Stan: The concentration of particles in fluids is important in processes such as drinking water purification, beer quality control, crystallization, fermentation and bioreactor cell concentration control. There are relatively inexpensive, reliable turbidity sensors to measure these concentrations online and inline.
Greg: I especially appreciate the key role of turbidity measurements in the pharmaceutical industry using units of optical density to provide an inferential measurement of cell concentration. We're particularly fortunate to have Eric Chan, product manager for Mettler Toledo in Urdorf, Switzerland, to give us the essential aspects of understanding and applying turbidity measurements.
Stan: What is the basic principal of operation?
Eric: Turbidity measurements are based on an optical principle and consequently take advantage of the interaction of light with undissolved (suspended) particles. Turbidity is determined by measuring the intensity of the scattered (reflected) light at a particular angle to its source, or by the loss of light intensity in transmitted light. The basic principle for scattered light measurement (backscattered, forward-scattered or 90°-scattered) is, the higher the particle concentration, the higher the intensity of the scattered light and, consequently, the higher the signal (or to be more specific, photocurrent) produced in the photodiode. On the other hand, for a transmitted measurement such as optical density (OD), the higher the particle concentration, the greater the loss of direct light intensity and consequently, the smaller the current produced in the photodiode.
Greg: What affects the performance?
Eric: The presence of air bubbles, changes in medium flow velocity and medium color can interfere with turbidity readings. Sudden pressure drops should be avoided because of the risk of air bubble formation. Any air bubbles present in the medium will scatter light and typically contribute to a higher turbidity measurement. For inline turbidity measurement, avoid installing the sensor at a position where air may be trapped. A degasser/air vent may be required if air bubbles are an issue. Sample flow velocity should be maintained at a constant rate. Sudden sample velocity changes or turbulent flow could flush out particles that precipitate on the long sampling line tube, and raise the turbidity value. In many cases, colored medium will absorb some of the wavelength/light emitted from the light source. This will typically contribute to a higher turbidity reading.
Stan: What are the considerations in selection and installation?
Eric: Ninety degree and forward-scattered light measurements are sensitive for very low turbidity measurements. Backscattered light measurements are more suitable for medium to high turbidities due to their linearity over a very wide concentration range.
Some turbidity probes on the market have a small optical path length (OPL). This kind of construction may be susceptible to clogging or dirt capture in the OPL gap. Therefore, avoid these designs in applications if a high particle load is anticipated. Turbidity probes with a long OPL may lose sensitivity in highly turbid liquids.
The dielectric properties of the suspended particles (absorption, fluorescence, refraction or a combination of these properties) influence the intensity of the scattered light. Therefore, the light source should be chosen carefully to minimize particle absorption or fluorescence. Moreover, the larger the refraction index of a particle, the more it scatters light. If particles are colored and also absorb in the wavelength range of the illuminating beam, the intensity of the scattered light is attenuated.
In hygienic applications (pharma, biotechnology, food and beverage), a turbidity probe (turbidimeter) with a stainless steel body and smooth surface (Ra ≤ 0.8 µm) or electropolished finish are recommended. For these same applications, cleaning-in-place (CIP) and sterilization-in-place (SIP) regularly occur before starting a new production batch. High temperatures with aggressive chemicals (sodium hydroxide, sulfuric acid or others) are used. Depending on process requirements, the process temperature can reach up to 120 °C. The chosen materials and equipment construction must be able to resist aggressive chemicals and withstand high process pressure and temperature.
The turbidimeter can be installed in a pipeline, sample line, recirculation line or directly in the process equipment. Process connections between the turbidimeter and process pipe or vessel can vary considerably. Typically, Tri-clamp, Neumo (aseptic), Ingold DN25, or Varivent process connections are preferred in food and beverage or hygienic applications. Other flow-through installations with flange or wafer types of process connections may also be available for other special applications. For high-flow, high-particle-load applications, sapphire sensor window construction may be required to protect turbidimeter optics. For inline probes, avoid installation in a high point or other position where air bubbles may be present. Further, the flow direction recommended by the manufacturer should be considered during installation.
In fermentation or cell culture-related processes, air bubbles in the medium can't be avoided. In such cases, the installation point must be chosen where the minimum quantity of air bubbles will be present. Installation and sensor tip clearance guidance provided by the manufacturer should be strictly followed. In certain cases, a special setup in the vessel may be required.
Greg: What about calibration and testing?
Eric: Many manufacturers perform sensor factory calibration before shipment. As long as a turbidimeter stays in good condition, the sensitivity curve will not alter and no recalibration is required. However, when the optics become fouled or light intensity changes, the turbidity probe may need to be recalibrated.
Particle accumulation on turbidimeter optics reduces measurement reliability. Cleaning should be performed when required. The use of chemicals or solvents on optical windows should be avoided. Clean or distilled water should be sufficient.
In regulated utilities or a drinking water facility, sensors or devices for monitoring turbidity should be subject to primary suspension standard calibration at least every four months to prevent instrument drift. A secondary suspension standard should be used daily to check the calibration of the instruments (such as sealed sample cells filled with labeled suspension or metal oxide particulates in polymer gel, or a turbid glass cube). If they vary by more than 10%, the system should be recalibrated, so its performance is acceptable.
The most common liquid reference standard is formazine (a heterocyclic polymer). A stock solution of 4000 FTU is available in the market. Alternatively, commercial stock solutions in different concentrations from various suppliers are available. Diatomaceous earth is also used for turbidimeter calibration.
Other process-specific reference standards, such as metal oxides, also can be used. Particles in these standards solutions may tend to become unevenly distributed over time. The samples must be stirred to ensure homogenized distribution of particles.
When using a turbid glass cube for verification, avoid any dirt or fingerprints on the surface of the cube. Also note that turbid glass cube properties can change on exposure to high or low temperatures. As long as the verification kit stays in perfect condition, the measured values should coincide with the assigned values.
In processes where the same product is produced in multiple batches, operators may consider an in-situ, multipoint calibration before the first batch. Prior to the next batch, “one-point” verifications may be carried out by means of a verification kit or against a reference system to ensure the inline turbidity instrument is still within good operating conditions. A history of changes in calibration can be used to determine frequency of cleanings and if a better selection or installation is needed.