Pressure switch deadband isn’t an unknown, but neither is it a known value when you take a new switch out of its shipping box.
Experienced users will tell you that three identical units of the same pressure switch can have three different deadbands. All three switches will trip at the same pressure setpoint, but reset at different points. To complicate matters, the deadband reset point changes with the pressure setpoint. And, they’ll also say, there’s no way to know, out of the box, what the deadband is without applying pressure and checking.
Who cares about deadband? Not everyone. The exact deadband reset point is critical in some applications, and inconsequential in others.
Why is getting a known, repeatable deadband such a challenge with conventional electromechanical pressure switches, and is there anything to be done about it?
First, what is deadband?
Some processes require a pressure switch to trip at a setpoint and then to reset at a pressure so many PSI (Kpa) away from the setpoint. The difference between the set and reset points is the deadband (also called hysteresis).
Deadband is essential to prevent the microswitch from chattering when the pressure remains right at setpoint, so all pressure switches have some deadband.
Every pressure switch (depending on sensor and range) has a deadband specification, defined by the manufacturer as the range over which any particular switch of that model is considered ‘in spec’.
For example, take the United Electric H100-702 with a setpoint range of 3 to 100 psi. Its deadband covers a range of 1.0 psi to 4.0 psi. That means any given switch might have a deadband of 1.0 psi all the way up to 4.0 psi. It’s likely to be around the midpoint of the range, 2.5 psi, but there’s no telling until the switch is tested.
Manufacturers give a deadband range instead of an exact point to compensate for the fact that manufacturing tolerances of the mechanical components combine. The best they can offer reliably is that the deadband falls somewhere within a range. Statistically, deadbands for a given model are distributed on a Gaussian curve, so most are somewhere around the center deadband value, but on occasion one falls at the edge. That’s why three pressure switches of the same model can each have a different deadband.
There is an option for adjustable deadband with an independent deadband adjustment on some series, as shown in the tables below.
But making the adjustment is, at best, an iterative and very time-challenging process. The set and reset points are interactive, so getting an exact deadband value requires setting and checking both the setpoint and reset points several times over.
UE’s adjustable deadband option (1519/1520) allows for tweaking the static deadband for a given switch. But even with adjustable deadband, there’s no guarantee that adjustment be made across the entire static deadband range.
So, what do people who need a known and repeatable deadband do? Is there any solution for getting a specific deadband response from a pressure switch?
That’s where an electronic pressure switch, like the UE One Series, makes sense. The deadband is both viewed and set up with the keypad and display.
There’s no need to apply pressure (wet calibration), or to go back and forth to tweak and re-adjust. You just enter the values.
To learn more about the UE One Series, visit the Lesman website: http://bit.ly/UEOneSeries