LEVEL MEASUREMENT

TYPES OF LEVEL MEASUREMENT

1. Direct Liquid Level Measurement :

Directly, by using the varying level of the liquid as a means of obtaining the measurement. Example : Sight Glass, Floats, Dip Rod & Bob and tape etc.

2. Indirect Liquid Level Measurement :

Indirectly, by using a variable signal like pressure, radio wave, Ultrasonic wave, Mass weight etc., in respect of liquid level. Many types of indirect level measuring devices.

Example : Pressure Gauge, Bubbler method, Level Transmitter (PT & DPT),Radioactivity,  Ultrasonic Waves, Weight-ment of entire vessel etc.

DIRECT LIQUID LEVEL MEASUREMENT

METHODS OF INDIRECT LEVEL MEASURMENT

  1. DIFFERENTIAL PRESSURE
  2. VIBRONICS
  3. CAPACITANCE
  4. CONDUCTIVITY
  5. ULTRASONIC
  6. ELECTRO-MECHANICAL
  7. RADIOMETRIC
  8. TANK GAUGING
  9. HYDROSTATIC
  10. RADAR-LEVEL

DIFFERENTIAL PRESSURE BASED LIQUID LEVEL MEASUREMENT

The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. This is termed hydrostatic pressure. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly.

Mathematically, we have:         P = d H

where 

P = Pressure (Pa) ,     d = density of the liquid ,H = Height of liquid column(m), P(absolute) = P (atm) + Hydrostatic pressure of liquid                             

d = Density = Weight of Liquid / Volume of Liquid

  The level of liquid inside a tank can be determined from the pressure reading if the weight density of the liquid is constant.

Differential Pressure (DP) capsules are the most commonly used devices to measure the pressure at the base of a tank.

Density of Water = 1.00 Kg / Liter

Density of Mercury = 13.6 Kg / Liter

INDIRECT LIQUID LEVEL MEASUREMENT

1. Open Tank Level Measurement  :

If the tank is open to atmosphere, the high-pressure side of the level transmitter will be connected to the base of the tank while the low-pressure side will be vented to atmosphere. In this manner, the level transmitter acts as a simple pressure transmitter. We have:

                             Phigh = Patm + d H

                             Plow = Patm

          Differential pressure          P = Phigh – Plow = d H

The level transmitter can be calibrated to output 4 mA when the tank is at 0% level and 20 mA when the tank is at 100% level.

2. Closed Tank Level Measurement  :

Compensation can be achieved by applying the gas pressure to both the high and low-pressure sides of the level transmitter.

                                        Phigh = Pgas + d H

                                        Plow = Pgas

P  = Phigh – Plow = d H

DRY LEG : The effect of the gas pressure is cancelled and only the pressure due to the hydrostatic head of the liquid is sensed. When the low-pressure impulse line is connected directly to the gas phase above the liquid level, it is called a dry leg.

DRY LEG SYSTEM

If the gas phase is condensable, say steam, condensate will form in the low-pressure impulse line resulting in a column of liquid, which exerts extra pressure on the low-pressure side of the transmitter.

A technique to solve this problem is to add a knockout pot below the transmitter in the low-pressure side as shown in Figure.

A technique to solve this problem is to add a knockout pot below the transmitter in the low-pressure side as shown in Figure.

          One example of a dry leg application is the measurement of liquid poison level in the poison injection tank, where the gas phase is non-condensable helium.

WET LEG SYSTEM

WET LEG : In a wet leg system, the low-pressure impulse line is completely filled with liquid (usually the same liquid as the process) and hence the name wet leg.

At the top of the low pressure impulse line is a small catch tank. The gas phase or Vapour will condense in the wet leg and the catch tank. The catch tank, with the inclined interconnecting line, maintains a constant hydrostatic pressure on the low-pressure side of the level transmitter. This pressure, being a constant, can easily be compensated for by calibration.

LEVEL COMPENSATION

It would be idealistic to say that the DP cell can always be located at the exact the bottom of the vessel we are measuring fluid level in. Hence, the measuring system has to consider the hydrostatic pressure of the fluid in the sensing lines themselves. This leads to two compensations required.

