Calibration Calibrating pressure switches

Pressure switches are common in the process industry, and various kinds of pressure switches are available. Like many instruments, pressure switches need to be calibrated to ensure their accuracy and reliability. Switches are a bit more difficult to calibrate than transmitters. The wrong kind of calibration can cause many errors in the calibration result. In this article, Beamex’s Heikki Laurila looks at how to properly calibrate pressure switches.

programmed to operate at a certain pressure. For example, it can be set so that when no pressure is connected (open to atmosphere) the switch is closed, but when pressure increases up to 10 psi, the switch opens. Again, when the pressure drops below 10 psi, the switch closes.


PRESSURE SWITCH TERMINOLOGY Let us first very briefly discuss the related terminology. Some switches have the switch terminals open when no pressure is connected, called normally-open (NO) or a closing switch. The opposite is normally-closed (NC) or opening switch. The selection depends what kind do of circuit you want to drive with the switch. What is “normally”? There is some debate about the definition of the normally open/closed switch. Most commonly it is defined as the state where the pressure switch output is when it's not connected to any pressure, i.e. it has no physical stimulation. Others may define the “normal” state as the state where the switch is during the normal operation of the process (un-tripped). A normally-open switch is open when no pressure is connected. When enough pressure is applied, the switch closes:

riefly stated, a pressure switch is an instrument that measures pressure and that has an electrical switch function

A switch will always have some deadband, which is the difference between the two operating points (opening and closing points). Deadband is required, because if a switch would open and close at the same point, it could start oscillating when the pressure is on that limit. Also, it could control the circuit on and off with a high frequency if there was no deadband. For example, a closing (NO) pressure switch may close at 10 psi pressure and open again at 9.5 psi pressure, so there is a 0.5 psi deadband. Some switches operate at rising pressure,

others with falling pressure. Sure, you always get one of the functions with rising and other with falling, but the primary desired function happens in one direction.

There are pressure switches that operate with different pressure types: gauge, absolute, differential or vacuum pressure. Some older switches are mechanical (or

A normally-closed switch is closed when no pressure is connected. When enough pressure is applied, the switch opens:

even pneumatic), so inside the switch the pressure is causing the switch to change its state. Most newer types are electronic or digital, so they measure the pressure and control the switch output accordingly. Many modern switches are programmable, so it is easy to set the desired operating points. While mechanical switches do not need any power supply, the electrical ones need to have one. When selecting the switch type, the state should be considered so that should the power supply fail, or a cable becomes loose, the switch status should remain safe. And in the case of a safety switch, it should be configured so that in case a cable comes loose, the alarm goes on. For example, if it is a normally-open (closing switch), you will not notice anything if the cable comes loose, the switch is still open, but it will not make the desired action when the switch closes. So all in all, you should design it to be Fail Safe. We also talk about dry and wet switches.

A dry switch has the connections being open or closed, so it is working like a mechanical switch. A wet switch has two different voltage values representing the two output states. The output of an electrical wet switch can

be a voltage signal with two levels, a current signal, or an open collector type signal. Sometimes the switch function can be also done in the control system, by measuring the current signal from a transmitter and programming the switch-like function to


control something based on the signal level. In practice, industrial switches often have

double switch contacts that can be programmed separately. This can be the normal Lo and Hi points, but also “Lo Lo” and “Hi Hi” points. While the Lo and Hi are the normal control points, the Lo Lo and Hi Hi are alarm limits that will control for more serious alarm activities.

SAFETY PRESSURE SWITCHES Safety switches are switches used in the safety instrumented systems (SIS), and these switches have certain safety classifications. Also, the calibration of these safety switches is regulated. A big difference with these switches is that these switches stay static most of the time without ever functioning. So, they do not toggle open and closed in normal usage, they are just waiting if the safety alarm level is met, and then they operate. As these switches very rarely operate, there

is a risk that they will get stuck and not work when they should. When calibrating, do not exercise these

safety switches prior to calibration, instead capture the very first point when the switch operates. It can happen that the first operation requires more pressure than the operations after a few exercises. Normal switches are typically exercised a few times before calibration, but that should not be done for the safety switches. In a safety switch, the operation point is critical,

but often the return point is not that relevant and may not even require to be calibrated.

HOW TO CALIBRATE PRESSURE SWITCHES Let us discuss how to calibrate pressure switches.

Preparations and safety

If the switch is installed in the process, it is very important to make sure it is isolated from the pressure line. You also need to make sure to disconnect any circuit that the switch is controlling - you do not want big valves to start opening/closing, or pumps to start operating, nor generate a safety alarm. Some switches may have mains voltage, or another dangerous voltage, across the switch terminals when they open, so make sure that it is isolated.

June 2020 Instrumentation Monthly

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74