What Is a Densitometer?
Densitometers are used to measure the concentration of a specific substance in a gas or liquid.
By measuring the amount of light absorbed or reflected at specific wavelengths obtained through a filter or light (spectrum) divided into fixed increments of wavelengths, the concentration of the substance to be measured is obtained. If the object to be measured is fixed, the sample is irradiated with light of a specific wavelength that is highly absorbed by the object, and the densitometer measures the amount of transmitted (attenuated) light to determine its concentration. Pulse oximeters are also a type of absorbance densitometer that uses light transmission.
Uses of Densitometers
The primary use of densitometers is to measure the concentration of specific substances of interest in liquids and gases. They are used in various industrial fields.
1. Food and Beverage Industry
In the food industry, there are many products whose quality deteriorates due to oxidation reactions caused by oxygen. In the packaging process, it is necessary to keep oxygen concentration low to prevent quality deterioration. Oxygen analyzers are used to monitor whether the settings of filling, sealing, and packaging machines are correct and whether gas filling is being performed reliably.
2. Environmental Measurement
In environmental monitoring applications, densitometers are used to monitor the concentration of various gases such as NOX, SO2, CO, CO2, and HCl in the air. Oxygen analyzers for environmental monitoring are also used for safety confirmation, especially in manholes, sewers, tanks, ships, and other places where there is a risk of oxygen deficiency.
3. Medical Applications
In medical applications, various instruments are used to measure blood oxygen concentration (pulse oximeters) and glucose concentration.
4. Industrial Applications
In industrial applications, densitometers are used to control the concentration of liquids in production lines. Examples include controlling the concentration of resist strippers and acid-alkali cleaning solutions in the wafer manufacturing process and controlling the concentration of recycled chemicals and oxide/nitride film etchants in the semiconductor manufacturing process.
Trace oxygen analyzers are used in semiconductor manufacturing processes, Ar welding, etc., and can measure the concentration of very small amounts of trace oxygen in industrial gases. Other special devices include those used to measure the color density of ink in printing, and salt concentration meters for ready-mixed concrete.
5. Experimental Science
In experimental science, the color reaction caused by an enzyme reaction is measured to convert the value of enzyme activity. Densitometers are also used to measure the concentration (quantitative analysis) of proteins and nucleic acids.
Principle of Densitometers
Densitometers used to measure liquid concentrations are often based on the principle of absorbance spectrophotometry.
1. Absorbance Spectrophotometry
In the absorbance spectrophotometry method, light of a certain wavelength is irradiated into a container (cell) containing a sample to be measured. The light transmitted through the cell is detected by an electrical signal from a detector, and the light transmittance (attenuation) is used to measure the substance’s densitometer.
When the intensity of irradiated light is I0, the intensity of transmitted light is I, the molar concentration is C, the optical path length (thickness) is l, and the molar absorption coefficient is ε, and -logI/I0 is defined as absorbance A, the following equation is established: absorbance A is proportional to sample concentration C. By creating a calibration curve from this relational equation, the concentration of an unknown substance can be determined. (A = εCl)
2. Spectrophotometer
There are several methods for obtaining a specific wavelength from the white light of a light source, including the filter method, which uses a filter such as colored glass, the prism method, which uses a prism made of quartz or quartz, and the grating method, which uses a diffraction grating to obtain continuous spectroscopy.
For example, in the filter type, a complementary color filter is placed between the sample and the light-receiving part, and light is measured through a red, green, or blue filter.
3. Light Source and Detector
The main light sources used are deuterium discharge tubes, tungsten, and LEDs. Detectors include photosemiconductors (photocells) and photomultiplier tubes (photomultipliers). Since the wavelength range that can be measured is divided according to the method of spectroscopy and type of detector, it is necessary to select the equipment and cell material according to the purpose.
On the other hand, in the case of densitometers such as oximeters, which have a specific substance to be measured, only specific wavelengths that are highly absorbed by the target substance are irradiated from the light source, and the light obtained by the detector is converted into an electrical signal and displayed as a concentration.
Types of Densitometers
Densitometers, as mentioned above, have a variety of applications, including those that measure gases and those that measure liquids. Among densitometers that measure liquids, in addition to absorbance spectrophotometry, some measure concentration from the viscous resistance created by vibrating a piece of sensor part in a liquid, and multi-component densitometers that combine ultrasonic waves and electrical conductivity.
There are also various types of oxygen analyzers. For example, there are those for physical and chemical applications that measure concentrations in chambers, glove boxes, and incubators, those for environmental measurements, those for detecting trace amounts of oxygen in production lines, and those for measuring oxygen concentrations inside the packaging. Each application has a different measurement range and limits, so it is necessary to select the right one for the application.
Other types of densitometers include dissolved hydrogen meters used to monitor the hydrogen concentration of dissolved hydrogen gas solutions and ozone meters.