Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N11/00—Investigating flow properties of materials, e.g. viscosity, plasticity; Analysing materials by determining flow properties
  • G01N11/10—Investigating flow properties of materials, e.g. viscosity, plasticity; Analysing materials by determining flow properties by moving a body within the material
  • G01N11/16—Investigating flow properties of materials, e.g. viscosity, plasticity; Analysing materials by determining flow properties by moving a body within the material by measuring damping effect upon oscillatory body

Definitions

• the invention relates to a simple and effective method of determining the viscosity of a fluid and in particular to a such a method using piezoelectric elements.
• GB 2342445 describes the use of a piezoelectric bimorph to measure viscosity.
• the temperature of the fluid is also measured and used, in conjunction with the output from the bimorph, to calculate the viscosity of the fluid.
• the problem with this method is that two parameters have to be measured to a certain degree of accuracy in order to obtain the viscosity measurement.
• the inventor has overcome these problems by formulating an effective method to determine viscosity by mean of a simple relationship.
• This simple and inexpensive device allows the use of such a fluid condition monitoring sensor in large volume applications and in harsh environments, neither of which are possible with existing systems.
• the relationship also has the advantage that the measurements are not dependent on temperature up to around 150°C.
• the invention comprises a method of determining the viscosity of the fluid by the steps of: a) determining a linear relationship between viscosity and a constant ⁇ , b) inserting a piezoelectric element into said fluid and exciting said element to cause it to deflect; c) releasing the piezoelectric element and measuring the voltage produced, d) determining the constant ⁇ from the exponential decay of said voltage where
• V V 0 e- n P'
• V being voltage produced after n oscillations after a point in time when the voltage is V 0 from ⁇ and step a) determining viscosity of said fluid.
• Figure 1 shows a piezoelectric element suitable for use in the method of measuring viscosity according to the invention.
• Figure 2 shows the damped oscillation of a released excited piezoelectric viscometer.
• Figure 3 shows how energy dissipation is linearly related to viscosity.
• Figure 1 shows a representation of a bimorph piezoelectric viscometer, cantilevered onto support 2.
• the bimorph comprises two individual piezoelectric elements 3 and 4 sandwiched together. This ceramics are chemically bonded by sintering them together with a metal-loaded ink. This allows them to be used at high temperatures where adhesives would be non-linear or fail..
• the elements are electrically polarised in opposite directions. As is clear to the skilled person electric excitation will cause the bimorph to bend. Such a configuration allows large bending amplitudes and has low resonant frequencies.
• a pulse signal is used to deflect the bimorph and using a charge or voltage amplifier in series with the bimorph, a signal from the bimorph as it decays.
• a strained piezoelectric element such as that of figure 1
• it will undergo damped harmonic motion as shown in figure 2.
• the damping motion can be described by the intrinsic damping associated with the bimorph and the external force of the fluid in which it is immersed. In most situations the extrinsic contribution will dominate and allow the determination of the viscoelastic properties of the fluid to be determined from an analysis of oscillatory decay of the bimorph. The more viscous the fluid, the more damping and the quicker the decay curve flattens out. By measuring the decay constant the viscosity of the fluid can be determined.
• V is the voltage at n cycles thereafter, and ⁇ is linearly related to viscosity of the fluid as shown in figure 3.
• V may be suitably measured by measuring the voltage peaks.
• V 0 e ⁇ ' describes the envelope 5 of the signal (fig 2).
• ⁇ is linearly related to viscosity of a fluid and the viscometer may be calibrated by measuring ⁇ at two points, e.g. in air, water or with a standard oil of known viscosity.
• Figure 3 shows how energy dissipation is linearly related to viscosity.

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Citations (4)

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• 2000
  • 2000-02-19 GB GB0003804A patent/GB2359368A/en not_active Withdrawn
• 2001
  • 2001-02-15 WO PCT/GB2001/000599 patent/WO2001061312A1/en active Application Filing
  • 2001-02-15 AU AU2001232104A patent/AU2001232104A1/en not_active Abandoned

Patent Citations (4)

Non-Patent Citations (3)

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