Chapter 6: Pressure Measurement

Overview

Pressure is measured as a force per unit area. Pressure measurements are important not only for the monitoring and control of pressure itself but also for measuring other parameters, such as level and flow (through differential pressure). Pressure measurement is one of the most common measurements made in process control. It is also one of the simplest in terms of which measuring device to select. One of the key items to consider is the primary element (i.e., strain gage, Bourdon tube, spiral, etc.). Primary-element materials should be selected to provide sufficient immunity from the process fluids and at the same time the required measured accuracy under the process conditions they will encounter.

This chapter provides some of the basic knowledge plant personnel will require to select the correct pressure-measuring device. However, it is essential that the instrument selector take into consideration the users' experiences.

Pressure-measuring instruments are really pressure transducers that convert the pressure energy into a measurable mechanical or electrical energy. Pressure measurement is always made with respect to a reference point. There are basically three types of pressure-sensing configurations (see figure 6-1).

1. Gage pressure, where the reference is atmospheric pressure

2. Absolute pressure, where the reference is complete vacuum

3. Differential pressure, which represents the difference between two pressure levels (note that gage pressure is a differential pressure between a value and atmospheric pressure)

Figure 6-1: Absolute and gage pressure.

In certain cases, pressure devices must conform to specific requirements. For example, on pressures greater than 15 psig (103...