Say whether the following statements are true or false

1. Indirect reading gages measure some property of the gas that changes with the density of the gas and usually produces an electric output.

2. The thermal conductivity increases from a nearly constant value above ~1 torr to essentially 0 at pressures below 10–2 torr.

3. As the pressure changes the thermal conductivity is stable.

4. The Pirani gage is perhaps the oldest indirect gage used today.

5. Thermistor gage can not measure the same pressure range as the thermocouple.

6. Modern thermistor gages use constant-temperature techniques.

7. Thermocouple gage is an expensive device.

8. The thermocouple gage can be operated in the constant-temperature mode.

9. Operation of the thermocouple gage in high partial pressures of organic molecules such as oils is recommended.

10. Sensitive electronics measure the current and are calibrated in pressure units.

Fill in the table

Using the table speak about different types of vacuum gages.

Unit 12

VOLUME MEASUREMENT

1. Memorise the spelling of the following words:

Formula, fluid, entirely, irregularly, clinician, diagnosis, tumor resection, muscle, plethysmography, acoustic, frequencies, suitable, throughout.

2. What are the units of measurement for volume? Discuss in a group.

Read and translate text A.

TEXT A

For simple geometric shapes, volume measurements can be performed analytically by measuring the dimensions of the object in question and using the appropriate formula for that shape.

Volume can also be measured by fluid displacement. The object whose volume is to be measured is placed in a container filled with fluid and the initial and final volumes are measured. The object’s volume is equal to the final volume minus the initial volume. This technique is especially useful for irregularly shaped objects.

Fluids can also be used to measure volume of cavities within objects by filling the cavity entirely with a fluid and then measuring the volume of the fluid after it is removed from the cavity.

Quantitative volume information can be of importance to the clinician in the diagnosis of a variety of diseases or abnormalities. It can also improve the understanding of the physiology of the patient. Information on volume may be used in various applications such as cardiac monitoring, diagnosis of prostate diseases, follow-up during organ transplantation, surgery for tumor resection, blood flow measurements, plastic surgery, follow-up of preterm infants, sports performance analysis, etc. The techniques can be invasive or noninvasive, and are based on either direct or indirect measurements. Each technique has its own advantages and disadvantages, and the application determines the selection of volume measurement method.

One of the earliest techniques to measure (changes of) body volume was Plethysmography , originally developed by Glisson (1622) and Swammerdam (1737) to demonstrate isovolumetric contraction of isolated muscle. The measuring technique consists of surrounding the organ or tissue with a rigid box filled with water or air. The displacement of the fluid or the change in air pressure indicates the volume

René G. Aarnink University Hospital Nijmegen Hessel Wijkstra changes of the tissue due to arterial in-flow. Two major types of plethysmography exist, and these can be distinguished by the technique used to measure the volume change. These are volume plethysmography (direct-measurement displacement plethysmography including water and air types), and electrical plethysmography (strain-gages, inductive and impedance plethysmographs). The physical condition in which the measurements should be performed determines the plethysmographic method chosen.

Advances in medical imaging have provided new possibilities for noninvasively extracting quantitatively useful diagnostic information. Images are constructed on a grid of small picture elements (pixels) that reflect the intensity of the image in the array occupied by the pixel. Most medical images represent a two-dimensional projection of a three-dimensional object.

Currently, the most commonly used medical imaging modalities are ultrasound, nuclear magnetic resonance imaging, X-rays and X-ray computer tomography.

Ultrasound imaging is based on the transmission and reflection of high-frequency acoustic waves. Waves whose frequencies lie well above the hearing range can be transmitted through biological tissue and will be reflected if they cross a boundary between media of different acoustic properties. These reflected signals can be reconverted to electrical signals and displayed to obtain a two-dimensional section.

Magnetic resonance imaging is based on the involvement of the interaction between magnetic moment (or spin) of nuclei and a magnetic field. The proton spins are excited by an external radio frequency signal and the return to an equilibrium distribution is used to construct cross-sectional images. The volume of a fluid-filled region can be calculated if a known quantity of indicator is added to the fluid and the concentration measured after it has been dispersed uniformly throughout the fluid. The selection of the indicator used to measure the volume depends on the application and can be based on temperature, color, or radioactivity. Finally, volume (changes) can also be performed directly using water-displacement volumetry , a sensitive but time-consuming method to measure the volume of an extremity.

Date: 2016-04-22; view: 897