Sampling frequencies

The recommended sampling frequencies in Tables 6.2 and 6.3 are based on several factors. The first, and most important, factor to consider is the question being addressed. Some questions require veiy frequent sampling, whereas others require much less frequent sampling. 

The second factor is the temporal variation in concentrations in different ecosystem compartments. For example, sediments and prey fish exhibit less temporal variation in mercuiy concentration than do air or water, and thus statistically valid estimates of their status can be collected with less frequent monitoring (e.g., annual sampling for prey fish vs. daily or hourly sampling for atmospheric concentrations of mercury). 

The fourth factor affecting sampling frequency is efficiency, that is, to collect the minimum number of samples needed to address the key questions. The costs of collecting and analyzing samples are substantial, and the oversampling of a particular indicator would reduce resomces available for monitoring other indicators or other sites. 

One of the most important considerations in using molecular dynamics for a conformational search is determining the sampling interval. HyperChem lets you sample the simulation in two ways  

If the sampling interval is too short, the total number of structures that belong to unique structural families is small compared to the total number available. If the sampling interval is too large, the total number of available structures is small, and you can miss unique conformations. 

You must consider these factors when deciding on sampling interval. There is no ideal interval for all molecules however, intervals of 0.5-1.0 ps are often used. 

A detailed study of all the factors that influence the sampling frequency complemented with a computerized procedure for optimization of continuous segmented configurations was reported by Angelova and Holy [13]. 

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Sample Frequency. Defines the sampling frequency, in MHz, to be used in the digitisation of the A-scan signal. 

The sample frequency of the ADC (analogue to digital converter) should be 8 times higher than the test frequency (centre frequency of the spectrum). In dependence of the application, different ADC- boards are used. A standard board (20520) provides 8 bit resolution and up to 100 Msamples/s in single shot mode. For manual tests, up to 400 Msamples/s can be reached in the repetition mode. For scanning systems with high frequencies boards up to 400 Msamples/s (single shot) are available. 

Fig. 4 A-scan of a 0.5 mm steel plate (RF-recording) with 400 MHz sample frequency (HILL-SCAN 3010HF), 0.1 V/div. and lOOns/div.

The UT-Acquisition system hardware consists of a 1 channel ultrasonic board, for puls/echo, one or two crystal probes It operates within a frequency range of 0,5 - 15 Mhz and with variable sampling frequencies in steps of 20,40 or 80 MHz... 

Construct a graph of pH as a function of time, and suggest an appropriate sampling frequency for a long-term monitoring program. 

Analyte Sample Volume Sample (liL) Concentration Range Sampling Frequency (h-1)... 

The generation of photons obeys Poisson statistics where the variance is N and the deviation or noise is. The noise spectral density, N/, is obtained by a Fourier transform of the deviation yielding the following at sampling frequency,... 

The proposed PAS-FTIR procedure allows the determination of 30 samples h does not require any sample pre-treatment and avoids waste generation. So, it can conclude that the PAS-FTIR methodology is a reliable alternative to long and tedious classical method of analysis of Mancozeb and improves the sample frequency achieved by the FTIR procedure based on the measurement of KBr disks using internal standard. 

The number of accumulated scans and nominal resolution have been tested in order to ensure a compromise between sensitivity and sampling frequency. 

On considering the solvent consumption and waste generation, it is clear that NIR measurements reduce drastically the amount of CH CN consumed compared with the HPLC procedure. On the other hand, the sample frequency obtained by the NIR methodology was higher than those obtained by HPLC. 

DCS sampling frequency too low or the response time for some analog instruments may be too slow for proper control of transient nature of batch processes and may lead to a process upset. 

Verify suitability of sampling frequency for process dynamics (can vary from process to process)... 

Are all water discharge outfalls, sources, discharge sites and sample frequencies known ... 

Figure 4.10 Representation of the Uansforniation of data from a single-column data suing to a mauix form, based on the sampling frequency and modulation time. The data points acquired for each modulation period are placed in a separate row of the mauix. The matrix data are then in a suitable format to read into an appropriate plotting package such as the Transfoim program.

Sample frequency is a function of the mean time to failure from the onset of an abnormal wear mode to catastrophic failure. For machines in critical service, sampling every 25 hours of operation is appropriate. However, for most industrial equipment in continuous service, monthly sampling is adequate. The exceptions to monthly sampling are machines with extreme loads. In this instance, weekly sampling is recommended. 

Ward, R.C. Nielson, K.S. Evaluating the Sampling Frequencies of Water Quality Networks, Environmental Protection Agency, EPA 600-77-78-169, 1978. 

LDV, PM, and microphone output voltages are simultaneously recorded at a sampling frequency/a = 16,384 Hz during a period of 2 s. It is thus possible to record at least 100 periods for the lowest driving frequency/g = 50 Hz... 

This technique is invasive however, the particle can be designed to be neutrally buoyant so that it well represents the flow of the phase of interest. An array of detectors is positioned around the reactor vessel. Calibration must be performed by positioning the particle in the vessel at a number of known locations and recording each of the detector counts. During actual measurements, the y-ray emissions from the particle are monitored over many hours as it moves freely in the system maintained at steady state. Least-squares regression methods can be applied to evaluate the temporal position of the particle and thus velocity field [13, 14]. This technique offers modest spatial resolutions of 2-5 mm and sampling frequencies up to 25 Hz. 

The use of confidence intervals is one way to state the required precision. Confidence limits provide a measure of the variability associated with an estimate, such as the average of a characteristic. Table I is an example of using confidence intervals in planning a sampling study. This table shows the interrelationships of variability (coefficient of variation), the distribution of the characteristic (normal or lognormal models), and the sample frequency (sample sizes from 4 to 365) for a monitoring program. 

The role of meteorological and climatic factors and their variability. Ambient air quality (e.g., oxidant concentrations) and deposition that can affect the emissions to indicator relationship. Sampling frequency is also an important attribute of a monitoring program that strongly influences the ability to detect trends. 

The considerably higher costs for operation and sample analysis of an event-based network currently preclude a higher sampling frequency. In addition, data from the networks are limited to wet deposition. Methodologies for determining dry deposition are subject to greater uncertainties, and can be more costly than wet deposition... 

Hebert CE, Weseloh DVC. 2003. Assessing temporal trends in contaminants from long-term avian monitoring programs the influence of sampling frequency. Ecotoxicology 12 141-151. 

Study type Required pre- or post-registration Use of radiolabel Identification of unknowns Need for mass balance Importance of low detection limits Importance of adjustable sampling frequency... 

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