In order to understand the use of high pressure specialty gas cylinder regulators, it is important to first understand the basic principles of calibration gas regulator operation & design.
Controlling Calibration Gas Flow and Pressure
The simplest way to control the flow & pressure of gas is to restrict the size of an opening (the orifice) through which a pressurised gas travels. Because only so many molecules can fit through an opening at a certain point in time, the pressure & flow of a gas can be controlled by changing the size of the orifice (figure 1). In the simplest of terms, this is how a pressure regulator operates.
Another way to control the flow & pressure of a gas is to vary the gas source pressure through the same sized orifice. A higher pressure will increase the amount of gas moving through the orifice. This means if the source pressure drops, so does the pressure & flow on the outlet side. Or, in other words, as a cylinder empties, the gas pressure & flow will be reduced (figure 2). While this is true in theory, practice shows that high pressure regulators have an inverse relationship between cylinder pressure and outlet pressure.
The above diagrams represent a very simplistic way to look at pressure regulation. Let’s look at the workings of a specialty gas regulator and how each part connects together.
Specialty Gas Regulator Operation:
Gas pressure regulators are designed to offer more stable flow and pressure output using several key design features (figure 3). The key elements of regulator design are:
1) The Loading Mechanism – the pressure adjustment knob
2) The Sensing Element – the diaphragm
3) The Control Element – the poppet and seat, which control the orifice size
The Three Key Forces of High Pressure Gas Regulators
The loading mechanism, sensing element and control element features work around three key forces acting inside the regulator:
1. Inlet Pressure Force
The force of the pressure of the gas in the cylinder. This force attempts to shut the poppet, or close the orifice. (figure 4)
2. Outlet Pressure Force
The force of the pressure of the gas after it passes the orifice. This pressure applies to the sensing element, and also attempts to close the orifice. (figure 4)
3. Loading Mechanism Force
The force of the pressure adjustment knob applied to the sensing element. This force is applied in opposition to the first two forces, and attempts to open the orifice. (figure 4)
If the Loading Mechanism applies a force greater than the combined Inlet Pressure and Outlet Pressure forces, the diaphragm will deflect, and allow the poppet and seat to open, which allows gas to flow through the orifice (figure 5)
Single and Dual Stage Gas Regulation
Single and dual stage regulators perform the same task. The difference between the two is that a single stage regulator has a single set of control elements as described in the previous section, whereas a dual stage regulator has two sets of control elements.
We have seen previously that when gas flows through an orifice, the inlet pressure will have a direct effect on the outlet pressure. So, as a cylinder empties, the outlet pressure will drop. While this is true in theory, in practise, the opposite becomes true.
- The outlet pressure of a pressure regulator will increase as the cylinder empties
- As the cylinder empties, the inlet pressure force and the outlet pressure force (the forces trying to close the orifice) drop, while the Mechanical Loading Force (the force trying to open< the orifice) remains constant.
- The Mechanical Loading Force becomes stronger relative to the other two forces, and the orifice opens wider, allowing more gas to pass through.
In order to counter this effect, we use a dual stage regulator. The addition of a second set of control elements, or stage, reduces the pressure differential, so that the forces remain balanced, despite changes to the pressure in the cylinder. In the first stage, the Loading Mechanism Force is pre-set to a value above the desired final outlet pressure range (e.g 10BAR on a 0-8BAR regulator). In this way, the second stage is fed a much more stable Inlet Pressure Force and therefore also a more stable Outlet Pressure Force.
In practical terms, if a single stage regulator is used, periodic adjustments of the Mechanical Loading Pressure (the pressure adjustment knob) will be required to maintain a constant outlet pressure. When using a dual stage regulator, the pressure adjustment knob can be set to the desired value, which will be supplied until the cylinder is empty.
If a stable and constant outlet pressure is required, a dual stage regulator should be used. However, it is important to note that if the regulator is monitored, and periodic adjustments are practical, a single stage regulator can suit the same applications.
This provides some of the key elements of how a speciality gas regulator works and why single and dual stage regulators exist. Other key issues including regulator material, inlet connector selection, outlet pressure range and outlet connectors should all be considered in selecting a high pressure specialty gas regulator.