Fixed-Point Detectors

The operation of a catalytic sensor depends on the oxidation of the flammable gas on the detector which is an electrically heated catalytic filament. This detector is usually made of fine platinum wire that is coated with one of several alternative substances—palladium/platinum/thoria catalysts. These catalysts help to enhance the catalytic activity and prolong the life of the detector by enabling it to operate at lower temperatures. The detector filament is connected to an identical, but inactive, unit in a Wheatstone bridge, and is located adjacent to the active detector. The identical inactive unit allows for ambient temperature compensation. 

When a flammable gas enters a catalytic cell, the gases are combusted on the active filament, causing an increase in temperature and a change in the balance of the Wheatstone bridge. The output signal for flammable gases in air in this type of cell is linear up to the lower explosive limit (LEL). 

Many industrial atmospheres contain contaminants such as silicones, organic lead, sulfur compounds, and halogens which can poison catalytic sensors. Manufacturers go to great lengths in the design of these units to minimize loss of sensor activity from such influences.1 Means taken to keep water, corrosive liquids, and dirt from entering sensors involve the installation of membranes before the flame arrestor in front of the detector filament.2 

The reason that manufacturers of these detectors go to such lengths to protect the instruments from poisoning and burnout is that a failed detector 

2 Product brochure. Teledyne Analytical Instruments, City of Industry, CA. 

---

For a leak-detection system that features fixed-point detectors to be reliable, it must be monitored and calibrated frequently. As discussed in Section... 

K is the same as for unpolarized incident light with a fixed-point detector. The second and higher concentration terms of equation 1 are usually negligible at the low concentrations used in SEC. In this case. 

X-Ray detectors may be classified as point, linear or area, depending on whether they record the diffraction pattern in zero, one or two spatial dimen -sions. Point detectors must be scanned to measure the diffraction pattern, whereas linear or area detectors can be fixed. Point detectors are easily compatible with post-sample optical elements. Linear and area detectors allow the data to be acquired much faster, but as more open systems they are prone to detecting parasitic scatter from the air or sample environment. Both linear and area detectors are types of position sensitive detector (PSD). 

Thermal Methods Level-measuring systems may be based on the difference in thermal characteristics oetween the fluids, such as temperature or thermal conductivity. A fixed-point level sensor based on the difference in thermal conductivity between two fluids consists of an electrically heated thermistor inserted into the vessel. The temperature of the thermistor and consequently its electrical resistance increase as the thermal conductivity of the fluid in which it is immersed decreases. Since the thermal conductivity of liquids is markedly higher than that of vapors, such a device can be used as a point level detector for liquid-vapor interface. 

Sonic Methods A fixed-point level detector based on sonic-propagation characteristics is available for detection of a liquid-vapor interface. This device uses a piezoelectric transmitter and receiver, separated by a short gap. When the gap is filled with liquid, ultrasonic energy is transmitted across the gap, and the receiver actuates a relay. With a vapor filling the gap, the transmission of ultrasonic energy is insufficient to actuate the receiver. 

There are two common types of heat detectors - fixed temperature and rate of rise. Both rely on the heat of a fire incident to activate a signal device. Fixed temperature detectors signal when the detection element is heated to a predetermined temperature point. Rate of rise detectors signal when the temperature rises at a rate exceeding a pre-determined amount. Rate of rise devices can be set to operate rapidly, are effective across a wide range of ambient temperatures, usually recycle rapidly and can tolerate a slow increase in ambient temperatures without providing an alarm. Combination fixed temperature detectors and rate of rise will respond directly to a rapid rise in ambient temperatures caused by fire, will tolerate a slow increase in ambient temperatures without effecting an alarm, and recycle automatically on a drop in ambient temperature. 

Where process equipment is provided with fixed-temperature detectors, these should be located as near as possible to the potential fire source for example, above flammable liquid pump seals, immediately over a solvent draw-off point, or mounted above a crude tank mixer stuffing box. As a general rule, fixed-temperature detectors directed at a potential hazard should be considered only for process equipment where specific fire problems are anticipated. 

Portable detectors for toxic or flammable gases are essential for use by an emergency response team so that they will know the hazards they are facing when responding to a release. There are many portable analyzers that utilize the fixed-point and sampling system technology described in Sections 6.2.1 and 6.2.2. One other method of portable detection is the use of chemical tubes. 

Fig. 5) This figure shows a sketch of the investigated detector concept. An irradiated high mobility two-dimensional electron gas device is subjected to a constant magnetic field Bo, where Bo is chosen to correspond to a fixed point (marked as a dot on the top inset) of the resistance oscillations for incident radiation at a frequency f. The detector device function is realized by superimposing on the static magnetic field, a small time varying component, which has been shown here in blue. Then, a high harmonic, tuned band Terahertz sensor is realized by detecting the device resistance at a odd-harmonic multiple of the field modulation frequency, as the detector is illuminated by Terahertz radiation.

Figure 8 The detector response at the 3 harmonic as a function of the frequency, when the static magnetic field Bo = 1 Tesla, and the working point is the second fixed point, i.e., n = 2.

In this section, we present some of the commercially available detectors for CWAs and toxic chemicals that are based on IMS or DMS. These include a large variety of devices ranging from handheld, or even pocket-size, instruments to larger fixed-point monitors that can even be placed in the ventilation system of buildings. The devices mentioned here are for demonstration purposes and are not to be understood as recommended by us. 

---