Gaging the Inside of a Pressurized Tank
So you’ve been tasked with measuring the strain on the inside of a pressurized tank. A couple of concerns come to mind right away.
First it is very important to determine how the tank will be pressurized. If a gas is used for pressurization it must be filtered and dry. Wet gases should never be used as moisture will certainly find their way through most protective coatings with the help of pressurization.
If the pressurization will be in liquid form the same filtering is recommended although drying may not be an option if the pressurizing fluid is water. If at all possible the use of water should be avoided as it is difficult to keep gages sealed for any extended time. If possible use common mineral oil, available at any pharmacy, for liquid pressurization as it is not harmful to gages even under pressure. Silicone oil is also used but it needs to be dried prior to use as it can absorb water during storage.
Another important consideration is the strain gage sensors to be used, the selection of which is typically driven by the specifics of the application. One of the first things to remember is that a strain gage is an averaging device. If the strain field is uniform or uniformly changing, the size of the gage is not critical. If there are stress concentrations, then the size of the active sensing grid should be commensurate with the size of the high strain/stress area. If you use a large gage pattern over a small stress concentration, the reported strain will be averaged down, reporting lower strains than are actually present. This is almost universally bad.
If you are not certain of the direction of the strains, a 3-element rosette is the best way to determine both direction and magnitude of the maximum and minimum principal strains at a particular location. For more information concerning the use of 3-element rosettes, please see Micro-Measurements’ Tech Note TN-515: Strain Gage Rosettes: Selection, Application and Data Reduction.
Where practicable — meaning your instrument has the proper bridge completion — gages with resistance of 350 W or higher are preferred. The higher the resistance, the greater the excitation that can be applied to the gage without grid self-heating. This results in a better signal-to-noise ratio. For more help with determining the proper excitation voltage see Micro-Measurements’ Tech Note TN-502: Optimizing Strain Gage Excitation Levels.
Another consideration is the material from which the tank is made and its CTE (coefficient of thermal expansion). The CTE is required in order to select the proper STC (self-temperature compensation). For example, if the tank is made of aluminum, the CTE for most aluminum alloys is nominally 13. The matching STC for the gage would also be 13. For more details concerning STC selection and other issues encountered when the test temperature changes, see Micro-Measurements’ Tech Note TN-504-1: Strain Gage Thermal Output and Gage Factor Variation with Temperature. For more detailed gage selection help, see Micro-Measurements’ Tech Note TN-505-4: Strain Gage Selection: Criteria, Procedures, Recommendations.
Finally, the leadwire system must be considered. Teflon insulated wires tend
s to have fewer pin holes than vinyl, so it is preferred. Getting the leadwires out of the tank will require liquid-proof bulkhead connectors rated for the anticipated test pressure.
By taking these considerations into account, you can properly plan ahead and avoid excessive “pressure” from your boss by having your tank pressure test go off without a hitch.