In combination with other imperfections in an NMR tube, a tube with an OD that is too large could make contact with the probe insert, damaging the probe, a costly repair. When the failure to meet OD or ID tolerances is localized in a small area of the tube, portions of the sample might fall outside the 'perfect' cylindrical volume and be exposed to a portion of the magnetic field which may not be homogeneous with the remaining portions of the sample within the 'perfect' cylinder. You'll know this is the case if you must perform extensive shimming from one sample to another. In some cases, you may not be able to achieve satisfactory resolution with further adjustments in magnetic field homogeneity. A measure of the lack of wall uniformity, concentricity can be thought of as the degree to which the cylinders defined by the inner and outer surfaces of the tube are parallel and overlap.
Concentricity is zero for a 'perfect' tube.
Failure to conform to concentricity tolerances causes portions of the sample to fall outside the 'perfect' cylindrical volume, exposing that portion to a magnetic field which may not be homogeneous with portions of the sample within the 'perfect' cylinder. This leads to modulation sidebands, which can quickly become significant. Camber A measure of the lack of straightness of a tube, camber is determined with indicating gauges by measuring the deflection at the middle of the tube held at the ends and rotated. Higher frequency NMR spectrometers have placed greater importance on the Camber of tubes in recent years and stricter tolerances have been required to meet the needs of spectroscopists fortunate enough to be using the most sophisticated NMR spectrometers. Modulation sidebands are prominently seen when tubes with poor Camber are used.