Probe Adjustment

As was discussed earlier in the chapter, probes typically include two coils H and X nucleus (e.g., or N). The inner (or observation) coil is more sensitive and requires more careful adjustment. Normal NMR spectra usually have such good signal-to-noise ratios that probe tuning is not critical for relatively concentrated samples. Tuning, however, is very important for both one- and two-dimensional X-nucleus-detected experiments and for many two-dimensional H-detected techniques. A surprising number of these experiments have failed simply because the X coil was not tuned to the correct nucleus In addition, if X-nucleus detection is to be conducted with proton broadband decoupling, as is usually the case (Section 1-5), then it is important that the H decoupling coil also be tuned optimally. 

In the former case, an internal lock is established at the deuterium frequency of the solvent by adjusting the frequency of the lock transmitter until it matches that frequency. The operator typically observes a decreasing number of interference-pattern sine waves as the lock transmitter frequency approaches that of the deuterium nuclei in the solvent. A null appears when the two frequencies are identical the operator then turns the lock control to On. On most modem spectrometers, autolocking procedures are also available that search for the deuterium resonance and automatically lock the spectrometer when the signal is found. 

It is important to avoid lock instability due to either saturation or suspended particles, because instability interferes with magnet field regulation by the lock channel and, in severe cases, can result in loss of the lock signal altogether. When a stable lock level is achieved somewhere around midrange, the lock phase (see next subsection) should be maximized. Only an approximately maximum lock phase, however, is desired at this point, because the lock phase is dependent on the homogeneity of the magnetic field. 

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Use for diluting probe, adjusting the probe concentration, and/or inactivating alkaline phosphatase (AP) enzyme. 

A further improvement is possible with the help of an x-ray monochromator 29> 30>, however, the inherent loss in sensitivity and the problems of probe adjustment create a number of difficulties which still have to be overcome in practise. Also the spectra obtained with the help of a monochromator clearly indicate that the gain in resolution for the test standard graphite is around 0.2 eV and the remaining 0.7 eV are obviously due to other parameters 30). 

Shaft position and vibration are monitored continuously during operation by eddy-cinrent proximity probes adjusted to within 1-1.5 mm of the shaft. Radial vibration is generally not symmetrical. Ideally, vibration should be measured in more than one plane. 

The depth adjustment capability has another use besides avoiding damage. It allows the measurement of the temperature profile across the depth of the flow channel using only one probe. Adjustable upstream temperature probes are currently commercially available, e.g., by Goettfert. However, their application in commercial extruders is still rather limited. 

The operation is quite simple One sets the frequency to the lowest value, adjusts the gain and phase to the desired sensitivity using a special calibration standard discussed below and performs a zero-compensation on a defect free zone of the standard. Now one is ready to test. As one slides the probe across the surface of an aluminum structure, a signal response will be indicative of the presence of corrosion or of the presence of a subsurface edge. 

By variation of ceramic volume fraction and selection of the best fitting PZT material we can as well adjust the dielectric constant of the piezocomposite within a wide range. Therefore, we can choose the best piezocomposite material for each probe type to get optimum pulse form and amplitude. 

To increase the sensitivity, direction of amplitude variation of probe output signal in defective area must coincide with the one after CCF processing. If the defect decreases the probe signal (single contact probe) A((/should be set Ai// = 0, in the opposite case (twin contact probe) it should be set Aif/= n. So the instrument should be supplied with a device to adjust A((/ and to sustain it constant. 

The laboratories for ultrtisonic testing must use standard blocks for adjustment of the electronic component of the device and of the probe every time before testing. The blocks must comply with the requirements of normative documents being in force. 

The procedure approved by Gosstandart may have status of state standard, for example, GOST Non-destructive Testing. Ultrasonic Testing of rails , where types of devices, probes, procedure of the instrument adjustment are specified. Such kind of procedure may be used on site. 

At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

Figure Bl.19.40. The scanning ion-conductance microscope (SICM) scans a micropipette over the contours of a surface, keepmg the electrical conductance tlirough the tip of the micropipette constant by adjusting the vertical height of the probe. (Taken from [211], figure 1.)...

Uniformity of the rate of feed will be ensured by a constant-weight feeder density control may be automatically obtained through a measuring probe on the media-return line that adjusts delivery of the nec-essai y volume of media from the densifier or media thickener the viscosity can be controlled automatically by continuously testing a predetermined volume of return media and adjusting the divider under the drainage screen for media cleaning as needed pH control can be automated by conventional methods. 

All vibration equipment shall eonform to API 670. The probes shall be properly loeked to prevent movement during operation. Conneetions from oseillator/demodulators to the Purehaser s wiring shall terminate at suitable terminal bloeks loeated within junetion boxes mounted at the edge of the unit or baseplate. The probes shall be externally removable and adjustable without unit shutdown. 

If the probe velocity is less than the stack velocity, particles will be picked up by the probe, which should have been carried past it by the gas streamlines. The inertia of the particles allows them to continue on their path and be intercepted. If the probe velocity exceeds the stack velocity, the inertia of the particles carries them around the probe tip even though the carrying gases are collected. Adjustment of particulate samples taken anisokinetically to the correct stack values is possible if all of the variables of the stack gas and particulate can be accounted for in the appropriate mathematical equations. 

Two-dimensional C//correlations such as C//COSY or HC HMQC and HSQC provide the Jqh connectivities, and thereby apply only to those C atoms which are linked to H and not to non-protonated C atoms. Modifications of these techniques, also applicable to quaternary C atoms, are those which are adjusted to the smaller Jqh and Jqh couplings (2-25 Hz, Tables 2.8 and 2.9) Experiments that probe these couplings include the CH COLOC (correlation via long range couplings) with carbon-13 detection (Fig. 2.16) and HC HMBC (heteronuclear multiple bond coherence) with the much more sensitive proton detection (Fig. 2.17)... 

The Nernst equation shows that the glass electrode potential for a given pH value will be dependent upon the temperature of the solution. A pH meter, therefore, includes a biasing control so that the scale of the meter can be adjusted to correspond to the temperature of the solution under test. This may take the form of a manual control, calibrated in 0 C, and which is set to the temperature of the solution as determined with an ordinary mercury thermometer. In some instruments, arrangements are made for automatic temperature compensation by inserting a temperature probe (a resistance thermometer) into the solution, and the output from this is fed into the pH meter circuit. 

Remove the electrode assembly (and the thermometer probe if used), rinse in distilled water, and place into a small beaker containing a little of the second buffer solution. If the meter reading does not agree exactly with the known pH, adjust the Slope control until the required reading is obtained. 

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