Zero Suppression

In some cases, it is not possible to mount the level transmitter right at the base level of the tank. Say for maintenance purposes, the level transmitter has to be mounted h meters below the base of an open tank.

Phigh = Patm + d.hm + d.H ,

Plow = Patm

∆P = Phigh – Plow = d.hm + d.H

The pressure on the high-pressure side is always higher than the actual pressure exerted by the liquid column in the tank (by a value of d.hm). This constant pressure would cause an output signal that is higher than 4 mA when the tank is empty and above 20 mA when it is full. The transmitter has to be negatively biased by a value of – d.hm . So that the output of the transmitter is proportional to the tank level (d H ) only. This procedure is called Zero Suppression and it can be done during calibration of the transmitter.

Zero Elevation

When a wet leg installation is used the low-pressure side of the level transmitter will always experience a higher pressure than the high-pressure side. This is due to the fact that the height of the wet leg (h) is always equal to or greater than the maximum height of the liquid column (H) inside the tank. When the liquid level is at H meters,

we have:

Phigh = Pgas + d H

Plow = Pgas + d h

P = Phigh – Plow = d H  – d h      = – d (h – H)

The differential pressure    P sensed by the transmitter is always a negative number (i.e., low pressure side is at a higher pressure than high pressure side).

P increases from P = -d h to P = -d (h-H) as the tank level rises from 0% to 100%. 

Note : If the transmitter were not calibrated for this constant negative error (-d h), the transmitter output would read low at all times.  To properly calibrate the transmitter, a positive bias (+d h ) is needed to elevate the transmitter output.  This positive biasing technique is called zero elevation.

VIBRONICS LEVEL SENSOR

A sensor in the form of a tuning fork is made to vibrate at its resonant frequency by a piezo-electric crystal drive. The frequency changes when the fork comes into contact with the liquid. The change is evaluate and converted into a switching signal.

CAPACITENCE TYPE LEVEL SENSOR

The Probe and vessel wall form the two plates of a capacitor, the capacitance of which is determined by their surface areas, the distance between them as well as the type and dielectric properties of the product to be measured. When the vessel is filled, the capacitance increases. The capacitance is measured and a level proportional signal is produced in the electronic insert of the probe. This signal is then evaluated by other electronic units connected to the system..

CONDUCTIVITY TYPE LEVEL SENSOR

The difference in conductivity of liquids is measures with air as the reference point. A very small alternating voltage (AC Voltage) is applied between two probe tips or between the probe and the vessel wall.The circuit is closed and the level is indicated when the liquid reaches the probe tip. The voltage and current used are so small that no dangerous shock-hazard voltages can occur. The use of alternating voltage prevents electrolysis occurring.

ULTRASONIC LEVEL SENSOR

The Ultrasonic measuring systems measure the level of all kinds of liquids and solids including those in hazardous areas. The sensor is not directly in contact with the material, thus the unit is wear and maintenance free.

          The emitter in the sensor is excited electrically and sends an ultrasonic pulse in the direction of the surface of the product which partially reflects the pulse. Thus echo is detected by the same sensor, now acting as a directional microphone, and converted into a electrical signal. The transmission and reception of the pulse (the sonic run time) is directly proportional to the distance between the sensor and the product surface. This distance is determined by velocity of sound c and run time t using the formula:    D = ( c.t ) / 2

D= Distance from sensor to surface of material

c = Velocity of sound

t = The sonic run Time 

E = Zero Point of measurement  ( 0 % Empty)

F=  Maximum Level ( 100 % Full)

BD = Blocking Distance

Electro Mechanical Type Level Sensor

A measuring tape with a sensing weight attached to the end is driven down into the bunker. When the weight touches the surface of the material, the tape slackens and the motor reverse. The weight returns to the start “parked”position. During the weight’s downwards or upwards journey, the transducer emits pulses equivalent to the length of the extended tape. The pulses are decoded by D/A converter and the measurement is stored until the next measurement cycle. This is initiated by time circuit or the start button on the timer.

Radiometric Type Level Sensor

The gama source, either a caesium or cobalt compound, emits radiation which is attenuated as it passes through materials.

