FLAME DETECTOR / SENSOR

Flame detection is the technology for detecting flames, using a flame detector. Flame detectors are optical equipment for the detection of flame phenomena of a fire. There are two categories of flame detection:

Flame detector for the detection of a fire in a fire alarm system

Flame scanner for monitoring the condition of a flame in a burner

THE DETECTION RANGE

Detection Range

The range of a flame detector is highly determined by the mounting location. In fact, when making a projection, one should imagine in what the flame detector “sees”. A rule of thumb is, that the mounting height of the flame detector is twice as high as the highest object in the field of view. Also the accessibility of the flame detector must be taken into account, because of maintenance and/or repairs. A rigid light-mast with a pivot point is for this reason recommendable. A “roof” on top of the flame detector (30 x 30 cm, 1 x 1-foot) prevents quick pollution in outdoor applications. Also the shadow effect must be considered. The shadow effect can be minimized by mounting a second flame detector in the opposite of the first detector. A second advantage of this approach is, that the second flame detector is a redundant one, in case the first one is not working or is blinded. In general, when mounting several flame detectors, one should let them “look” to each other not let them look to the walls. Following this procedure blind spots (caused by the shadow effect) can be avoided and a better redundancy can be achieved than if the flame detectors would “look” from the central position into the to be protected area. The range of flame detectors to the 30 x 30 cm, 1 x 1-foot industry standard fire is stated within the manufacturers data sheets and manuals, this range can be affected by the previously stated de-sensitizing effects of sunlight, water, fog, steam and blackbody radiation.

THE SQUARE LAW

Square Law

The square law applies to flame detection and relates the flame area and the distance from the flame to the flame detector: If a flame detector can detect a fire with an area A on a certain distance, then a 4 times bigger flame area is necessary if the distance between the flame detector and the fire is doubled. In short:

Double distance = four times bigger flame area (fire). This law is valid for all flame detectors, also for the ones, which are based on camera

technique. It is a law, which is valid for all optical detectors. The maximum sensitivity can be determined by dividing the maximum flame area A by the square of the distance between the fire and the flame detector: c = A/d2. With this constant c can, for the same flame detector and the same type of fire, the maximum distance or the minimum fire area be calculated: A = cd2 and d = √(A/c). It must be emphasized, however, that the square root in reality is not valid anymore at very high distances. At long distances other parameters are playing a significant part; like the occurrence of water vapour and of cold CO2 in the air. In the case of a very small flame, on the other hand, the decreasing flickering of the flame will play an increasing part.

TYPES OF FLAME DETECTORSThere are several types of flame detector. The optical flame detector is a detector that uses optical sensors to detect flames. There are also ionization flame detectors, which use current flow in the flame to detect flame presence, and thermocouple flame detectors.

Optical Types

Ultraviolet

Ultraviolet (UV) detectors work with wavelengths shorter than 300 nm. These detectors detect fires and explosions within 3–4 milliseconds due to the UV radiation emitted at the instant of their ignition. False alarms can be triggered by UV sources such as lightning, arc welding, radiation, and sunlight.

In order to reduce false alarm a time delay of 2-3 seconds is often included in the UV flame detector design.

Near IR Array

Near infrared (IR) array flame detectors, also known as visual flame detectors, employ flame recognition technology to confirm fire by analyzing near IR radiation via the pixel arrayof a charge-coupled device (CCD).

Infrared

Infrared (IR) flame detectors work within the infrared spectral band. Hot gases emit a specific spectral pattern in the infrared region, which can be sensed with a specialized thermal imaging camera (TIC) for firefighting, a type of thermographic camera. False alarms can be caused by other hot surfaces and background thermal radiation in the area as well as blinding from water and solar energy. A typical frequency where single frequency IR flame detector is sensitive is in the 4.4 micrometre range. Typical response time is 3-5 seconds.

UV/IR

UV and IR flame detectors compare the threshold signal in two ranges in "AND" configuration and their ratio to each other to confirm the fire signal and minimize false alarms.

IR/IR flame detection

Dual IR (IR/IR) flame detectors compare the threshold signal in two infrared ranges. In this case one sensor looks at the 4.4 micrometre range the other sensor at a reference frequency.

IR3 flame detection

Triple IR flame detectors compare three specific wavelength bands within the IR spectral region and their ratio to each other. In this case one sensor looks at the 4.4 micrometre range and the other sensors at reference bands above and below. This allows the detector to distinguish between non flame IR sources, and flames that emit hot CO2 in the combustion process (which have a spectral characteristic peak at 4.4 micrometre). As a result, both detection range and immunity to false alarms can be significantly increased. IR3 detectors can detect a 0.1m2 (1ft2) gasoline pan fire at up to 65m (215ft) in less than 5 seconds.

