Gauge tapes, bobs and water gauge bars shall conform to the requirements set forth in API Manual of Petroleum
Measurement Standards (MPMS), Chapter 3.1A, “Manual Gauging of Petroleum and Petroleum Products”. Gauge
tapes shall be of continuous length and of chrome clad steel construction. Gauge tapes, bobs and water gauge bars
shall have graduations on one side in feet, inches and 1/8 inch (meters, centimeters and millimeters).
GAUGE TAPES, BOBS AND WATER GAUGE BARS
The accuracy of each tape and bob combination shall be verified before initial use and at least once per year thereafter
by a recognized independent inspection agency using the procedure outlined in API Manual of Petroleum
Measurement Standards (MPMS), Chapter 3.1A:
The verification shall be conducted by comparing readings from the working tape and bob against readings from
a reference tape at uniform intervals not to exceed 15 feet (5 meters). The difference between the working tape
and bob and the reference tape shall not exceed ±1/16 inch (±2 mm) at any length up to 100 feet (30 meters).
The uncertainty of the reference tape shall have been established within the previous five years and shall be
traceable to a national weights and measures standard. The reference tape shall have an uncertainty of ±0.01
inch (±0.3 millimeters) for any length up to 100 feet (30 meters).
An innage gauge tape and bob are designed to measure the liquid or free water level from the datum plate near the
bottom of the tank or tank bottom. The zero point for an innage gauge tape and bob is the tip of the bob. The
graduations increase from the zero point upwards. Gauge tapes used in conjunction with water gauge bars must be
modified to account for the additional length of the bob.
An outage gauge tape and bob are designed to establish the liquid level or free water level from the reference point at
the top of the tank. The zero point for an outage gauge tape and bob is the point of contact between the tape snap clasp
and the eyelet of the bob. The graduations increase from the zero point downwards.
A water gauge bar is designed to measure free water from the datum plate or bottom of the tank. The zero point for a
water gauge bar is the tip of the bar. The graduations on a water gauge bar increase from the zero point upwards.
FIGURE 1
Gauge Tapes, Bobs and Water Gauge Bars
