PRESSURE AND LEVEL MEASUREMENT SYSTEMS

ABSTRACT

                        The sensor is a transducer that converts the measurand into signal carrying information.

                        The present paper describes the construction of a fiber optic microbend sensor for the measurement of pressure and a water level sensor, which is a pulley and counterweight version of the float type level sensors. Optical fibers used for transmission of digital signals have a constant and lower attenuation. However, attenuation in optical fibers still occurs. Losses in fiber optic cables can be due to absorption, scattering or excessive bending. Bending losses due to external forces can be used to sense changes in the measurand for a fiber optic sensor. Micro bend sensors are intensity modulated type of fiber optic sensors. In these sensors, the light emitted from an optical source is carried along a fiber, its intensity is modified at the transducer and the light is returned to an optical detector.

                         Level measurement is an integral part of process control, and maybe used in a wide variety of industries. Float type sensors are liquid level sensors, which can operate well in a wide variety of liquids. The float and pulley gauge provides an ex­cellent method of measuring large changes in level with accuracy.  It has the advantage that the scale can be placed for convenient reading at any point within a reasonable distance of the tank or vessel. This paper suggests a float type of sensor that gives the level change as a function of the resistance change.

1.  INTRODUCTION

The sensor is a transducer that converts the measurand (a quantity or a parameter) into signal carrying information. The nature of the output signal can be electrical, optical and mechanical.

Instrumentation systems using sensors can be categorized into measurement and control systems. In measurement systems, a quantity or property is measured and its value is displayed. In control systems, the information about the measurand is used to control it so that its measured value should equal a desired value. The actuator converts a signal into an action in order to modify the measurand. The design characteristics of a sensor include the ratings and descriptions of the major features.

            The performance characteristics of the sensor can be static, dynamic and environmental. Static characteristics are usually given for the room conditions. Dynamic characteristics define the sensor’s response to a time variation in the measurand. Environmental characteristics pertain to the performance of the sensor after or during exposure to specific external conditions. Some of the sensor parameters are:

Range, Calibration, Linearity, Sensitivity and Resolution.

The present paper describes the construction of a fiber optic microbend sensor for the measurement of pressure and a water level sensor which is a pulley and counterweight version of the float type level sensors.

2.  FIBER OPTIC MICROBEND PRESSURE SENSOR

2.1 INTRODUCTION

            Fiber optic sensors use light beams to transfer the sensed process parameter from the sensing element to the instrumentation where it is processed. The main advantages of fiber optic instrumentation over conventional sensing systems include their small size, low mass, high accuracy and fast dynamic response capabilities.

2.2 OPTICAL FIBER ATTENUATION

One of the major advantages of optical fibers over electrical lines is their lower attenuation or loss. Also, optical fibers used for transmission of digital signals have a constant attenuation in relation to their frequency whereas electrical attenuation changes with frequency. These two factors allow optical repeaters to be placed at much greater distances from each other than electrical repeaters. However, attenuation in optical fibers still occurs. Losses in fiber optic cables can be due to absorption, scattering or excessive bending.

The bending losses are due to macro bending and micro bending. The figure shows the macro bending which results in the attenuation of optical signal in multi mode step index fiber. The normal, which is perpendicular to the core to cladding interface, changes throughout the bending region. Therefore as some of the modes enter the bending region, they strike the core to cladding interface at angles of incidence that are lower than the critical angle. This results in some refraction into the cladding which constitutes a loss in optical power output.

The other type of bending is micro bending. This is caused by ripples or imperfections in the core to cladding interface or external forces exerted on the fiber. The reason for attenuation is similar to that for macro bending. Bending losses due to external forces can be used to sense changes in the measurand for a fiber optic sensor.

2.3 FIBER OPTIC PRESSURE SENSORS

Two distinct methods may be utilized by the fiber optic sensing system to measure a process variable. These methods are referred to as extrinsic and intrinsic sensing. In an extrinsic sensor, the fiber optic cables are only used to supply light to and from and off-fiber transducer. The fiber optic cable may be viewed as strictly providing light to a black box. After the black box has modulated the light signal with information about the measurand, a second fiber optic cable, or alternatively the original cable, transmits the information to a remote interface unit. In an intrinsic sensor, the modulation occurs inside the fiber. In this sensing mode, the measured property is allowed to deform the fiber which changes its optical properties resulting in modulation of transmitted light.