Most IR detectors are designed to ignore constant background IR radiation, which is present in all environments. Instead they measure the modulated part of the radiation. When exposed to modulated non flame IR radiation, IR and UV/IR detectors become more prone to false alarms, while IR3 detectors become somewhat less sensitive, and more immune to false alarms. Triple IR, like other IR detector types, is susceptible to blinding by a layer of water on the detector's window.

Visible sensors

In some detectors a sensor for visible radiation is added to the design in order to be able to discriminate against false alarms better or improve the detection range. Example: UV/IR/vis, IR/IR/vis, IR/IR/IR/vis flame detectors.

Ionization current flame detection

The intense ionization within the body of a flame can be measured by means of the current which will flow when a voltage is applied. This current can be used to verify flame presence and quality. They are normally used in large industrial process gas heaters and are connected to the flame control system and act as both the flame quality monitor and the "flame failure device".

Thermocouple flame detection

Thermocouples are used extensively for monitoring flame presence in combustion heating systems and gas cookers. They are commonly used as the "flame failure device" to cut off the supply of fuel if the flame fails. This prevents the danger of a large explosive mixture building up, or the hazard of asphyxiation in a confined space due to exclusion of oxygen.

PRESSURE TRANSMITTER

A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as afunction of the pressure imposed. For the purposes of this article, such a signal is electrical.

Pressure sensors are used for control and monitoring in thousands of everyday applications. Pressure sensors can also be used to indirectly measure other variables such as fluid/gas flow, speed, water level, and altitude. Pressure sensors can alternatively be called pressure transducers, pressure transmitters, pressure senders, pressure indicators, piezometers and manometers, among other names.

TYPES OF PRESSURE MEASUREMENTS

silicon piezoresistive pressure sensors

Pressure sensors can be classified in terms of pressure ranges they measure, temperature ranges of operation, and most importantly the type of pressure they measure. Pressure sensors are variously named according to their purpose, but the same technology may be used under different names.

Absolute pressure sensor

This sensor measures the pressure relative to perfect vacuum.

Gauge pressure sensor

This sensor measures the pressure relative to atmospheric pressure. A tire pressure gauge is an example of gauge pressure measurement; when it indicates zero, then the pressure it is measuring is the same as the ambient pressure.

Vacuum pressure sensor

This term can cause confusion. It may be used to describe a sensor that measures pressures below atmospheric pressure, showing the difference between that low pressure and atmospheric pressure (i.e. negative gauge pressure), but it may also be used to describe a sensor that measures low pressure relative to perfect vacuum (i.e. absolute pressure).

Differential pressure sensor

This sensor measures the difference between two pressures, one connected to each side of the sensor. Differential pressure sensors are used to measure many properties, such as pressure drops across oil filters or air filters, fluid levels (by comparing the pressure above and below the liquid) or flow rates (by measuring the change in pressure across a restriction). Technically speaking, most pressure sensors are really differential pressure sensors; for example a gauge pressure sensor is merely a differential pressure sensor in which one side is open to the ambient atmosphere.

Sealed pressure sensor

This sensor is similar to a gauge pressure sensor except that it measures pressure relative to some fixed pressure rather than the ambient atmospheric pressure (which varies according to the location and the weather).

PRESSURE-SENSING TECHNOLOGYThere are two basic categories of analog pressure sensors,

Force collector types These types of electronic pressure sensors generally use a force collector (such a diaphragm, piston, bourdon tube, or bellows) to measure strain (or deflection) due to applied force (pressure) over an area.

Piezoresistive strain gauge

Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to

applied pressure. Common technology types are Silicon (Monocrystalline), Polysilicon Thin

Film, Bonded Metal Foil, Thick Film, and Sputtered Thin Film. Generally, the strain gauges

are connected to form a Wheatstone bridge circuit to maximize the output of the sensor and

to reduce sensitivity to errors. This is the most commonly employed sensing technology for

general purpose pressure measurement. Generally, these technologies are suited to

measure absolute, gauge, vacuum, and differential pressures.

Capacitive

Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain due to

applied pressure. Common technologies use metal, ceramic, and silicon diaphragms.

Generally, these technologies are most applied to low pressures (Absolute, Differential and

Gauge)

Electromagnetic

Measures the displacement of a diaphragm by means of changes

in inductance (reluctance), LVDT, Hall Effect, or by eddy current principle.

Piezoelectric

Uses the piezoelectric effect in certain materials such as quartz to measure the strain upon

the sensing mechanism due to pressure. This technology is commonly employed for the

measurement of highly dynamic pressures.

