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Vibrating Tuning Fork Liquid Level Switch Successfully Used in Level Control of Fermenter
Bio-fermentation technology is common in the production of beer, soy sauce and monosodium glutamate. In the whole fermentation process, it is necessary to detect temperature, PH value, dissolved oxygen concentration and liquid level. The detection of liquid level to achieve timely feeding is crucial for maintaining the nutrition needed in biological growth.
The feeding system is mainly composed of feed diaphragm valve, metering tank, high / low liquid level switch, discharge diaphragm valve, etc. During fermentation process, microbes usually grow more vigorously. After feeding and stirring, a lot of bubbles are generated, which causes interference to the accurate liquid level measurement. This puts forward high requirements for liquid level switch, that is, the measurement of liquid level should not be affected by bubbles and the change of dielectric constant, so as to ensure accurate liquid level measurement.
Selectable level sensors:
At present, common liquid level sensors are: float level indicator, capacitive liquid level switch, RF admittance liquid level switch, external cage switch, photoelectric liquid level sensor and vibrating tuning fork liquid level switch. Based on different working principles and product performance, each of them are designed to apply in different working conditions.
Float level indicator is based on the principle of buoyancy and static magnetic field. Due to magnetic effect, as the float moves, the proximal reed switches are magnified by the permanent magnet in the float and change the status from “open” to “closed”. The resistance value in the sensor changes linearly.
For capacitive liquid level switch, the liquid of tank acts as the dielectric medium, which will affect the capacitance created between the two electrodes. If the liquid level rises and falls, this will change the capacitance measured by the probe in a predictable way, and therefore it can be used very effectively as a measurement of liquid level.
The working principle of RF admittance level switch is similar to that of capacitive liquid level switch. But as an upgraded technology from capacitive liquid level switch, RF admittance level switch has higher reliability due to the anti-hanging circuit.
External cage switch is a kind of non-contact liquid level measuring instrument which uses "frequency conversion ultrasonic technology" to measure the liquid level outside a tank. The measuring probe installed on the outer wall of a vessel emits ultrasonic waves and detects its residual vibration signal. When level overflows the probe, the amplitude of residual vibration signal becomes smaller; when instrument detects the change of residual vibration signal, it outputs a switch signal to achieve the purpose of liquid level alarm.
Photoelectric liquid level sensor operates using optical principles——the principle of refraction and reflection of light, and infrared ray to measure the liquid level. The light is reflected or refracted at the interface of two media. When measured liquid is at a high level, the measured liquid forms an interface with sensor; when at a low level, another interface forms between the air and the sensor. The liquid sensor is equipped with an interface adapter for compatibility with the level interface.
Vibrating tuning fork liquid level switch operates by the principle of detecting the change in the frequency of vibration of forks as a result of the presence or absence of a fluid at the forks. In operation a piezoelectric oscillator is used to vibrate the tuning fork at its natural frequency. When a fork which vibrates in harmonic resonance comes in contact with the measured liquid, its vibrating frequency will greatly decrease.
Jiwei vibrating tuning fork liquid level switch ---- Ring-11 Liquid Level Switch
Adaptability comparison of various liquid level sensors
|
Type |
Foam/bubble interference |
Dielectric constant interference |
Stability |
Easy installation |
Price |
|
Float level indicator |
No |
No |
Mechanical，bad stability |
Yes |
Low |
|
Capacitive liquid level switch |
Moderate |
Yes |
Parameters need to be adjusted according to the dielectric constant, bad stability |
Yes |
Moderate |
|
RF admittance liquid level switch |
Moderate |
Yes |
Parameters need to be adjusted according to the dielectric constant, bad stability |
Yes |
Moderate |
|
External cage switch |
No |
No |
Good |
No |
High |
|
Photoelectric liquid level sensor |
Yes |
No |
Bad |
Yes |
Low |
|
Vibrating tuning fork liquid level switch |
No |
No |
Good |
Yes |
Moderate |
According to the comparison table, it is obviously that:
1. Float level indicator: buoyancy principle is not affected by foam/bubbles or dielectric constant. Low cost is also attractive to users with limited budgets. However, the frequent usage of mechanical connecting rod structure tends to damage, and the service life is relatively short.
2. Capacitive liquid level switch and RF admittance liquid level switch: in the process of biological fermentation, the high process temperature maintains for a period of time, and this changes the dielectric constant of material, and leads to the change of capacitance value, which makes the sensor continuously calibrate these changes. In addition, the foam/bubbles with strong adsorption tend to adhere on the probe, and this also interferes with accurate measurement.
3. Photoelectric liquid level sensor and external cage switch: under the condition of large amounts of foam/bubbles, cannot accurately measure the liquid level. Besides, the particular installed positions are complex and also hard to maintain.
4. Vibrating tuning fork liquid level switch: no movable parts, and no need to adjust dielectric constant. With strong disturbance resistance ability, not influenced by foam, bubbles, viscosity, vibration and other liquid characteristics. So, vibrating tuning fork liquid level switch is a good choice for the liquid level measurement of fermenter.
After comparison, a Guangdong biotechnology company decided to select Jiwei vibrating tuning fork liquid level switch. Jiwei Ring-11 Liquid Level Switch has following advantages that comparable to imported products:
● With only 40mm fork length, specially designed for pipelines and other applications with constricted space.
● High sensitivity due to the precisely pre-adjusted harmonic resonant frequency of the fork, the lowest measurable medium density can be as low as 0.5g/cｍ³.
● With strong disturbance resistance ability, not influenced by foam, bubbles, viscosity, vibration and other liquid characteristics.
● Industry-leading performance for high temperature working environment. Process temperature can reach up to 250℃.
● Three optional output methods: relay output, two-wire system and Namur.
● Optional air-tight high pressure protection insulation sleeve, special for pressurized or corrosive liquids level measurement.
● Meet the food safety standards and applicable for liquids level measurement in food industry.
● Certified for both Intrinsic Safety and Flameproof Enclosure explosion protection.
● High reliability due to the detection of vibration frequency.
Ring-11 Liquid Level Switch fully meets the user's requirements, and finally selected by the end user as the level measuring instrument for fermenter. With excellent tuning technology, accurate measurement, stable and reliable performance, Ring-11 Liquid Level Switch has been widely applied in fermenters.
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