A detector mounted on the opposite side of vessel or pipe, converts this radiation into an electrical signal. The strength of the signal is determined by the distance between the radiation source and the detector, and also the thickness and density of the material. The distance and the vessel or pipe walls through which the radiation penetrates are constant values. These must be calculation when selecting the strength of the radiation source. The actual measuring principle is based on the absorption of the radiation by the product to be measure.

TANK GAUGING

The tank gauging system is based upon the principal of displacement measurement . 

           A small displacer is accurately positioned in the liquid medium using a servo motor. The displacer is suspended on a stainless steel wire which is wound onto a finely grooved drum housed within the transmitter unit.

          The drum is driven via two coupling magnets which are completely separated by the drum housing. One magnetic ring is connected to the wire the other is connected to the drive motor. As the inner ring turns, its magnetic attraction causes the outer ring to turn as well, thus turning the entire drum assembly.

          The weight on the wire puts torque on the outer ring and this torque is detected by a unique electromagnetic transducer on the inner ring.
         
          The change of magnetic flux generated between the drum assembly and servo-driven coupling magnets are converted into a voltage.

          At the operator command, the displacer is lowered and as it touches the liquid, the weight of displacer is reduced because of the buoyant force of liquid. As result, the torque in the magnetic coupling is changed and this changed is measured by a Hall detector. The signal, an indication of the position of the displacer, is sent to the motor control circuit. As liquid level is rises or fall, the position of displacer is adjusted by the drive motor and hence continuous measurement up to 0.9 mm accurately.

HYDROSTATIC LEVEL SENSOR

The weight of a column of liquid generates a hydrostatic pressure. At constant density, the hydrostatic pressure is a function of the height h of the column of liquid only.

Phydrostatic = h.d      

h = distance between the surface of the liquid and the centre of the process diaphragm.

d = density

Bubbler Level Measurement System

If Process liquid have corrosive , radioactive, contains solid particle, etc properties, it is desirable to prevent it from coming into direct contact with
the level transmitter. In these cases, a bubbler level measurement system,
which utilizes a purge gas, can be used.

a bubbler tube is immersed to the bottom of the vessel in which the liquid level is to be measured. A gas (called purge gas) is allowed to pass through the bubbler tube. Consider that the tank is empty. In this case, the gas will escape freely at the end of the tube and therefore the gas pressure inside the bubbler tube (called back pressure) will be at atmospheric pressure. However, as the liquid level inside the tank increases, pressure exerted by the liquid at the base of the tank (and at the opening of the bubbler tube) increases. The hydrostatic pressure of the liquid in effect acts as a seal, which restricts the escape of, purge gas from the bubbler tube. As a result, the gas pressure in the bubbler tube will continue to increase until it just balances the hydrostatic pressure (P = S⋅H) of the liquid. At this point the backpressure in the bubbler tube is exactly the same as the hydrostatic pressure of the liquid and it will remain constant until any change in the liquid level occurs. Any excess supply pressure will escape as bubbles through the liquid.
As the liquid level rises, the backpressure in the bubbler tube increases
proportionally, since the density of the liquid is constant.
A level transmitter (DP cell) can be used to monitor this backpressure. In an
open tank installation, the bubbler tube is connected to the high-pressure
side of the transmitter, while the low pressure side is vented to atmosphere.
The output of the transmitter will be proportional to the tank level. A constant differential pressure relay is often used in the purge gas line to
ensure that constant bubbling action occurs at all tank levels. The constant
differential pressure relay maintains a constant flow rate of purge gas in the bubbler tube regardless of tank level variations or supply fluctuation. This ensures that bubbling will occur to maximum tank level and the flow rate does not increase at low tank level in such a way as to cause excessive
disturbances at the surface of the liquid. Note that bubbling action has to be
continuous or the measurement signal will not be accurate. An additional advantage of the bubbler system is that, since it measures only the backpressure of the purge gas, the exact location of the level transmitter is not important. The transmitter can be mounted some distance from the process. Open loop bubblers are used to measure levels in spent fuel bays.

