Sensor response time

The strategy depends on the situation and how we measure the concentration. If we can rely on pH or absorbance (UV, visible, or Infrared spectrometer), the sensor response time can be reasonably fast, and we can make our decision based on the actual process dynamics. Most likely we would be thinking along the lines of PI or PID controllers. If we can only use gas chromatography (GC) or other slow analytical methods to measure concentration, we must consider discrete data sampling control. Indeed, prevalent time delay makes chemical process control unique and, in a sense, more difficult than many mechanical or electrical systems. 

McDonagh C., Bowe P., Mongey K., MacCraith B., Characterisation of porosity and sensor response times of sol-gel-derived thin films for oxygen sensor applications, J. Non-Cryst. Solids 2002 306 138-148. 

In principle, there are two possible ways to measure this effect. First, there is the end-point measurement (steady-state mode), where the difference is calculated between the initial current of the endogenous respiration and the resulting current of the altered respiration, which is influenced by the tested substances. Second, by kinetic measurement the decrease or the acceleration, respectively, of the respiration with time is calculated from the first derivative of the currenttime curve. The first procedure has been most frequently used in microbial sensors. These biosensors with a relatively high concentration of biomass have a longer response time than that of enzyme sensors. Response times of comparable magnitude to those of enzyme sensors are reached only with kinetically controlled sensors. 

The performance of common multisensor arrays is ultimately determined by the properties of their constituent parts. Key parameters such as number, type and specificity of the sensors determine whether a specific instrument is suitable for a given application. The selection of an appropriate set of chemical sensors is of utmost importance if electronic nose classifications are to be utilised to solve an analytical problem. As this requires time and effort, the applicability of solid-state sensor technology is often limited. The time saved compared with classic analytical methods is questionable, since analysis times of electronic nose systems are generally influenced more by the sampling method utilised than the sensor response time [185]. 

The condition Jq > Jg (Jg — Ig/(area of electrode)) is favored by small values of <7l- However, small values of lead to long sensor response times (see below). 

Enzyme concentration Temperature Sensor response time... 

Figure 7. Microparticle Sensor Response Time. The arrows show when the sensor was placed in the respective pH buffer. Measurements were taken at various intervals over 20 minutes.

Figure 4. Sensor response time constant for film (17 nm Ag/330 nm W03/100 nm Pd) over a period of 2 days. The sample was exposed to 5% H2 in air for periods of about 2 minutes, then to dry air. The response time constant was measured away from the SPR, at 800 nm. The fitted curve varies as the square root of time.

Li et al. [37-39] described the use of the bacterial species Bacillus subtilis and Bacillus licheniformis entrapped between a polycarbonate membrane and a Teflon-covered DO probe. The differences between the steady-state signals before and after exposure to the test samples were used as a measure of the sample BOD levels. Riedel et al. [32-33] described the use of the yeast Trichosporon cutaneum, or both T. cutaneum and B. subtilis, sandwiched between a dialysis membrane and a polyethylene-covered DO probe. In these cases, the sensor response times were speeded up by measuring the initial rates of change of the signals. In this way, measurements could be made within 30 s rather than within 15-20 min for the steady-state approach [33]. 

The particulate sensors will be highly sensitive to atmospheric tests (threshold lower than 1 kt) and will provide unambiguous evidence of a nuclear explosion, although sensor response time is slow and localization accuracy limited. For... 

Besides the obvious advantages, there are also some limitations the most important one is the fact that a customary SAW is very sensitive towards high viscosity and loading. This makes it virtually impossible to operate it in liquid phase, as the surface wave is completely damped by the sample matrix. The second consequence is that sensitive layers for SAW coating usually are thinner than for QCM, however, the highly increased device sensitivity by far overcompensates the lower amount of interaction sites. Additionally, thin layers lead to very favourable sensor response times. 

Carbonic anhydrase catalyses the hydration and dehydration reactions of CO2 and accelerates this reaction rate approximately 5000-fold. The results of earlier attempts to use carbonic anhydrase to improve sensor response time were mixed, probably because the response times of previous sensors were limited by slow diffusion through the gas-permeable membrane, and not by reaction kinetics [29, 30]. 

In order to improve the sensor response time, a thin hydrogel layer was directly deposited onto the backside of the bending plate covered with a 220 nm thick PECVD silicon nitride film and with a 17 run thick adhesion promoter layer (Fig. 2c). The final thickness of the dried and then cross-linked hydrogel layer was 4... 50 pm. 

Typical monitor data, such as response time, accuracy, or sensitivity, do not depend only on sensor characteristics. For example, the overall response time is composed of the sensor response time, of delays due to ambient influences such as water, of sample flow in the case of sidestream sensors, or of gas exchange time of the cell or detector volume. 

To test for sensor response time, a high shaft speed was desirable. Errors in response time resulting from liquid boil-off and measurement tolerances on actual sensors position could be kept below 2 msec by using shaft speeds between 23 and 60 in./sec. The available 6-in. stroke was sufficient to ensure that all sensors were completely submerged at the bottom end of the stroke and completely out of liquid at the top. 

Both sensors were heated to their operational temperature of 700°C by the Pt-heater. A temperature modulation frequency of 0.156 Hz was applied to the modulation heater. The thermopower was determined by a continuous regression analysis over two periods. Due to this low modulation frequency, the sensor response time is limited to 12.8 seconds. 

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