Manganin Pressure Gages
QUESTION: What are manganin gages?
ANSWER: Manganin gages are composed of a thin metal foil grid laminated to a plastic insulator (backing). In many ways they resemble strain gages. In use, however, their function is not to measure strain but to measure applied pressure. With thin, strain gage like construction, manganin pressure gages can be installed in very confined areas where other pressure measuring devices would not be usable.
QUESTION: What is manganin alloy?
ANSWER: Manganin is composed of approximately 84% copper, 12% manganese, and 4% nickel. It has long been used in electrical instruments because of its favorable mechanical properties and the low effect of temperature upon its resistance. It is valued as a high pressure sensor since its electrical resistivity is a linear and repeatable function of applied pressure. Being without mechanical linkages, it can also respond to explosive pressure increases.
QUESTION: How sensitive is the manganin gage to pressure changes?
ANSWER: The nominal pressure sensitivity for manganin gages is 0.0027 ohms/ohm per kilobar. For example, a change in pressure of 1 kilobar (14,500 psi or 100 kN/m2) on a 50.0 ohm manganin gage would result in a change in resistance of approximately 0.135 ohms. Clearly, this low sensitivity does not allow for very good resolution below a 1k bar pressure level.
QUESTION: How effectively can the manganin gage measure static fluid pressures like in hydraulic cylinders or pressure tanks?
ANSWER: Usually not very well. The resistance change in the manganin grid is quite low unless pressure is extremely high. With low sensitivities, resolution of changes in pressure are difficult to measure. In some applications of high pulse pressures, like would occur in diesel fuel injection nozzles, manganin gages can be used. This type of use is complicated, however, by the problems of routing leadwire from high pressure environments. In addition, the use of an electrically non-conductive and inert fluid is required since protecting the gage from high pressure conductive or corrosive fluids is not usually possible.
QUESTION: What is an ideal application for a manganin pressure gage?
ANSWER: The area in which the manganin gage has found its most useful application is the measurement of propagating internal stress waves. These propagating stress (pressure) waves are the result of impact on or explosive blast adjacent to a test object. In these applications the gage is usually mounted on the impacted surface or internal to the test object either by laminating or by adhesive bonding.
With the proper instrumentation and placement of gages, both the magnitude of the stress (pressure) and the speed at which the stress wave is traveling can be measured. To prevent the gage backing material and bonding adhesive from impeding the normal propagation of the stress wave, they are selected to have mechanical properties similar to the material being tested. These material properties are determined from Hugoniot data relating to shock wave propagation.
The information the manganin gage yields can be useful to designers of structures that undergo shock loading resulting from impact or explosive blast. Pressures as high as 400 kilobar have been measured with manganin gages. Consequently they have found wide usage in testing of military ballistics and their targets.
QUESTION: What are the requirements for the application of manganin grids?
ANSWER: Installation of manganin grids is usually accomplished with epoxy adhesives. It is important when bonding and overcoating the gages all voids be carefully filled. This includes the area between the actual grid strands of the manganin gage. Cleanliness is essential as particles of dirt will upset the uniform coating of the grid. Adhesive should be forced into any voids and bubbles de-aired before cure.
QUESTION: What precautions are necessary when selecting and installing leadwire between the manganin grid and the instrumentation?
ANSWER: In its normal application the manganin gage is subjected to very high shock forces. It is, therefore, necessary to adequately protect the leadwire connections to the grid. This is usually accomplished by removing the actual lead attachment point from the shock environment; a primary reason for supplying manganin gages with long tabs. Copper plating of the tabs is available to lower the tab resistance and its subsequent affect on gage sensitivity.
Fast rise time pressure measurements produce signals that approach radio frequencies. Shielded 50 ohm coaxial cables are therefore recommended. The gage resistance has been specifically designed for impedance matching with 50 ohm coaxial cable.
QUESTION: What instruments are used to measure manganin gages?
ANSWER: Due to their low sensitivity and usual high frequency applications, manganin gages normally require a pulsed power supply and high speed recording equipment. Pulsed excitation voltage is timed to be applied just before the pressure pulse is experienced by the gage and removed just after the pressure pulse has passed the grid. Since the output from the manganin gage is directly proportional to the applied voltage input, a pulsed high excitation voltage results in higher circuit output without the normal detrimental effect of gage self-heating. In this way the power supplies can be adjusted to apply many times the voltage allowed for continuous gage excitation. Pulsed excitation has been used as high as 300 volts.