RADAR LEVEL TRANSMITTER

Radar level transmitters work based on the time of flight (TOF) measuring principle or time domain reflectometry (TDR). To start with, we can measure the distance from the reference point to the surface of a liquid. Then the meter sends a high-frequency signal from an antenna or along a probe.

Radar level transmitter’s working principle

When the product surface reflects the pulse, the meter receives the reflection. Then the device calculates how long it took the pulse to return and translates that time delay into a level measurement.

Before we apply a radar meter, we need to know the dielectric constant (DC) of a product, as that has a direct impact on the quality of the reflections. In fact, products with high DC values will reflect strong, clear pulses. On the other hand, a product with a low DC value will absorb more of the pulse, reflecting less and reducing accurate readings.

Radar level measurement is a safe solution even under extreme process conditions (pressure, temperature) and vapours. Radar level transmitters can also be used in hygienic applications for non-contact level measurement. Radar level transmitters versions are available for different industries like for water/wastewater, the food industry, life sciences or the process industry. Various antenna versions for every kind of radar applications are available.

Basic radar level transmitter setup

The basic radar level transmitter setup isn’t too hard. Regardless, nowadays they come with a “setup wizard,” which makes them even easier. Usually, the setup wizard will walk us through the setup. For example, it’ll often start by asking which product we want to measure. Then it’ll go on to ask for the dielectric of the product, then the type of tank, and so on.

A radar level detector basically includes:

  • A transmitter with an inbuilt solid-state oscillator
  • A radar antenna
  • A receiver along with a signal processor and an operator interface
  • The operation of all radar level detectors involves sending microwave beams emitted by a sensor to the surface of the liquid in a tank. The electromagnetic waves after hitting the surface of the fluid returns back to the sensor which is mounted at the top of the tank or vessel. The time taken by the signal to return back i.e. time of flight (TOF) is then determined to measure the level of fluid in the tank.

Yes, devices that use radar can have significant problems with radar buildup. That’s because when the buildup increases, the signal strength will drop, giving bad measurements.

Thus, proper cleaning of the antenna will fix this problem and get us back to reliable measurements. However, depending on the device, we can also control the clean up with the device itself or our programmable logic controller.

If a device doesn’t have the automated option, then we’ll have to clean it manually. Therefore, we may want to consider upgrading the radar device. Today’s transmitters can measure even with buildup and perform their own cleanings when necessary.

Also, some devices on the market have algorithms to reduce the interference caused by buildup. That means we can maintain the device’s accuracy, even with high levels of contamination, without the need for cleaning. The ability of the transmitter to control the cleanup process using compressed air will also save us money and reduce unplanned downtimes.

Types of radar level transmitters

We have two kinds of radar level transmitters:

  • Noninvasive or Non-contact Systems
  • Invasive or Contact System

Noninvasive radar level measurement

Radar level measurement is based on the principle of measuring the time required for the microwave pulse and its reflected echo to make a complete return trip between the non-contacting transducer and the sensed material level. Then, the transceiver converts this signal electrically into distance/level and presents it as an analogue and/or digital signal. The transducer’s output can be selected by the user to be directly or inversely proportional to the span.

Pulse radar has been used widely for distance measurement since the very beginnings of radar technology. The basic form of pulse radar is a pure time of flight measurement. Short pulses, typically of a millisecond or nanosecond duration, are transmitted and the transit time to and from the target is measured.

Everything inside the tank that conducts energy, such as level switches or heater systems, can reflect the signal. If the product has a low dielectric level, then the radar may find a false level. We may also wind up with bad readings from vapour, foam, or other product conditions.

We can find many solutions to avoid this issue – high-frequency radar level transmitters, echo analysis, stilling wells, and more. This type of radar level measurement can be very accurate.

Invasive or contact radar level measurement

The invasive method used for liquid level measurement is called Guided-wave radar i.e. GWR method. In this method, a cable or rod is employed which act as a wave guide and directs the microwave from the sensor to the surface of the material in the tank and then straight to its bottom. “The basis for GWR is time-domain reflectometry (TDR), which has been used for years to locate breaks in long lengths of cable that are underground or in building walls.

