Shimming

Most NMR spectrometers have 12 to 18 shim controls (Churmny and Hoult 1990). Each user will adopt their own procedure but the aim is to produce the minimum linewidth consistent with a good lineshape. In practice, some shims are much more significant than others and for particular probes different shims will be important. For solid-state operation, shimming usually needs to be carried out relatively infrequently. One possible procedure for probes tuned to H is to crudely shim on H2O. If there is no proton channel most multinuclear probes will tune to D, so D2O can be used. For CP-MAS probes that tune to - C, adamantane is a useful compound which should be shimmed under spinning and H decoupling conditions. A typical resolution for in admantane of 3-4 Hz at 7.05 T and 10 Hz at 11.7 T should be achieveable. 

In any shimming procedure the gradients should be varied systematically (Churmny and Hoult 1990) and it should be borne in mind that even with very good shim sets the gradients will be interdependent. The speed of the Fourier transformation in modem spectrometers allows the spectrum itself to be used for shimming, rather than the FID. There will always be a compromise between the very narrowest lines that can be produced and a good lineshape (free of shoulders and humps). 

First optimize the z-gradient to maximum lock level. Note the maximum value obtained. 

Then adjust the 2 -gradient, and note carefully the direction in which the z -gradient is changed. 

Again adjust the z-gradient for maximum lock level. 

Check if the strength of the lock level obtained is greater than that obtained in step 1. If not, then readjust z , changing the setting in a direction opposite to that in step 2. 

Readjust the z-gradient for maximum lock level, and check if the lock level obtained is greater then that in steps 1 and 3. 

field lock. The maintenance of a constant applied field strength through the use of an active feedback mechanism. 

The deuterium lock channel is the part of the NMR instrument that monitors the frequency of the H s in the sample and adjusts the strength of the applied field so that the frequency of whatever nuclide is being observed is known. The frequency of the NMR signal is monitored every 500 ns and applied field strength is adjusted to maintain a constant value. 

The field lock is established by slightly varying the strength of the applied field Bq until the frequency being generated in the NMR console is the same as the Larmor frequency of the H s of the solvent in the sample. At this point, a phase-locked loop circuit is used to lock onto the frequency. From then on, magnet drift is compensated for, unless the limit of the ability of the instrument to adjust the applied field strength is reached. 

Field lock. Syn. deuterium field lock, lock, lock. The holding constant of the strength of the applied magnetic field through the monitoring of the Larmor frequency of one nuclide (normally H, but possibly F) In the solution and making small field strength adjustments. 

Shim (n). One of a number of colls of wire surrounding the sample and probe wrapped so that a current passing through this coll Induces a change In the strength of the applied magnetic field with a prescribed geometry. 

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Shim M G and Wilson B C 1997 Development of an in vivo Raman spectroscopic system for diagnostic applications J. Raman Spectrosc. 28 131-42... 

Control of the core is affected by movable control rods which contain neutron absorbers soluble neutron absorbers ia the coolant, called chemical shim fixed burnable neutron absorbers and the intrinsic feature of negative reactivity coefficients. Gross changes ia fission reaction rates, as well as start-up and shutdown of the fission reactions, are effected by the control rods. In a typical PWR, ca 90 control rods are used. These, iaserted from the top of the core, contain strong neutron absorbers such as boron, cadmium, or hafnium, and are made up of a cadmium—iadium—silver alloy, clad ia stainless steel. The movement of the control rods is governed remotely by an operator ia the control room. Safety circuitry automatically iaserts the rods ia the event of an abnormal power or reactivity transient. 

Chemical shim control is effected by adjusting the concentration of boric acid dissolved ia the coolant water to compensate for slowly changing reactivity caused by slow temperature changes and fuel depletion. Eixed burnable poison rods are placed ia the core to compensate for fuel depletion. 

These are made of boron carbide ia a matrix of aluminum oxide clad with Zircaloy. As the uranium is depleted, ie, burned up, the boron is also burned up to maintain the chain reaction. This is another intrinsic control feature. The chemical shim and burnable poison controls reduce the number of control rods needed and provide more uniform power distributions. 

J. S. Shim and co-workers. Cancer Epidemiol Biomarkers Prev. 4(14), 387—391 (June 1995). 

M. Adsawa,. Oh shim a, andH. B. Mamyama, Chem. Pharm. Bull 30, 3333 (1982). 

T. Nishino, N. Ishii, T. Tanino, S. Oh shim a, and T. Yamaguchi, 19th Interscience Conference on Antimicrobial Agents and Chemotherapy, Boston, Mass., 1979, Abstr. No. 241. 

Y. Mi shim a, The Second Japan-Mustralia International Workshop on Thermal Neutron Capture Therapyfor Malignant Melanoma, Vol. 2—4, Kobe, Japan, 1989, pp. 223-386. 

F. Tsutsumi, M. Sakakibara, and N. O shim a, Paper presented at MCS Rubber Division Meeting, Cincinnati, Ohio, Oct. 18, 1988. 

Lemon hore or elliptical. This bearing is bored with shims split line, which are removed before installation. The resulting shape approximates an ellipse with the major axis clearance approximately... 

Sometimes in practice the dial indicators are mounted on the couplings, but it is best to mount and fix the indicatttrs onto the shafts because the couplings may be eeeentrie to the shaft centerlines. Rotate the shafts and obtain the displacement readings. Project these readings graphically (tr mathematically to the motor base t(t determine the adjustments required, and the spacing shims under each foot. 

This method is most usefiil when only one of the shafts can be rotated for the alignment procedure, or when the two shaft ends are very close to each other. Obtain the displacement readings with the dial on the rim (OD) of the coupling and the coupling face. Project these readings mathematically or graphically to the motor base to determine the required adjustments and shims for each foot. This method is not as precise and may have a built-in error, if the coupling center is eccentric from the shaft centerline. 

At Pump Installation Be sure the motor shaft centerline is below the pump shaft centerline so that it can be shimmed upward. Make sure the motor mount boltholes have sufficient play to allow for some lateral adjustment. Many pumps and motor assemblies are shipped from the factory on a common channel iron base plate. The manufacturer alleges that they arc already aligned at the factory. You need to verify and correct this alignment in all cases. 

Shims should be free of dirt and corrosion. They should be replaced from time to time because they can become deformed with time and weight. 

Spacer shims should be made of 304 stainless steel, exeept with chlorine and hydrochloric acid service. In these services, use Mylar shims to resist corrosion. 

It is best to use the thickest shim possible instead of numerous thin shims, which can suffer from compression. Never stack more than 3 shims under an equipment foot. 

Measure shims to verify their thickness and toleranee, especially thin shims (those less than 0.005 inch). 

Avoid the use of shims with the thiekness stamped on the shim face. 

Use shims large enough to completely cover the equipment footprint. 

Install the shims sliding them under the machinery footprint, until contact is made with the anchor bolt. Then move the shim back away from the bolt shaft to avoid interference with the threads and to assure tolerance. 

To correct soft foot, place shims under the foot in the thickness corresponding to the movement of the dial indicator. 

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