Heat sensors

Fig. 19. (a) Low level sensor. In the absence of media, the heated sensor tip causes a temperature differential between the two sensors, (b) High level sensor. As media contacts the sensing assembly, heat is dissipated and temperature differential decreases. 

Sensors following the principle of heat of reaction , so-called Pellistors , see Fig. 3.17, serve as indicators for flammable gases. With the catalytic conversion of fuel gases on the surface of a heated sensor, its electrical resistance changes proportionally to the concentration of the gas. 

When dealing with an entire fire detection system that utilizes more than one type of detector, a Detonator Module greatly expands the flexibility and capability of the system. An individual Detonator Module can accept multiple inputs from UV and IR controllers, other Detonator Modules, manual alarm stations, heat sensors, smoke detectors or any contact closure device. In the event of a fire, any of these devices will cause the internal fire circuitry of the module to activate the detonator circuit, sound alarms, and identify the zone that detected the fire. When properly used, a Detonator Module will add only one millisecond to the total system response time. See Figure 8 for an illustration of a fire detection system with a Detonator Module. 

Photomultiplier tubes or photodiodes (light sensors) are used as detectors in UV-VIS spectrophotometers, while thermcouples (heat sensors) are used as detectors for infrared (IR) spectrometry. This is the reason UV-VIS instruments are called spectrophotometers while IR instrument are called spectrometers. 

Fire detection/fire alarm systems consist of different types of fire detection devices and fire alarm systems. These systems may detect fire, heat, smoke, or a combination of any of these. For example, a typical fire alarm system might consist of heat sensors, which are located throughout a facility and which detect high temperatures or a... 

Volatile samples were collected and analysed in the field using the sampling system in conjunction with an array of metal oxide gas sensors. The sensors were calibrated using vapour from an ethanol/water solution and this was used as a reference. A sample of room air was used as a control -background odour. The SPME sampler was introduced to the suspect area, and after sampling the fibre was desorbed into the heated sensor block. When many response patterns are compared it is difficult to visualise... 

Cold-flow velocity field measurements were made, employing hot-wire anemometry to determine the flow characteristics without the influence of combustion. With this diagnostic technique, fluid velocity is measured by sensing the changes in heat transfer from a small, electrically heated sensor exposed to the fluid motion. Under conditions in which fluid temperature, composition, and pressure are constant, the dominant mechanism of heat transfer from the wire is through forced convection, which is directly related to the velocity of the fluid. By measuring the voltage required to maintain the desired hot-wire temperature, the fluid velocity can be accurately determined. 

Attention has to be paid to the material used for hoods and ducts since the gases may be at high temperatures when the intake of the collection system is positioned close to the furnace. The heating required by radiation or convection from the molten metal bath needs to be taken into account in the design stage. Proper maintenance in combination with heat-sensors reduces the risk of fire. 

The heat transfer rate from a flush-mounted shear stress tensor depends on the near-wall flow, i.e., the magnitude of the velocity gradient. For a laminar two-dimensional thermal boundary layer developing over the heated sensor with an approaching linear velocity profile (Fig. 2) and negligible free convectimi effect, the heat loss from the thermal element can be derived from the thermal boundary layer equation as... 

A thermoresistive flow sensor refers to the flow measurement element based on monitoring the thermal states of an electrically heated sensor whose dependence on the heat loss represents the flow rates of the surrounding fluid. The thermoresistivity of a sensor reflects its variation in electrical resistivity with temperature. A sensor element should usually have a sufficiently high-temperature coefficient of resistance (TCR). Typical sensor materials are platinum, nickel, or polysilicon with a specific TCR in the... 

WE-1 Fire in Zone 2A Electrical Limited number of electrical systems Limited Inventory of flammables IV Visual, heat sensor, CAMS Nitrogen system, low volatility of material, metallic containers Access control Worker 0 Collocated D Public D Environment D 4 4 4 4 ... 

Heat sensors and/or smoke detectors may be part of the building safety equipment. If designed into the fire alarm system, they may automatically sound an alarm and call the fire department, they may trigger an automatic extinguishing system, or they may only serve as a local alarm. Because laboratory operations may generate heat or vapors, the type and location of the detectors must be carefully evaluated in order to avoid frequent false alarms. 

Figure 5 Convenient low-power laser pyrolysis system A, injection needle B, stainless steel body Cu, copper cylinder FL, flash lamp G, gas H, heater HS, heated sensor I, injector K, pyrolysis chamber LB, laser beam M, HR mirror Ma, output mirror N, Nd-GGG medium OF, optical fiber OR, O-ring P, probe PC, pumping chamber PS, power supply QC, quartz capillary S, sample Sh, shield Sw, Swagelok T, trigger V, voltage supply. (Reproduced with permission from Cecchetti W, Polloni R, Bergamasco G, et al. (1992) Journal of Analytical and Applied Pyrolysis 23 165-173 Elsevier.)...

Differential scanning calorimetry (DSC) Heat sensor interfere Requires that the heat of dissociation of the water is... 

Fixed Temperature Fixed temperature heat sensors operate when a sensor reaches a preset temperature. They are a variety of temperature settings. Because there is some mass in the sensors themselves, the fixed temperature sensors take some time to respond to conditions. The air surrounding the device will reach the trigger temperature at some time before the sensor elements do. The time lag depends on the device. Because of the lag, fixed temperature sensors are not suitable for fires that develop quickly. 

Smoke Detectors In many fires there is smoke before there is any significant heat buildup. As a result, smoke detectors will usually detect fire before heat sensors. Smoke detectors operate on one of two principles ionization or photoelectricity. 

CTR thermistor heat sensor element Thick-film sensor infrared sensor Varistor noise elimination, surge current absorber, lightning arrestor, etc. 

Chen, M., Yuan, L. and Liu, S. (2007). Research on low temperature anodic bonding using induction heating. Sensors and Actuators A, Vol. 133, pp. 266-269. 

Permselective memWane, chemical valve, affinity chromatography Thermostat, bimetal, pH control, functional electrode heat sensor, thermal actuator... 

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