Kemtrak DCP007 Absorbance Photometer for Color, Concentration & Detection Applications

In-line photometers use light to provide real-time measurement of the color or concentration of components in a liquid (or gas) process stream. Photometers can be configured for a variety of purposes, with the configuration and technique used depending upon the specific measurements needed. Simply put, an industrial photometer is used to measure light after it has passed through an industrial process stream.

Analytical photometry has been around for many decades with many earlier photometers offline units based upon visual standards. They utilized glass plates of specific colors that were compared stereoscopically to process fluid samples with the naked eye. As technology progressed, in-line analyzers using photodetectors were developed that measured light more accurately, repeatably and in real time.

The most common type of photometers, of which the Kemtrak DCP007 is a notable example, uses light absorbance techniques to determine and report the concentration of a particular component in a fluid stream. They can be configured to operate with broadband light, broad NIR, or a discrete wavelength in the UV, VIS, or NIR light regions depending upon the application. The Beer-Lambert law—which relates the reduction in intensity of light as it passes through a material to the concentration of the material absorbing the light—applies in this application. The optical path length must be configured to suit the expected absorbance measurement range.

Turbidimeters are often referred to as photometers, mainly because they are manufactured and supplied by absorbance photometer producers. In-line turbidimeters use scattered light, absorbance and backscattered light techniques to determine the amount of particulate or solid in a flowing stream.

They measure process parameters such as color, concentration, turbidity, solids concentration and fluorescence. Each photometer consists of an advanced electronic unit, with a light bench constructed using high-performance light source technology, rugged industrialized fiber optic cables, and an in-line flow cell or probes with wear-resistant sapphire windows.

Photometers are used in many industries, including chemical, oil and gas, food and beverage, biotech, pharmaceutical and water. They are deployed where a specific property or constituent of the stream is to be monitored and used as a control variable in some way.

The most common measurement flow cell for lightly colored applications, such as drinking water, is a long pass stainless steel Z-path cell, shown here with optional windows cleaning nozzles. The cleaning sequence can be fully automated using the inbuilt DCP007 controller software. Wastewater monitoring is typically higher colored samples and a shorter OPL measurement cell is required.

Kemtrak stainless steel long pass (OPL = 100mm) G 1” (DN 25) pipe thread measurement cell with optional G 1/8” (DN 6) sapphire window cleaning nozzles.

A typical measurement cell for this application is a 1” NPT type cell, like this Kemtrak stainless steel NPT 1” tapered pipe thread measurement cell with sapphire measurement window.

Installation. Wherever possible, always install a photometer in a pipe where flow is vertically upwards. This will ensure the most homogeneous sample is passing through the photometer light beam.

Flow conditioning. Aeration is the number one interfering factor in photometer performance. In absorbance systems, it makes measurement noisy and creates errors. In turbidity systems, gas bubbles look just like particulate matter, so readings become inaccurate and can be erratic. Suppressing aeration in the flow stream greatly improves the quality of the measurement.

Fouling compensation. Photometers use windows to introduce and extract light from pipelines. Over time, these windows can become contaminated with process material forming a film or layer on their surfaces. Mechanical cleaning devices or spray jets can help alleviate the problem, but make sure your photometer also has a means of electronically adjusting for window contamination so the measurement remains stable.

Instrument light decay. Today, LEDs are widely available and older style incandescent/gas discharge lamp photometers are essentially obsolete. Whatever the light source in your unit, make sure it has the ability to self-check and adjust the light output so that it remains constant. Changing light intensity looks like a change in measurement to a photometer and contributes greatly to drift.

Stray light. Incidental light getting into your photometer’s optical circuit can create havoc with the measurement. This is not usually a problem with metal pipe installations, but can be quite extreme when installing photometers in plastic or fiberglass pipes. Ambient light can travel through non-black materials and enter the light circuit. If you observe trends like measurements declining every time the sun comes out, you have an issue with light contamination. Install blackout material upstream and downstream of your sensor—it may be necessary to do so for quite a distance—to prevent ambient light from being a problem.

APHA, Platinum-Cobalt (Pt-Co) and Hazen are different names for the same color scale, which is specified by ASTM D 1209, BS5339:76 (1993), DIN 53409 and ISO 6271-1:2004(E). The APHA color scale measures the yellowness of a liquid, and is well suited for quality control and contamination detection. It is regularly used to evaluate pollution levels in wastewater and for drinking water quality control and regulation. The APHA scale can also be used to measure and control the concentration of iron in groundwater.

Originally a visual standard using glass slides to compare against process samples inside a stereoscopic viewer, APHA has evolved to being electronically measured with portable testers.  A further evolution has moved measurements online so APHA color can be monitored in real time using photometers.

