a clear cause.

The previously mentioned zero error, also known as a zero shift calibration error, is typified by the resulting readings being offset at the same percentage. For example, a pressure transmitter which is functioning incorrectly as the result of a zeroing error can be corrected by a corresponding zero adjustment. After the adjustment and the transmitter being calibrated back to the correct zero point, the error will disappear. Another common type of systematic error is span shift calibration error. Unlike the zero error, span error can impact readings from the instrument repeatedly, but not necessarily identically. Similarly, by correcting the corresponding setting on the transmitter, in this case the span adjustment, the instrument can be correctly programmed once again by measuring the readings against a properly configured reference.

Hysteresis error occurs when the instrument in question returns erroneous responses as the input variable changes. The antidote to this kind of systematic error is to check the instrument against a pre-defined set of calibration points, first by increasing the input, and then subsequently decreasing the input in sequence to determine how the instrument responds as the input changes. Mechanical friction has been known to be a common culprit for hysteresis errors.

Understanding the capabilities and limitations of whatever instrument is relied upon for delivering process information is essential to successful operation.

Bleaching of Pulp in the Paper Making Process

The purpose of the bleaching process is to enhance the physical and optical qualities (whiteness and brightness) of the pulp by removing or decolorizing the lignin. Two approaches are used in the chemical bleaching of pulps. One approach called brightening, uses selective chemicals, such as hydrogen peroxide, that destroy chromatographic groups but do not attack the lignin. Brightening produces a product with a temporary brightness (such as newspaper) that discolors from exposure to sunlight or oxygen. The other approach (true bleaching) seeks to almost totally remove residual lignin by adding oxidizing chemicals to the pulp in varying combinations of sequences, depending on the end use of the product. This creates a longer lasting (sometimes permanent) whiteness, but it weakens the fibers and reduces sheet strength. The most common bleaching and brightening agents are chlorine, chlorine dioxide, hydrogen peroxide and sodium hydroxide.

Typically, the pulp is treated with each chemical in a separate stage. Each stage includes a tower, where the bleaching occurs; a washer, which removes bleaching chemicals and dissolved lignins from the pulp prior to entering the next stage; and a seal tank, which collects the washer effluent to be used as wash water in other stages or to be sewered. Bleaching processes use various combinations of chemical stages called bleaching sequences.

The first stage in the bleaching process is the chlorination stage, whose primary function is to further delignify the pulp. Chlorine reacts with lignin to form compounds that are water-soluble or soluble in an alkaline medium, which aids in delignifying the pulp before it proceeds to the next bleaching stage.

The next stage after chlorination is typically the extraction stage. This stage and the remaining stages serve to bleach and whiten the delignified pulp. The extraction stage removes the chlorinated and oxidized lignin by solubilization in a caustic solution.

Chlorine dioxide is often used in bleaching, either in the chlorination stage (as a substitute for some of the chlorine usage - chlorine dioxide substitution) or as an additional chlorine dioxide stage. Chlorine dioxide has 2.63 times greater oxidizing power (on a pound per pound basis) than chlorine and is used for nearly all high brightness pulps.

The next stage is the actual bleaching stage. Hypochlorite is a true bleaching agent that destroys certain chromophobic groups of lignin. It also attacks the pulp so high cellulose degradation occurs in Kraft pulp. Application of hypochlorite to Kraft pulp is usually used only as an intermediate stage of the sequence or to produce semi-bleached pulps. In the bleach process, residual chlorine must be removed through washing in vacuum washers.

Abstracted from Washington State
Air Toxic Sources and Emission
Estimation Methods