Pressure Gauges and Differential Pressure Gauges: Types and Working Principles

Pressure is one of the most important parameters of technological and thermal processes, characterizing both the internal energy of a medium and the condition of equipment. It is among the least inertial parameters, and the pressure of saturated steam is closely tied to temperature.

Pressure is a force that acts uniformly and normally across an area. The unit of pressure is the ratio of a unit of force to a unit of area. The accepted unit of pressure is the Pascal — the pressure exerted when a force of 1 N acts uniformly and normally on an area of 1 m².

How are pressure-measuring instruments classified?

Pressure-measuring instruments are classified by several criteria. By the type of measured quantity they divide into differential manometers and vacuum gauges. By operating principle they are liquid, deformation, hydrostatic, electrical, and float types. By accuracy they range from reference (standard) and laboratory instruments to technical ones.

A manometer is an instrument for measuring pressure. The choice among these classes depends on the magnitude of pressure to be measured, the required accuracy, and the operating conditions of the process.

How do liquid pressure instruments work?

The operating principle of liquid instruments is based on balancing the measured pressure, or pressure difference, against a hydrostatic column of liquid. Depending on the magnitude of the pressure or vacuum being measured, liquids such as water, alcohol, mercury, and mineral oils are used. The advantages of liquid instruments are their simplicity, low cost, high accuracy, reliability, and clear readability.

Figure 1 — Two-tube pressure instruments. Liquid instruments are either two-tube or single-tube. A two-tube pressure instrument (Fig. 1) consists of a U-shaped glass tube (1) with a diameter of 5–10 mm, filled to half its height with the working liquid (3), and a scale (2).

The system is in equilibrium when the hydrostatic pressure of the open limb balances the measured pressure P acting on the other limb of the instrument tube: P_a·S = P_б·S + H·S·ρ·g. Dividing the equation by S gives the expression for excess pressure: ∆P = (P_a − P_б)/S = ρgH, where S is the cross-sectional area of the tube in m²; P_a and P_б are the heights of the liquid columns in the two-tube manometer; H is the difference in liquid levels in the tubes, in mm; ρ is the density of the liquid in kg/m³; and g is the acceleration due to gravity in m/s².

Excess pressure is therefore proportional to the difference in the heights of the hydrostatic liquid columns in a two-tube manometer. The drawback of two-tube instruments is the doubled reading error from taking two scale readings.

What are deformation manometers and differential manometers?

Deformation differential manometers are the most widely used instruments for measuring pressure. They are simple and compact in construction, reliable in operation, and offer a wide measurement range — from 0.1 to 160 MPa — with sufficient accuracy. Their sensing elements are plain and corrugated membranes, bellows, and single-turn and multi-turn tubular springs. The operating principle of deformation instruments is based on the elastic deformation of these sensing elements under the influence of pressure or vacuum.

Figure 2 — Membrane differential manometer. The sensing element of this differential manometer (Fig. 2) is a membrane block consisting of two membrane boxes (1) and (3), connected by a channel (2) and placed in separate chambers. The membrane boxes are filled with distilled water. The higher pressure P₁ is fed to the lower chamber and the lower pressure P₂ to the upper one. A core (4) of a differential-transformer converter (5) is rigidly attached to the centre of the upper box membrane (3). The core sits inside a separating tube (6) made of stainless steel.

When the differential pressure ∆P = P₁ − P₂ is applied to the instrument, liquid from the lower membrane box (1) flows through channel (2) into the upper box (3). As a result, the upper box's volume increases, the upper membrane moves upward, and it displaces the plunger (4).

The differential-transformer converter (DTP) generates an electrical signal proportional to the pressure difference. The Ivano-Frankivsk production association "Promprylad" manufactures differential manometers of type DM 3583M with differential-transformer converters and DMT 3583M with current converters (0–5 mA), for pressure differences of 1.6; 2.5; 4; 6.3; 10; 16; 25 kPa; and 0.04; 0.063; 0.1; 0.16; 0.25; 0.4; 0.63 MPa. The accuracy classes of these instruments are 1 and 1.5.

Self-check questions

1. The concept of pressure and vacuum.
2. Units of pressure and the relationships between them.
3. Classification of methods and instruments for measuring pressure and vacuum.
4. Liquid pressure instruments: single-tube and two-tube.
5. Liquid differential manometers: construction, operating principle, types, and applications.
6. Membrane differential manometers: their characteristics, applications, construction, and operating principle.
7. Tubular-spring manometers: types, applications, construction, and operating principle.

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