Surprisingly, we get asked this question a lot.
I would have thought, if you needed one of these measuring devices, you would know which one you need, since manual liquid or oil tank gauging is an activity undertaken by professionals, who have been trained in safety procedures and proper techniques. So, this question and answer is really geared towards distributors of products generally used in the oil and gas industry.
Now, back to Innage vs. Outage…
Simply stated, there are 2 techniques to manually gauge the level of liquids in a large storage tank using a tank gauging tape measure-Innage or Outage. Innage is the more prevalent technique used.
THE INNAGE METHOD
The Innage method will tell you the actual depth of the liquid in the tank. An oil gauging tape measure with a plumb bob is lowered down from the reference point just to the point that the bob touches the bottom of the tank or datum plate. Care should be taken to lower slowly so the bob does not reach the bottom and then tilt over. This will cause an inaccurate measurement. Also, if the bob hits the bottom of the tank with too much force, it can damage or puncture the tank. (We have a new plumb bob that has a rubber end tip to help avoid this kind of damage).
When the tape is reeled back up the depth of the liquid in the tank is read where the tape is wet (like a dipstick in a car). Sometimes water-finding paste or petroleum paste can be used to help show the mark in very thin liquids. Innage oil gauging tapes are used with Innage plumb bobs. The zero point of an Innage gauging tape is at the bottom of the bob.
THE OUTAGE METHOD
The Outage (or Top gauging) method will tell you the measurement of the part of the tank that is NOT filled with liquid (the level that has been taken out, or the “ullage”). Because the exact top of the liquid is hard to find, the graduated plumb bob is lowered slowly from the reference point just until it comes into contact with the liquid. Outage gauging tapes are used with an Outage plumb bob.
The graduations on the outage plumb bob start at the point where the bob is attached to the swivel end clip of the gauging tape, and then get higher. The zero point on an Outage gauging tape is where the plumb bob and the end clip of the gauging tape meet. The reading on the bob (marked by the liquid) is added to the reading on the tape at the reference point (at the top of the tank). This will give you the total empty portion of the tank. Knowing the total depth of the tank, you can subtract the space NOT filled, from the total tank depth to determine the depth of the liquid.
The Outage gauging method is helpful when measuring caustic or thick liquids, as the actual tape measure blade never gets submerged but it is not as accurate as the Innage method for determining the actual level of liquid in the tank.
CHOOSING OIL GAUGING TAPE
So, now you know a bit more about manual tank gauging and some of the nuances of Innage vs. Outage measuring and be better able to stock the proper gauging tapes and bobs for your customers. We offer gauging tapes in lengths from 18’ to 100’, in measurement scales including 8ths, 10ths, 16ths, metric, and English/metric. We also have different blade styles (chrome plated, double duty, black etched, and etched stainless steel) for the varied liquids these gauging tapes will be used for. We have a large selection of plumb bobs to accompany them.