**Pressure and its measurement:**

Pressure is a prerequisite for circulation of liquids in heating or cooling systems, and it is created by pumps. The system pressure is positive (in terms of gauge pressure) in relation to the atmospheric pressure.

It is often expressed as a relative pressure with respect to atmospheric pressure (it also applies for the DMP 331 sensor), but it can also be expressed as an absolute pressure.

There are a number of units for measuring the pressure; the most commonly used are: Pa, kPa, bar and m. Pa, kPa, bar and m.

The ratios of these units are:

**bar and kPa**

1 bar = 100,000Pa = 100kPa (this value is approximately equal to the atmospheric pressure - 1000hPa)

Other units include lbf/in^{2}= psi (pounds per m^{2}) and mm of mercury (mmHG).

These units are specified as:

1psi = 6,895Pa

1mmHG = 133Pa

**m**

The m unit (the water column in meters) depends on the gravitational acceleration (g), which varies depending on the distance from the equator.

In standard usage, an approximate value of 1 m = 10kPa, which is equal to the gravitational acceleration of 10 m/s^{2}.

**Standard values of operating pressures:**

Public water main 200 ÷ 400 kPa

Central heating 150 ÷ 250 kPa

Solar circuit approx. 200 kPa (2 bars)

Pressure measurements can be utilized e.g. for monitoring of the heating system, where a lowering of pressure can indicate a leakage and an impending breakdown (some change in operating pressure of the heating system are normal - the pressure fluctuates with the temperature of the liquid).

The pressure in the heating system can only be monitored via a point level switch (although the precise pressure is not known, a decrease below a pre-set value can be measured) - see Chapter 11.9.1. Pressure can also be measured continuously, with the advantage of obtaining an instantaneous pressure value - see Chapter 11.9.2