Machine Design

Gages and Meters

Any fluid system may need to measure pressure, flow, or fluid level. Typically, measurements can be made with dozens of different types of gages connected directly to the system; alternatively, special sensors on the equipment can generate air or electrical signals that are displayed remotely on electrical meters or gages. If switches are necessary to initiate system action, they can be readily obtained; pressure and temperature switches are typically gaging elements with a snap-action or solid-state switch included.

Fluid level in the reservoir can be monitored inexpensively. The simplest level-monitoring devices are dipsticks and sight gages. Dipsticks are most common on mobile systems; sight gages, on industrial systems. Together, these two devices take care of level-monitoring needs of most fluid-power systems. Where fluid level is critical, or where the level is unlikely to be observed periodically by the operator, low-level alarms actuated by float switches should be used, sometimes in conjunction with remote-reading gages. If potential damage from loss of fluid could be costly, the alarm circuit can be wired to shut down the system.

An alternative to the analog electromechanical system consists of a microprocessor (MPU), an analog-to-digital converter, a piezoresistive pressure sensor, and an operational amplifier. In this system, the pressure sensor generates a voltage in response to the pressure applied to it by the liquid in the tank. The pressure is a function of the liquid level in the tank. The sensor signal is converted from analog form into a digital signal by the analog-to-digital (a/d) converter. Most a/d converters require the millivolt sensor output to be amplified. For this reason, an operational amplifier (op amp) is included in the system.

The digital signal that the MPU receives from the a/d converter is manipulated so that it can be compared with data in lookup tables, which are stored in ROM. This comparison is used to determine the volume of liquid in the tank. Measurement accuracy depends largely on the number of levels chosen to define the tank and system calibration. The MPU then drives a display, either digital or analog, to show the level of liquid in the tank.

Temperature monitoring is often omitted from mobile systems, although some systems have remote-reading thermometers with a readout on the operator's control panel. Industrial systems typically have a thermometer installed as part of the sight-level gage or as a probe in the suction or return line. High and low-temperature alarms operated by thermostats are often used and may automatically shut down the system. A word of warning: A thermometer installed in a sight-level gage does not indicate maximum system temperature. It indicates only the temperature of the fluid adjacent to that wall of the reservoir. Maximum system temperature can easily be as much as 130°F higher than that measured in the reservoir. The advantage of measuring reservoir temperature is that it indicates whether or not the system is operating normally -- a substantial benefit considering that the thermometer typically costs less than a dollar.

Manometers are the simplest of all pressure gages. They permit readings of both absolute and differential pressure, with excellent accuracy. Dynamic response is poor, but static response is excellent. In basic form, manometers consist of a U-shaped tube about half-full of liquid. Pressure to be measured is fed into one tube, while the other (the reference tube) is either left open to the atmosphere or connected to a differential source. The height difference between the fluid legs is proportional to the applied pressure.

Bourdon-tube gages are the most commonly used of all pressure gages. They are used almost exclusively for measurements in industrial hydraulic, and pneumatic systems, where pressures above 15 psi are expected. Accuracy of Bourdon gages is good to excellent, dynamic response is fair, and static response is excellent. The main working element in the gage is a tube (a Bourdon tube) shaped like a letter "C." As the pressurized fluid enters this tube, it tends to straighten it, moving the tube tip. The movement displaces a connecting linkage that actuates a pointer on the gage face.

Differential-pressure gages are available in Bourdon-tube mechanisms. Typically, they have two independent measuring chambers, each connected to a different pressure source. However, they have only one pointer, which shows the pressure difference between the two gages. The major types of Bourdon gages, and the qualities recommended for specific jobs, are:

  • Commercial gages are designed for low unit cost. They are usually less expensive to replace than repair and they have only modest accuracy.
  • Industrial gages are widely used for plant services such as steam, oil, and waterline pressure, and on equipment designed for installation in industrial plants, including commercial hydraulic, and pneumatic systems. Industrial gages have rugged cast cases and higher accuracy than commercial gages.
  • Process gages are used in equipment such as autoclaves, pressure vessels, piping, and oil refineries; they have cast metal or plastic cases and are highly accurate.
  • Test gages are the super-accurate specials used for calibrating other gages. Normal test gages are used for most such jobs; laboratory test gages are used for maximum accuracy and readability.

Diaphragm gages are based on a flexible diaphragm that is distended by pressurized fluid. As the diaphragm moves, it actuates a direct mechanical linkage attached to an indicator. Because of the way the diaphragm expands, these gages have a linear pressure-displacement relationship only over a comparatively narrow pressure range. They are normally used to measure pressures less than 15 psi.

Diaphragm gages can read both absolute and differential pressures with fair accuracy, fair dynamic response, and excellent static response. Diaphragm gages should not be confused with diaphragm-protected gages. In these gages, the diaphragm does not actuate the pointer, but merely isolates the gage from the fluid being measured. Diaphragm-protected gages are usually filled with a temperature-stable fluid.

Bellows gages work like diaphragms, but have much greater extension to cope with wider pressure ranges. They are most useful over maximum pressure ranges from 0.5 to 150 psi. In this range, they can be used to measure both absolute and differential pressures with fair to good accuracy. Dynamic response is only fair, but static response is excellent. Bellows gages are troubled somewhat more by hysteresis and zero shift than other gages.

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