The level, depending on the requirements, can be measured or signaled.
Level gauges have relaying inputs. They are applied, most often, to protect containers from over-filling (maximal level) or from being completly empty (minimal level).
Measures and level converters let measure the level without any brake. At the output of these appliances we get standard analogue signal. It let, depending on the requirements:
The rule of the measurement:
A sensor contains an ultrasound converter and a temperature sensor. A probe issues sequences of ultrasound impulses. Each impule is reflected from the surface of the medium and goes back to the converter as an echo. The electornic converts the echo.
A special filtration can differentiate true echos, that are reflected from the material and the false ones, that comes from electric and acustic disturbances and from stirrers that work inside of the containers.
The measured time, that the impulse needs to go from the probe to the surface of the medium and back is converted to the showed on the projector distance, converted to the current signal ate the output of the probe and it can be applied to set the transmitter in to the motion.
Ultrasound converters can be used to measure the level of the liquid if, beacuse of some reasons, direct contact with the medium is impossible.
Something like that can happen when:
- the medium is aggressive and can cause a corrosion of the material that the gauge is made of (for acids)
an appliance can be polluted by the medium (for wastewater)
an appliance can be polluted by the particles of the medium, that adhere to the gauge (for adhesive substances)
A rules of the measurement
The level measurement by using a hydrostatic probe is realized by using a simple dependence between the heigth of liquid column and the hydrostatic pressure.
A pressure measurement is realized at the level of the separating diaphragm of the dipped probe and related to the atmosphere pressure by the capilary that is placed in the cable. As a measuring element you can use pressure converters that are built for example at the bottom of the container, graduated in H2O.
A measuring element is a piezoresistance silicon sensor, that is separeted from the medium by the separating diaphragm. Electronic intensifier that cooperates with the sensor answers for making the signal standard.
An output intensifier of hydrostatic probe should have an anti-overvoltage system, that protects the probe from demages , that can be caused by inducted disturbances from atmospheric discharges or from electoenergetic cooperating appliances.
The probe that is dropped to the level of the reference can freely hang on the cable or lie on the bottom of the container. A cable with the capillary can be extended by any standard signal cable
The linkage of the cables should be placed in the non-hermetic can (the pressure inside should be the same as the atmospheric pressure), that protects the capillary from the water or other pollutions.
In the container, in that turbuleces are possible (a work of stirrers, rapid flow) the probe should be placed in the protecting pipe (for exmple PCV). If the probe is dropped lower than 100 m, the cable should be slung to the steel bearing cord.
A hydrostatic measurement of the level is an easy solution for steady monitoring of the liquid, liquid mass and wastewater level.
A simple installation and a compact, made of rust-proof steel, construction cause, that the probes are applied most often to measure the level in the deep wells and in the open containers where no possibilty to violation of the construction of the container is given.
Hydrostatic probes, that are used in the containers, in that drinkable water is, should be made on the special way, they should have a nonsticking wrapper on the cable. Probes, that are made for level measuring of wastewaters are appropriated for mediums, that conatain pollutions and suspensions and they can be applied in pumping-stations, in septic tanks and in settling tanks.
The rules of the measurement
By the capacitative level measurement one of the covers of the capaciter is isolated or non-isolated probe and the second one is metall wall of the container or auxillary probe (it's a concentric pipe around the probe or around a rod, that is placed near and pararell to the probe).
A dielectric is medium, a gas over the level of the medium and, eventually, an isolation of the measuring probe. Any changes in the level of the medium influences on the capacity of the ''capaciter”, what is changed than into output signal 4...20mA by the electronic block placed in the cap of the probe.
For conducting liquids an isolated probe must be applied.
For non-conducting liquids non-isolated probes are recommended.
A dielectric constant and density of the medium, thats level should be measured aren't usually known with well accuracy. Because of that the capacitive level measure should be always calibrated on the target container by the conditions of the normal work of the container. After the calibration the measure can be applied only by the same conditions as while the calibration.
The relative dielectric constant of liquids and loose material is contained in the range from 1 to 80, but most often it's 10. A dielectric constant of water solutions and wet materials is contained between 10 and 80, for water it's 80
While the level measurement by using the capacitive measure such facts should be taken into account: a dielectric constant of homogeneous materials depends on the temperature, the dielectric constant of non-homogeneous materials depends on temperature and proportions of particular elements. For the majority of the issues relative dielectric constant increases in proportion to the increasment of the temperature of the medium, however, for some materials, for example for plasctics, dielectric constant doesn't change to a specific temperature, and after exceeding it, the constant increases rapidly. You may omit such changes in the limited interval of work temperatures.
The level measurment that bases on the capacity measurement is possible only than, when the relative dielectric constant Er>1.5
On account of the construction and technical parameters capacitive probes can be applied to level measurements of liquids and loose material that have little granulation in temperature to 200C and in the pressure containers.
A signal pitchfork signalling
A construction of a pitchfork and the kind of vibrations assures a self-cleanig of the sensor.
A specific construction of the pitchfork and of the activating rezonator assures a lack of false indications of the maximal level. A signal from electronic system stimulates the cristal of the rezonator, and than activates the pitchfork to the vibrations with specific frequency. If the pitchfork is coverd up by any material, the system detects a change in the vibrations and generates an output signal in the character of the change of the transmitter state. When the pitchfork is set loose, normal vibrations return and the transmitter has the previous state.
The pitchfork annunciator is applied to signal the level of explosive and non-explosive liquids, aggressive (solvents, acids) liquids, liquids that have large viscosity; without taking a foam, turbulences or content of gases into account. It can be applied for dusts, powders and granulats.
A vibratory rod signalling
Such advantages as solid construction, self-cleaning by vibrations for the majority of mediums, high pressure and corosion resistance, that the vibratory annouciators offer cause, that they're an optimum solution to signal the level of loose materials.
Vibratory rod announciators may be aplied for materials of granulation not over 10 mm. They are applied as level announciators only for materials that have considerable inner friction. If they are applied to signal the minimal level, they should be protected from mechanical demages caused by the pressure of the material. To protect them you can use wrappers over the annunciator.
A conductive signalling
A conductive announciaters of the level, that detect changes of conductivity between probes may be applied to signal the level of liquids that have conductivity over 20S/m.
A level announciator consists of a probe with electrode with length corresponding to the signaled level and a transmitter, that detects a change of conductivity between the electrode and reference (a reference can be other electrode or metall wall of the container).
Float annunciators are applied very often in industry to signal a liquid level. The announciators work without any additional energy, and the move of the float is transmitted to the switch on the magnetic way.
Detection of the liquid level is done by the float with a magnet, that moves along the measuring probe. The probe has a character of pipe made of acid-proof steel, and having biult-in contractons.
The float with a magnet moves pursuant to the changes of level along the measuring probe, that is dipped in the liquid, and activates contractons that are placed inside of the probe. When the float passed it, a contracton saves its state.