Chapter 18: Metrology Flashcards
What is acceptance testing
first check that a device meets the manufacturers stated specification
Subsequent measurements that check the device still meets the specification is referred to as _____ or _____
performance monitoring or quality assurance
A third category of use of metrology is described as research which is
technological development, measurements are necessary to advance the science behind ultrasound technologies, repeatability, comparison with numerical models to check assumptions embedded in them actually reflect what happens in the real world
Hydrophones are usually made of what material?
PVDF (pzt is used in diagnosis)
Types of hydrophone?
Needle hydrophone Membrane hydrophone Lipstick hydrophone Fibre-optic hydrophone
Why are lipstick-shaped hydrophones curved at the tip?
to improve the frequency response by reducing the likelihood of interface waves propagating across it
Any hydrophone used for metrology will have a calibration certificate which is what?
Any hydrophone used for metrology will have a calibration certificate which is a chart giving the conversion factor from Volts to Pascals.
What three things is a hydrophone connected to for its setup?
an amplifier, a DAC, and perhaps an oscilloscope or a data acquisition card.
What two things does the calibration have to correct for when calibrating a hydrophone?
- The frequency-dependent amplitude scaling 2. The frequency-dependent phase shift.
Official hydrophones are obtained from?
The national measurement institutes (NMIs).
How does the NMI calibrate it?
They compare the hydrophone with a ‘reference’ hydrophone. This is done by placing them one after the other, in a known acoustic field. This procedure is known as a ‘secondary calibration’. (the primary is calibration of the ref elec).
There are two types of acoustic sources used in secondary calibrations. What are they?
- One is a large element planar transducer that is placed sufficiently far from the hydrophone to ensure the wavefield forms a shock by the time it reaches it. (advantages include far enough for multiple frequencies to develop, and a plane wave can be formed ) 2. A bowl transducer - to ensure the acoustic pressures are high enough to be measurable. (higher uncertainty because its more difficult to get the focus to be located at the hydrophone)
What is the primary calibration standard for the reference hydrophone?
The displacement or velocity of the pellicle is measured using an optical interferometer, a laser vibrometer. This means that, via the reference hydrophone, there is a traceable link from a direct physical measurement of the magnitude of the acoustic fluctuation to the calibration of users’ hydrophones.
How are the two acoustic pulses compared? Which defined measures are commonly used
p+ positive peak pressure p- peak negative pressure pi pressure square integral td pulse duration - 1.25 x time taken for cumulative integral to grow from 0.1pi to 0.9pi
What are the measures relating to acoustic intensity?
- Itp temporal peak: the maximum of the intensity pulse
- Ipa pulse average: this is the average over pulse duration td
- Ita temporal average: this is the average over an integer number of pulse repetitions. This is therefore lower than Ipa as it takes into account the time between pulses.
There is another spatial measure that is sometimes useful: an average of the Ita average over the beam. However, to give a fair measure of the average signal in the beam, it is necessary to say which points in the field are part of the beam. For this reason, the spatial average temporal average intensity is defined as
What is derating?
a way of standardizing measurements of a hydrophone over different materials by artificially adding the effect of some absorption to the measurements.