In-line instruments, such as the Kemtrak DCP007, use a measurement wavelength that is typically between 380 and 500nm depending upon the process color range to be monitored. Highly colored processes (> 500 APHA) require a short optical path-length (OPL) while lightly colored process streams (<100 APHA) require a long OPL. A reference wavelength of 650nm or 850nm is used to compensate for both turbidity in the process stream and window fouling over time. Optimum selection of wavelength and OPL is necessary for the best performance.

The optional Kemtrak NIST-traceable validation filter holder permits the use of either NIST-traceable reference standards or liquid samples in order to quickly check instrument performance on a regular basis to ensure the photometer is operating within satisfactory parameters without having to remove the flow cell from the process line.

The validation accessory is supplied either fitted to the optical flow cell or as a standalone accessory in the optical fiber chain. The validation filter holder is designed to use industry standard 10mm cuvette style NIST-traceable absorbance filters, the same type as used on many standard laboratory spectrophotometers.

Benefits of using a Kemtrak NIST-traceable validation filter holder include:

• Fast verification of measurement accuracy
• Industry standard 10mm cuvette size
• Same set of standard NIST filters as used in the laboratory
• Cross validation between laboratory spectrophotometer and Kemtrak in-line industrial process photometers
• Liquid samples can be measured
• Recognized by international quality systems such as GLP, ISO 9000 and ISO/IEC 17025

Four variants of the DCP007 are available—UV, VIS, NIR(L) and NIR(H). The UV, VIS and NIR(L) variants all utilize long life LED light sources. UV light sources are rated for >20,000 hours, while VIS and NIR(L) light sources are guaranteed for life. The NIR(H) unit uses a Peltier thermopile to generate the wavelengths needed for operation.

Each photometer consists of a DCP007 electronics unit configured for wavelength, an inline flow cell or probe with the optimum optical pathlength for the application and a pair of high quality fiber-optic cables. When used in reference mode, the DCP007 compensates for both optical window buildup and the presence of solids/turbidity in the measurement stream.

Cleverly designed amplification stages around the photometric detection circuitry of the DCP007 allows for linear absorbance performance with full measurement reolution up to 3A and an overall measurement range of up to 5A. This impressive measurement range ensures effectiveness in high absorbance applications, without compromising process flow rates. Where differential measurements are made, the depth of absorbance available improves and extends measurement capability.

Color and concentration photometers, often called colorimeters, are used to directly measure the color or the concentration of liquids in a process. While often used for online measurement in the food and beverage industries, color and concentration are also critical measurements in the oil, gas, and chemical industries.

Color is often related to visual standard scales, such as APHA/Hazen and Saybolt. Many color scales are used in industrial applications, sometimes being very industry specific (such as ICUMSA color for sugar), so configuration of the photometer to reproduce the visual standard online is very important. Concentration measurements range from the simple concentration of a binary fluid mixture to the amount of a specific chemical species within a multi-component process stream.

Typical applications for color analyzers include:

• Product color control for QA/QC
• Chemical concentration
• Filtration bed monitoring
• Ion exchange column monitoring
• Leak detection
• Contaminant detection
• Dilution and dosing control
• Interface detection

The Kemtrak DCP007-VIS absorbance photometer is designed to accurately measure color or concentration even in turbid or hazy flow streams. The unique referencing system used by Kemtrak absorbance photometers removes the effects of solids suspended in the flowing stream to give a “true color” measurement. In some cases, the DCP007 can provide outputs for both color/concentration and solids/turbidity from the same unit, making it a true multifunction instrument.

The Kemtrak DCP007-VIS achieves this by measuring at two wavelengths. A primary absorbing wavelength is used to measure the concentration of the chemical/component being monitored, while a non-absorbing reference wavelength is used to compensate for turbidity, air bubbles, and fouling. The DCP007 uses precision fiber optics to shine a precisely focused monochromatic light through the process medium and back to a photodetector. The attenuation of the transmitted light beam caused by absorbing substances in the process medium is mathematically described by the Beer-Lambert law.

Applying the Beer-Lambert law, the concentration of the sample can be calculated as:

A = ε · l · c

Where:

A = absorbance ( = – log transmittance)
ε = molar absorptivity coefficient
l = optical path length
c = concentration of absorbing species

Addition and dosing of chemicals (such as chlorine dioxide for sterilization) are easily monitored and controlled using the Kemtrak DCP007-VIS. The rock-steady performance of the unit in-line far exceeds that of other measurement techniques in similar applications.

An NIR absorbance photometer is used to measure how much emitted light is being absorbed at a specific near-infrared (NIR) wavelength when passed through a fluid. Using this tool, it is possible to determine the concentration level of the absorbing specied within that solution. NIR absorbance photometers are often used for measuring the concentration of water in solvents either in trace amounts, as in ethanol production, or in bulk as in beverage applications.

