Distance measuring device and method for measuring distance

A distance measuring device that measures, in a reactor in which a plurality of reaction tubes arranged parallel to each other is joined to a tube sheet, a distance from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube in a non-contact manner for at least some of the plurality of reaction tubes, the distance measuring device comprising:

• a measuring member holding a length measuring instrument;
• at least one base member having a surface on which the measuring member is movably disposed;
• an adjustment mechanism configured to adjust a measurement direction and an angle of the length measuring instrument, wherein the angle is adjustable so that the measurement direction is adjusted to be parallel to the axial length direction of the reaction tube when the axial length direction of the reaction tube is not perpendicular to the surface of the base member;
• and
• support legs that support the base member and allow the base member to be detachably attached above the tube sheet, wherein the support legs are stretchable, and a height of each of the support legs is adjustable,
• wherein when a straight line parallel to a direction in which the measuring member disposed on the base member moves is defined as a reference line, an angle formed by a straight line parallel to an axial length direction of the reaction tube and the reference line, which are on an identical plane, is constant for the plurality of reaction tubes disposed side by side along the reference line,
• the measurement direction of the length measuring instrument is parallel to the axial length direction of the reaction tube in a state where the measuring member is disposed on the base member, and
• the measuring member is disposed on the base member to be able to sequentially move from a position where the distance of one of the reaction tubes is measured to a position where the distance of another of the reaction tubes is to be measured.