Level measurements head devices

Head Devices A variety of devices utilize hydrostatic head as a measure of level. As in the case of displacer devices, accurate level measurement by hydrostatic head requires an accurate knowledge of the densities of both heavier-phase and lighter-phase fluids. The majority of this class of systems utilize standard-pressure and differential-pressure measuring devices. 

Liquid level measuring devices are classified into two groups (a) direct method, and (b) inferred method. An example of the direct method is the dipstick in your car which measures the height of the oil in the oil pan. An example of the inferred method is a pressure gauge at the bottom of a tank which measures the hydrostatic head pressure from the height of the liquid. 

Alternatives to the use of a mouse for computer input that are often used by persons with disabilities include trackballs, a head-controlled mouse, a continuous joystick, eye pointing, and flie use of the arrow keys on the numeric keypad (called mouse keys, see Table 30.2.).t Head control for mouse emulation employs an infrared system that detects a reflected beam to measure head position relative to a fixed reference point for the cursor (the center of the screen). As the user moves his or her head away from this point in any direction, the cursor is moved on the screen. Commercial systems I use a wireless approach in which a reflective dot is place on the user s forehead and serves as the reflective target. This allows the user to move around more freely. These head-controlled systems are intended for individuals who have limited arm and hand control and who can accurately control head movement. For example, persons with high-level spinal cord injuries who cannot use any limb movement often find these head pointers to be rapid and easy to use. On the other hand, individuals who have random head movement (e.g., due to cerebral palsy) or who do not have trunk alignment with the vertical axis of the video screen because of poor sitting posture often have significantly more trouble using this type of input device. 

Pressure/head devices such as the DP cell are the most popular of all level measurement devices. The DP cell can often be used where manometers are impracticable and floats would cause problems. The DP cell requires a constant product density for accurate measurement of level, or a way of compensating for density fluctuations. Figure 2.8 demonstrates a typical set-up for level measurement using a Rosemoimt Model 3095 level controller, which is essentially a combined DP cell and proportional controller. 

Research performed by Charles DiMarzio s students [29] at Northeastern University is on the study of blood oxygenation. For the first application, a fiber optic device has been built to measure blood oxygen in the brain. A NIR beam is shone on the patient s head at one point via a fiber optic probe, and a second probe is used to collect the energy from a second point. This technique is expected to be useful in brain surgery in newborns. The other application for a noninvasive NIR probe is for use in measuring oxygen levels in the skin. This second device would be useful for patients with bums, skin ulcers, and other skin problems. 

Liquid-Column Manometers The height, or head, p = phg/g to which a fluid rises in an open vertical tube attached to an apparatus containing a liquid is a direct measure of the pressure at the point of attachment ancf is frequently used to show the level of liquids in tanks and vessels. This same principle can be applied with U-tube gauges (Fig. 10-la) and equivalent devices (such as that shown in Fig. 

In many instances in process engineering and in agriculture, liquids are flowing in open channels rather than closed conduits. To measure the flow rates, weir devices are often used. A weir is a dam over which the liquid flows. The two main types of weirs are the rectangular weir and the triangular weir shown in Fig. 3.2-5. The liquid flows over the weir and the height /iq (weir head) in m is measured above the flat base or the notch as shown. This head should be measured at a distance of about 3/iq m upstream of the weir by a level or float gage. 

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