For such applications, South Fork Instruments develops measurement solutions using the Kemtrak DCP007-NIR. It is simple to use, and designed to detect and report trace and bulk water content in flows such as alcohol, ethanol, methanol, gasoline, and sugar solutions.

The DCP007-NIR generates stable levels of NIR wavelength light that is passed through the flow stream and measures intensity changes due to absorbance of that light from the varying water content. A temperature input is also optionally available to allow compensation of the measurement to remove variations due to temperature if necessary for the application.

Two versions of the DCP007-NIR are available:

• Low Range - the low range uses an LED source and is ideal for bulk water content across ranges between 0 and 100%
• High Range - a longer wavelength unit that utilizes a thermopile source to generate wavelengths up to 1 micron for trace water content down to trace (ppm) levels

The DCP007 is used connected to any one of the many styles of flow-through measurement cells Kemtrak offers. For tank or large pipe applications, a third-party fiber optic probe can also be used, usually without any special configuration or calibration requirements.

Typical applications for the DCP007-NIR include:

• Solvent concentration
• Moisture content analysis
• Alcoholic beverage content monitoring and control
• Water in alcohol monitoring
• Chromatography control (solvent mixing)
• Residual solvent concentration in pharmaceuticals
• Solvent recovery
• Residual moisture concentration
• Replacement of Karl-Fischer titration
• Sugar concentration

NIR absorbance photometers are particularly useful for measuring the presence of water in solvents and alcohol. Water has a strong absorption throughout the entire NIR band with absorption peaks at approximately 1440nm and 1930nm.

1440nm is recommended for water measurements in the range of 0-100% where a precision of approximately 0.1% is possible.

1930nm is recommended for trace (ppm) water in solvent measurements.

NIR is a preferred technique for accurately determining water in solvent concentrations, primarily because a high degree of accuracy is achievable due to a large difference between water and solvent absorption.

UV photometers (often called UV Analyzers) are used to measure the amount of light being absorbed by materials in solution at a specific UV wavelength(s). UV absorbance photometers are of particular use in biotechnological processes, where they are used to detect the presence of expressed protein molecules after separation.

For such applications, we use the DCP007-UV, an advanced dual-wavelength photometer designed to accurately measure absorbance in the UV region of the electromagnetic spectrum. Optical fibers are utilized to transmit light from a solid state light source within the DCP007-UV instrument, through an in-line sample cell, and back again to a sensitive detection system to calculate substance concentration in real time. The absence of electrical components on the inline cell makes them extremely small and there is no concern about fitting them in hazardous areas where electrical certification and approval is required. The DCP007-UV is an excellent and cost-effective solution for measuring flow stream absorbance and concentration in a process environment in place of far more expensive scanning type analyzers.

Traditional UV analyzers/photometers use mercury vapor lamps that continuously expose the process stream to high-intensity broad spectrum UV radiation and heat. This can result in destruction of valuable in-stream product and produce unknown and (potentially hazardous) byproducts. Traditional UV photometers also suffer from systematic drift requiring constant adjustment from optical filter erosion caused by high-intensity UV radiation from the lamp and continuously reducing lamp output resulting from aging.

The Kemtrak DCP007-UV absorbance photometer uses state-of-the-art solid state light sources at the exact wavelength required for analysis. Light reaches the process through precision fiber optics and is delivered with energy levels thousands of times less than that of a traditional UV photometer using a mercury vapor lamp. The ultra-low, modulated power, UV LED light source in the Kemtrak DCP007-UV does not systematically drift or expose the product stream to high levels of UV radiation and temperature. LED light sources are available at discrete wavelengths from 250nm to 400nm in the UV region, ensuring any specific wavelength requirement can be serviced.

A proprietary dual wavelength, four-channel measurement technique and advanced digital electronic design allows for deep UV absorbance measurement up to 5 AU (in a 1 cm optical path length) at up to two discrete wavelengths. A selection of shorter optical path-lengths allows for even deeper optical density measurements.

Kemtrak zero-dead-volume hygienic measurement cells contain no electronics or moving parts and are well suited for hazardous environments. In-line verification and QA are made possible using the Kemtrak Absorbance Photometer Traceable Verification Accessory. This simple accessory utilizes the same traceable or prepared liquid standards used for laboratory benchtop instrumentation verification and calibration and provides a direct correlation to offline measurement techniques for added confidence in measurement.

Typical applications for the DCP007-UV include:

• Chromatographic separation
• Micro- and nano-filtration systems
• Concentration of aromatic solvents

The DCP007-UV in-line process analyzer delivers measurement performance never before thought possible in an in-line UV absorbance measurement and peak detection instrument.