The inspection of spirit levels must be conducted in accordance with relevant standards. The temperature within the inspection room shall be maintained at 20 ± 2°C, and inspections must be performed under conditions that are free from vibration and located away from heat sources. Prior to inspection, all components of the spirit level must be thoroughly cleaned; the instrument is then placed upon a metal surface plate within the inspection room and allowed to stabilize at ambient temperature for a minimum of three hours.
Visual Inspection
For newly manufactured spirit levels, the working surface must be flat, smooth, and free from defects such as sand holes, pores, dents, scratches, or corrosion. Non-working surfaces must be free from paint chipping, rust, or other visible defects. The vial (bubble tube) itself shall be clean and transparent; its graduation lines must be clear, uniform, and free from discoloration, and they must be perpendicular to the longitudinal axis of the vial. The manufacturer's name, serial number, and graduation value shall be clearly marked on a non-working surface of the spirit level.
Inter-component Functionality
The main vial must be securely mounted, and the zero-point adjustment mechanism must ensure ease of use and reliability. The movement of the bubble shall be smooth, exhibiting no visually perceptible jumping or sticking. At a room temperature of 20°C, the length of the bubble shall equal the distance between two adjacent long graduation lines; the permissible deviation is limited to ±1 graduation interval for levels with a sensitivity of 0.02–0.05 mm/m, and to ±0.5 graduation interval for levels with a sensitivity of 0.06–0.10 mm/m. The smoothness of the bubble's movement and the bubble's length shall be verified using a spirit level calibrator.
Flatness of the Working Surface
The working surface of the spirit level must be free from any convexities (high spots); its flatness shall comply with the specifications outlined in Table 6-10-54. (Working Surface Length: 150–200 mm; Flatness Deviation: <0.003 mm | Working Surface Length: 250–300 mm; Flatness Deviation: <0.005 mm). For ground and lapped working surfaces, flatness shall be verified using the light-gap method with a Grade 0 straightedge (knife-edge ruler) having a length no less than that of the surface under inspection; this verification procedure must be performed at multiple positions across the working surface-specifically along its longitudinal, transverse, and diagonal axes. The largest visible gap is taken as the flatness deviation of the surface. When estimating the size of the gap, it may be compared against a standard gap formed by gauge blocks.
The flatness of a scraped working surface is verified using a Grade 0 surface plate via the bluing method. Within a square area with a side length of 25 mm, the number of contact spots shall be no fewer than 25 for spirit levels with a graduation value of 0.02–0.05 mm/m, and no fewer than 20 for spirit levels with a graduation value of 0.06–0.10 mm/m; the distribution of these spots shall be uniform.
The straightness of a V-groove working surface is verified using a verification mandrel via the bluing method. The mandrel, coated with red lead powder, is placed on the V-groove surface and rotated; the resulting contact lines visible on the V-groove surface must not exhibit any discontinuity exceeding 10 mm.
Zero-Point Verification
The deviation of the bubble from the central position shall not exceed one-quarter (1/4) of the graduation value. Verification of the zero point for the lower working surface of a spirit level may be performed on a Grade 0 surface plate or on a spirit level verifier; verification of the zero point for the lower V-groove working surface shall be performed using a dedicated fixture. Furthermore, for frame-type spirit levels, verification of the zero points for the upper flat working surface, upper V-groove working surface, side flat working surface, and side V-groove working surface shall also be performed using dedicated fixtures.
Regarding zero-point stability: after the zero point of the lower working surface has been verified as compliant, a re-verification of the zero point shall be conducted after an interval of 4 hours; the observed change must not exceed the permissible range of zero-point deviation.
Verification of Errors
This verification shall be conducted using a spirit level verifier. The difference between the measured average angular value and the nominal angular value shall not exceed 10% of the nominal angular value. The non-uniformity of the graduation values shall not exceed 20% of the graduation value; that is, the difference between any two adjacent readings must fall within the range of 0.8 to 1.2 graduations. Verification of the graduation value errors for a spirit level shall be performed on both the left and right scales of the bubble tube. To eliminate the influence of backlash (dead travel) in the fine-adjustment screw of the spirit level verifier, the fine-adjustment screw must be rotated in a single, consistent direction throughout the procedure.