Fiber optic sensor designs may be divided into four main categories depending on the properties of the light signal that are modulated. These include intensity-modulated, phase-modulated, and spectrum-modulated and time and frequency modulated sensors.

In intensity modulated sensors which are also known as intensity-type sensors, the measurand affects the intensity or brightness of the light transmitted along a fiber optic cable. Phase modulated or interferometric sensors encode the measurand in the phase difference between the light returning from a sensing optical path and light from a reference optical path. Spectrum modulated or wavelength-encoding sensors alter the spectral properties of the light. Other sensor types can modulate the frequency of light signal or use an optical phenomenon known as fluorescence.

2.4 MICROBEND PRESSURE SENSOR

Micro bend sensors are intensity modulated type of fiber optic sensors. In these sensors, the light emitted from an optical source is carried along a fiber, its intensity is modified at the transducer and the light is returned to an optical detector. These sensors are analog in nature, as the light intensity detected is proportional to the measured variable.

Micro bend loss has always been an undesirable effect that causes problems in fiber optic communication links. However this phenomenon can be exploited profitably in the fabrication of a variety of fiber optic sensors to measure pressure, temperature and displacement. Micro bending results in attenuation or loss due to some light beams refracting into the cladding. The higher order modes of light are the ones most affected by a micro bend since they encounter the core to cladding interface at angles only slightly greater than the critical angle. Additionally, upon encountering a micro bend, lower order light modes may be transformed into higher order modes which can be refracted into the cladding at the next micro bend. A series of micro bends can therefore lead to significant light losses. Essentially, micro bend sensors are based on coupling and leakage of modes that are propagating in a deformed fiber.

2.5 CONSTRUCTION

The micro bend sensor consists of a multi mode step index optical fiber which is squeezed between the grooved or corrugated surfaces. One of the corrugated surfaces is attached to the diaphragm and as the diaphragm is displaced, the fiber is squeezed and bent. As the fiber is bent, an amount of light proportional to the pressure applied to the diaphragm is lost due to micro bending attenuation. In general, as the number of bending points on the corrugated surfaces is increased, and as the spacing between corrugations is decreased, the sensitivity of the sensor is enhanced. The fiber optic cable in the micro bend pressure sensors is usually jacketed in a metallic or polymer buffer coating to protect the optical fiber from normal micro bend stress, high temperature and other environmental stressors. Also this coating may extend the mechanical life of the sensor.

2.6 BASIC COMPONENTS

LIGHT SOURCE: Light Emitting diodes (LEDs), laser diodes and He-Ne laser.

FIBER OPTIC CABLE: Depending on the characteristics of both the light source and fiber optic cable, one or many modes of light may enter and propagate through the core. The number of modes distinguishes how many individual light beams are propagating through the core at the same time. The types of modes used here are:

Single Mode Step Index Fiber and MultimodeGraded Index fiber.

CORRUGATED PLATES: Corrugated plates, called deformer plates squeeze the optical fiber under measurand perturbation and thereby induce micro bending. This results in leak of optical power from fiber.

OPTICAL DETECTOR: The optical detector receives the modulated light signals and converts them to electrical signals which are processed by electronic instrumentation. The optical detectors typically consist of a photodiode or a photovoltaic cell along with some signal conditioning circuitry which can amplify or buffer the electrical signal produced by the diode in order to interface properly with signal processing devices.

2.7 WORKING

The optical source launches light into a single multimode step index optical fiber and its output is detected by a detector which is a photovoltaic cell. The pressure applied causes the tapered teeth of deformer plates to produce microbending of the optical fiber. As microbending increases, the optical power received by the detector decreases. The optical detector and the light source are selected by matching the characteristics of the light signals produced by the source and the characteristics of the light signals which can be effectively sensed by the optical detector. This allows for maximum efficiency in the fiber optic microbend sensing system.

The output from the photovoltaic cell is interfaced to a data acquisition card and the output analog signal is digitized. The digitized signal is further processed according to the requirement.

