A pressure switch, as the name suggests, measures the pressure of gas or air in real time and outputs a switch signal to the system when the pressure exceeds the high or low limit, so that the system can take appropriate action.

This small pressure switch may seem simple in structure, but it actually has many functions that are often unknown or overlooked. Let's take a look at the introduction below.

Knowledge Point 1:

Pressure switches do not inherently distinguish between high pressure and low pressure.

Generally, we require the installation of two pressure switches on the main gas pipeline, called a high-pressure switch and a low-pressure switch, to provide high and low pressure protection for the gas. A pressure switch is also installed on the combustion air duct as an underpressure alarm.

Actually, pressure switches don't inherently have a high or low pressure distinction; they simply have different ranges. Generally, we call a small-range switch used for low-pressure alarms (or low-pressure cutoff) a low-pressure switch, and a large-range switch used for high-pressure alarms (or high-pressure cutoff) a high-pressure switch.

However, this difference in pressure is not absolute, but relative. The same pressure switch can function as both a high-pressure switch and a low-pressure switch under different operating conditions.

At this point, you might be getting impatient. You're saying, "Why do you need to explain something so simple? You're just wasting your breath."

Okay, everyone, please read on.

Knowledge Point 2:

The pressure switch can also measure negative pressure and differential pressure.

Pressure switches from different brands may vary slightly in structure and function, but pressure switches from mainstream brands on the market can simultaneously measure positive pressure, negative pressure, and differential pressure.

For example, in the following case: when measuring positive pressure, only 1 or 2 needs to be connected; if you want to measure negative pressure, connect 3 or 4. Therefore, to measure differential pressure, connect 1 and 3 (or 2 and 4). For so many years, it has been used as a positive pressure switch. Those of you who have never used it for negative or differential pressure, please stand in the corner for an hour...

Haha, just kidding. It's not really anyone's fault, since measuring negative and differential pressure isn't very common. But we must be aware of this function. If you ever need to measure negative or differential pressure on-site, stay calm and don't rush to buy a negative pressure switch or differential pressure switch. That's what it means to be prepared.

Knowledge Point 3:

When high-voltage and low-voltage switches are used for safety interlocking, the wiring is reversed.

The safety interlock mentioned here refers to a hard interlock directly connected in series with the power supply line of the main shut-off valve. This means that when the gas pressure is too low or too high, the gas supply is directly cut off, thus protecting the combustion system.

How do you do that? Some might say, why not just connect the pressure switch contacts in series?

So the question is: how to connect them in series? Which contact should be used? Normally open or normally closed?

As shown in the diagram above, each pressure switch has a set of contacts: NO represents normally open, NC represents normally closed, and COM represents the common terminal.

When there is no airflow in the pipeline, the contacts of both low-pressure and high-pressure switches remain unchanged. However, once airflow is introduced into the pipeline, under normal operating pressure, the contacts of the low-pressure switch change: normally open switches close, and normally closed switches open.

Therefore, to connect the shut-off valve circuit in series with the two pressure switches, the correct answer should be: connect the low-pressure switch to the normally open contact and the high-pressure switch to the normally closed contact.

Knowledge Point 4:

Two pressure switches can be installed together.

The common installation method involves running two thin pipes from the mains and then installing the low-voltage switch and high-voltage switch respectively. This is perfectly acceptable, but it takes up some space. In areas with limited installation space, the following connection method is the correct approach.

Specifically, the method for connecting the pressure switch itself is as follows: Remove the plugs on the positive pressure ports (port 2) on the sides of the two switches, connect the two ports together to make the two positive pressure chambers connected, then plug the positive pressure port (port 1) of one of them, and connect only the positive pressure port of the other switch to the pipeline.