Optical

Techniques include the use of the physical change of an optical fiber to detect strain due to

applied pressure. A common example of this type utilizes Fiber Bragg Gratings. This

technology is employed in challenging applications where the measurement may be highly

remote, under high temperature, or may benefit from technologies inherently immune to

electromagnetic interference. Another analogous technique utilizes an elastic film

constructed in layers that can change reflected wavelengths according to the applied

pressure (strain).[1]

Potentiometric

Uses the motion of a wiper along a resistive mechanism to detect the strain caused by

applied pressure.

OTHER TYPES

These types of electronic pressure sensors use other properties (such as density) to infer pressure of a gas, or liquid.

Resonant

Uses the changes in resonant frequency in a sensing mechanism to measure stress, or

changes in gas density, caused by applied pressure. This technology may be used in

conjunction with a force collector, such as those in the category above. Alternatively,

resonant technology may be employed by exposing the resonating element itself to the

media, whereby the resonant frequency is dependent upon the density of the media.

Sensors have been made out of vibrating wire, vibrating cylinders, quartz, and silicon MEMS.

Generally, this technology is considered to provide very stable readings over time.

Thermal

Uses the changes in thermal conductivity of a gas due to density changes to measure

pressure. A common example of this type is the Pirani gauge.

Ionization

Measures the flow of charged gas particles (ions) which varies due to density changes to

measure pressure. Common examples are the Hot and Cold Cathode gauges.

CAPACITIVE LEVEL SENSOR

LOADCELLA load cell is a transducer that is used to convert a force into an electrical signal. This conversion is indirect and happens in two stages. Through a mechanical arrangement, the force being sensed deforms a strain gauge. The strain gauge measures the deformation (strain) as an electrical signal, because the strain changes the effective electrical resistance of the wire. A load cell usually consists of four strain gauges in a Wheatstone bridge configuration. Load cells of one strain gauge (quarter bridge) or two strain gauges (half bridge) are also available.The electrical signal output is typically in the order of a few millivolts and requires amplification by an instrumentation amplifier before it can be used. The output of the transducer can be scaled to calculate the force applied to the transducer. The various types of load cells that exist include Hydraulic load cells, Pneumatic load cells and Strain gauge load cells.

Hydraulic load cellThe cell uses conventional piston and cylinder arrangement. The piston is placed in a thin elastic diaphragm. The piston doesn't actually come in contact with the load cell. Mechanical stops are placed to prevent over strain of the diaphragm when the loads exceed certain limit. The load cell is completely filled with oil. When the load is applied on the piston, the movement of the piston and the diaphragm arrangement result in an increase of oil pressure which in turn produces a change in the pressure on a Bourdon tube connected with the load cells. Because this sensor has no electrical components, it is ideal for use in hazardous areas. Typical hydraulic load cell applications include tank, bin and hopper weighing.

Pneumatic load cellThe load cell is designed to automatically regulate the balancing pressure. Air pressure is applied to one end of the diaphragm and it escapes through the nozzle placed at the bottom of the load cell. A pressure gauge is attached with the load cell to measure the pressure inside the cell. The deflection of the diaphragm affects the airflow through the nozzle as well as the pressure inside the chamber.

Strain gauge load cellStrain gauge load cells are the most common in industry. These load cells are particularly stiff, have very good resonance values, and tend to have long life cycles in application. Strain gauge load cells work on the principle that the strain gauge (a planar resistor) deforms/stretches/contracts when the material of the load cells deforms appropriately. These values are extremely small and are relational to the stress and/or strain that the material load cell is undergoing at the time. The change in resistance of the strain gauge provides an electrical value change that is calibrated to the load placed on the load cell.

Other types of load cells in industrial applications are piezoelectric, hydraulic (or hydrostatic), and semiconductor are probably the second most common. By example, a hydraulic load cell is immune to transient voltages (lightning) so these type of load cells might be a more effective device in outdoor environments. Piezoelectric load cells work on the same principle of deformation as the strain gauge load cells, but a voltage output is generated by the basic piezo material - proportional to the deformation of load cell. Most applications for piezo-based load cells are in the dynamic loading conditions, where strain gauge load cells can fail with high dynamic loading cycles.

Other typesOther types include piezoelectric load cells (useful for dynamic measurements of force), and vibrating wire load cells, which are useful in geomechanical applications due to low amounts of drift, and capacitive load cells where the capacitance of a capacitor changes as the load presses the two plates of a capacitor closer together.

RingingEvery load cell is subject to "ringing" when subjected to abrupt load changes. This stems from the spring-like behavior of load cells. In order to measure the loads, they have to deform. As such, a load cell of finite stiffness must have spring-like behavior, exhibiting vibrations at its natural frequency. An oscillating data pattern can be the result of ringing. Ringing can be suppressed in a limited fashion by passive means. Alternatively, a control system can use an actuator to actively damp out the ringing of a load cell. This method offers better performance at a cost of significant increase in complexity.

DENSITYMETER

AIR FILTER REGULATOR (AFR)