Detector System Response Times

For the systems described in the previous sections, there is a wide variation in response times. As some of the examples in Chapter 7 will show, quick detection of a release and timely activation of postrelease mitigation systems is essential. 

Fixed-point sensors, such as the catalytic, electrochemical, or infrared types, will respond rapidly to gas exposure and generate an output signal within 10 to 30 seconds of exposure to the gas.13 

11 Product bulletin. Control Instruments Corporation, Fairfield, NJ. 

If the sampling system is looking at multiple points, then each point will be looked at only once in the total time per sample analysis, times the number of sample points being monitored. 

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For the detection of slow-acting biological agents (which may not produce symptoms for several days), the system response time would depend on the frequency of sampling and analysis. The frequency of sampling and analysis would be determined by factors such as the cost of the assay, the frequency with which critical reagents need to be replaced, the robustness of the detector, and so on. The minimum response time would be determined by the time required to collect a sample, prepare it for analysis, conduct the assay, and report the results. In the event of an alarm from a detector with a significant false-alarm rate, additional time would be required to determine its validity and to decide on an appropriate response. 

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. 

The detector time constant can distort column efficiency when the peak width (in time units) becomes of the same order of magnitude as the response time. High-efficiency columns produce very sharp peaks, and detectors with response times greater than 0.5 s can contribute significantly to band broadening. Electronic filtering can increase response time and cause measurable broadening of sharp peaks. Refer to Reference 109 for an exhaustive discussion of extra-column effects in detector systems. 

ACOMP Delay Time and Response Time Because ACOMP involves continuous withdrawal, dilution, and conditioning of reactor liquid, there is inevitably both (i) a delay time between when a fluid element is withdrawn and when a measurement of its properties is made by the detector train, and (ii) a system response time associated with the various mixing processes involved. 

Finally, the speed of response of the detector sensor and the associated electronics once played an important part in optimum column design. The speed of response, or the overall time constant of the detector and associated electronics, would be particularly important in the analysis of simple mixtures where the analysis time can be extremely short and the elution of each peak extremely rapid. Fortunately, modern LC detector sensors have a very fast response and the associated electronic circuits very small time constants and, thus, the overall time constant of the detector system does not significantly influence column design in contemporary instruments. The instrument constraints are summarized in Table 2... 

The most important hardware items appeared to be the detectors themselves. The gas detection system gave frequent spurious alarms, and on both platforms the ultraviolet (UV) fire detectors were also prone to spurious activation from distant hot work for example, and had a limited ability to detect real fires. The tmreliability of these systems had a general effect on response time and would, overall, lengthen the time to respond. The second aspect which was related to hardware was fimction and performance testing of the emergency blowdown systems. It is critical that the workers believe the systems will work when required, and this can only be achieved by occasional use or at least fimction testing. 

In a defensive strategy that is based on the detection of a chemical/biological agent in order to initiate a response, the time required for authorities to respond to an attack has three components the inherent response time of the detection system, the time required to verify the validity of a detector alarm, and the time required to decide on what action to take in response to the alarm. These three elements are discussed in more detail below. 

Dynode strings can be constructed in many ways and the response time and range of linearity of the detector depend on the configuration. In the Venetian blind configuration (Fig. 2.21c) the dynodes are wide strips of material placed at an angle of 45° with respect to the electron cascade axis. This system offers a large input area to the incident primary particles. The advantage is that the dynodes are easily placed in line and the dimensions... 

Smoke detectors are employed where the type of fire anticipated and equipment protection needs a faster response time than heat detectors. A smoke detector will detect the generation of the invisible and visible products of combustion before temperature changes are sufficient to activate heat detectors. The ability of a smoke detector to sense a fire is dependent on the rise, spread, rate-of-bum, coagulation and air movement of the smoke itself. Where the safety of personnel is a concern, it is crucial to detect a fire incident at its early stages because of the toxic gases, lack of oxygen that may develop, and obscuration of escape routes. Smoke detection systems should be considered when these factors are present. 

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