A TDR generator develops more than 200,000 pulses of electromagnetic energy that travel down the wave guide and back.”
The dielectric constant of the process material will cause variation in impedance and reflects the wave back to the radar. Time taken by the pulses to go down and reflect back is determined to measure the level of the fluid.
In this method, the degradation of the signal in use is very less since the wave guide offers an extremely efficient course for signal travel. Hence, level measurement in case of materials having very low dielectric constant can be done effectively. Also in this invasive measurement method, pulses are directed via a guide; hence factors like surface turbulence, foams, vapours or tank obstructions do not influence the measurement.

The GWR method is capable of working with different specific gravity and material coatings. However, there is always a danger that the probe or rod used as a waveguide may get impaired by the agitator blade in the fluid under measurement. A typical guided wave radar system is shown in the figure below.

Effect of Pressure on Level Measurement

Level measurement systems that use differential pressure     P as the   sensing method, are    also   affected  by  pressure,   although   not to the   same  degree as temperature mentioned in the previous section.

Again the measured height H of a column of liquid is directly proportional to the pressure PL exerted  at the  base of the  column  by the  liquid  and  inversely  proportional  to the density d of the liquid.

 H ~ PL/d

Density (mass per unit volume) of a liquid or gas is directly proportional to the process or system pressure Ps.

d ~ Ps

Thus, for  any  given  amount  of liquid  in a  container, the pressure PL (liquid pressure) exerted at the base of the container by the liquid will remain constant, but the height will vary inversely with the process or system pressure.

 H ~ 1/Ps

Most liquids are fairly incompressible and the process pressure will not affect the level unless there is significant vapour content.

INSTRUMENT ERRORS

Any given instrument is prone to errors either due to aging or due to manufacturing tolerances. Here are some of the common terms used when describing the performance of an instrument.

1. RANGE
          The range of an instrument is usually regarded as the difference between the maximum and minimum reading. For example a thermometer that has a scale from 20 to 100oC has a range of 80oC. This is also called the FULL SCALE DEFLECTION (f.s.d.).

2. ACCURACY
          The accuracy of an instrument is often stated as a % of the range or full scale deflection. For example a pressure gauge with a range 0 to 500 kPa and an accuracy of plus or minus 2% f.s.d. could have an error of plus or minus 10 kPa. When the gauge is indicating 10 kPa the correct reading could be anywhere between 0 and 20 kPa and the actual error in the reading could be 100%. When the gauge indicates 500 kPa the error could be 2% of the indicated reading

3. REPEATABILITY
          If an accurate signal is applied and removed repeatedly to the system and it is found that the indicated reading is different each time, the instrument has poor repeatability. This is often caused by friction or some other erratic fault in the system.

4. STABILITY
          Instability is most likely to occur in instruments involving electronic processing with a high degree of amplification. A common cause of this is adverse environment factors such as temperature and vibration.

For example, a rise in temperature may cause a transistor to increase the flow of current which in turn makes it hotter and so the effect grows and the displayed reading DRIFTS. In extreme cases the displayed value may jump about. This, for example, may be caused by a poor electrical connection affected by vibration.

5 TIME LAG ERROR
          In any instrument system, it must take time for a change in the input to show up on the indicated output.This time may be very small or very large depending upon the system. This is known as the response time of the system. If the indicated output is incorrect because it has not yet responded to the change, then we have time lag error.

          A good example of time lag error is an ordinary glass thermometer. If you plunge it into hot water, it will take some time before the mercury reaches the correct level. If you read the thermometer before it settled
down, then you would have time lag error. A thermocouple can respond much more quickly than a glass thermometer but even this may be too slow for some applications.

          When a signal changes a lot and quite quickly, (speedometer for example), the person reading the dial would have great difficulty determining the correct value as the dial may be still going up whe n in reality the signal is going down again.

6 RELIABILITY
          Most forms of equipment have a predicted life span. The more reliable it is, the less chance it has of going wrong during its expected life span. The reliability is hence a probability ranging from zero (it will definitely fail) to 1.0 (it will definitely not fail).

7.DRIFT
          This occurs when the input to the system is constant but the output tends to change slowly. For example when switched on, the system may drift due to the temperature change as it warms up.

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