3.  WATER LEVEL SENSOR

3.1 INTRODUCTION

Level measurement is an integral part of process control, and maybe used in a wide variety of industries. Level measurement maybe divided into two categories, point level measurement and continuous level measurement. Point level sensors are used to mark a single discrete liquid height, a preset level condition. Different types of liquid level indicators are available to signal process control systems and to activate alarms.

3.2 FLOAT –TYPE LEVEL SENSORS

Float type sensors are one of the various liquid level sensors available which can operate well in a wide variety of liquids. The float and pulley gauge provides an ex­cellent method of measuring large changes in level with accuracy. It has the advantage that the scale can be placed for convenient reading at any point within a reasonable distance of the tank or vessel. The paper suggests a float type of sensor that gives the level change as a function of the resistance change.

3.3 FLOAT- PULLEY TYPE LEVEL SENSOR

In the pulley and counter-weight version of the float type level sensor, a counterweight provides tension to a rubber cable.  The bore of the pulley is connected to the shaft of a multi turn potentiometer. As the liquid level changes the pulley moves in clockwise direction or anti clockwise direction depending on the level rise or fall. The rotation of the pulley causes the shaft of the potentiometer to rotate which produces a change in the resistance. The voltage across the resistance can be measured and this is directly proportional to the level of the liquid.

3.4 CONSTRUCTION

The float-pulley type of water level sensor involves a pulley and a counter weight. The design parameters of the sensor include the weight of the float (W), the minimum water level or the maximum displacement of the float (L), diameter of the pulley (D), the number of revolutions required (N), and the resistance of the potentiometer.With an initial assumption of the number of revolutions required for the maximum displacement of the float (L), the diameter of the pulley can be decided by the equation governing the conversion of translational motion to the rotational motion.

L = N*Π*D

 The minimum torque (T) required for the pulley to rotate is given by the equation

T = W*D/2

       Where W is the weight of the float. Since the measurement of the torque is a difficult process, the weight of the pulley can be determined by the trial and error method of balancing the float with a counterweight. The weight for which the pulley makes a rotation is the required weight of the float. The counterweight is accordingly selected to balance the float and provide the required torque. The bore of the pulley is connected to the shaft of the potentiometer. The potentiometer should be selected appropriately to give a suitable change throughout the range of operation of the sensor.

The pulley should be made of a light weight material so that the torque required to rotate the pulley is minimum. Also the diameter of the pulley should be selected appropriately so that the linear motion of the float can be converted into the rotational motion of the pulley throughout the required range of water level. The material of the float should be such that it floats on the water without sinking. The cable connecting the float and the counterweight should be such that there is enough friction between the cable and the pulley. One way to provide the necessary friction is by using a rubber belt with one side glued to a cloth. The cloth prevents the rubber belt from elongating due to elasticity.

3.5 WORKING

Change in the level of water causes the float to rise up or down according to the water level change. This translational motion of the float is converted into the rotational motion by the pulley which in turn causes the shaft of the potentiometer to rotate. The output is the voltage across the variable resistance of the potentiometer which is essentially a weak signal. The signal is amplified to the required value using a non inverting amplifier or two cascaded inverting amplifiers. The signal conditioned output signal is interfaced to a data acquisition card and is digitized. The digitized signal can be further interfaced to a micro controller and processed according to the requirement.

           The general advantages of the float-pulley type of level sensor are that it is not affected by dirty water, water temperature or by foam. Also low maintenance is required and it can withstand freezing temperatures. There is no delay between the time when the power is first applied to the sensor and the first output. However a stilling well or tank is absolutely required for the sensor to avoid excess strain on the pulley due to water currents and waves. Also the cable may slip and is easily vandalized unless it is enclosed. Though the electronics are less complex, they still must be mounted directly over the water. If the water level fluctuates around a certain value for an extended period of time, the potentiometer may wear out quickly.

4.  CONCLUSIONS

Pressure applied on the optical fiber causes bending losses which are generally disadvantageous. but in this sensor it is not so. the variation in the output of the sensor is made used to estimate the pressure applied on it. these pressure sensors are used in various industrial applications due to their low mass, high accuracy and fast dynamic response capabilities.

The float-pulley type of level sensor is preferred as it is not affected by dirty water, water temperature or by foam and also it requires low maintenance. An improved mechanical arrangement of the float and pulley and proper selection of the potentiometer in the float-pulley type water level sensor provides the results more accurately.