Why is pressure switch deadband such a crapshoot?


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.

Both the setpoint and the deadband (DB) values are entered as digital numbers.  

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

, , , , , , , , , , , , , ,

  1. #1 by Mahmoud on November 6, 2012 - 7:38 am

    Greetings,

    We have issue with ACHCROFT pressure switch model: B4-50-B-XFS-XNH+50-200PM-04T-XCK, where set point is 85 Bar but reset point is differ ,not same for all switches.

    Trip happens before reset action.

    Thanking & waiting your advice.

    • #2 by danweise on November 6, 2012 - 11:32 am

      All I can do is to reiterate the theme – any mechanical pressure switch has a range of deadband and any particular pressure switch can fall anywhere within that range of deadband and still meet the manufacturer’s specs.

      For instance, the table in the blog above shows a deadband with a range of 1.0 psi to 4.0 psi for the United Electric H100-702 (with a setpoint range of 3 to 100 psi).

      If the deadband on a new switch (with fixed unadjustable deadband) is not within the published deadband range, then it is an issue the manufacturer should address because the product is outside of its published specs.

      If the deadband falls within the published deadband range, then the deadband is what it is on that particular switch.

      If the application requires a specific, settable, repeatable deadband, then an electonic pressure switch should be considered.

  2. #3 by John Trauger on March 20, 2012 - 4:06 pm

    I have a dead band issue with a Ashcroft B464S rated at 1000 psi on a steam header at our plant. I was doing a cal check and verified the switch opened at 650 psi as the previous cal indicated, but the reset which was supposed to be in the 10 psi range ended up at 500 psi.

    There appears to be an adjustment on the lever (not the large nut adjuster that is common with Ashcroft) but actually on the end of the switch lever but I did not want to mess with it until I had my ducks in a row.

    Any suggestions? I can’t find much on the product via Ashcroft website. Mostly list this switch “fixed dead band.”

    Thanks John

    • #4 by danstips on March 22, 2012 - 4:11 pm

      Per your model number, it appears you do have a fixed deadband switch. In the Ashcroft spec for that switch series, it’d have to be a B450 to have an adjustable deadband. Your B464 has dual general purpose SPDT switches. The deadband widens with multiple switches, because there’s more slop and more friction with dual switches than with a single switch. The stainless element introduces yet more deadband action.

      Still, a 10 PSI deadband is much more narrow than Ashcroft’s specs list on their published range table. Per spec, an Ashcroft series B464 dual general purpose switch in a 1000 PSI range with a stainless sensor element has a deadband range of 82-300 PSI.

      Your measured 150 PSI deadband is right smack in the middle of the deadband range, where it should be. The reported 10 PSI deadband was way low and off the map for even a Buna diaphragm single switch.

      My observation from your data is that the switch is performing exactly like Ashcroft says it will, and not all that differently than other brands I’m familiar with. The only switch I’m aware of that will do 10 PSI deadband on 1000 PSI is the UE One Series electronic switch. (http://bit.ly/UEOneSeries)

      But I have to wonder if you really want such a close deadband, because of the control response to the switch ‘chatter’ that is likely to happen with any appreciable amount of line pressure fluctuation.

  3. #5 by Rick Frauton on January 13, 2012 - 1:34 pm

    In October of 2009, United Electric Controls (UE) implemented a change to the firmware of the One Series electronic switch product line. The change was limited to and affects only the configurability of the Deadband setting. Prior to this change, it was possible to enter a value of zero (0) for the Deadband configuration. The change does not permit a zero to be entered. If attempted, the One Series display will show “ERR” and automatically replace the zero with the previous value.

    The reason for this change was made necessary by the definition of Deadband as it relates to a switch. The Set Point is where a switch will change state or trip (open if closed; close if open) as this value is reached at the sensor. The Deadband is where the switch will reset, returning to its normal (original) state. The One Series firmware algorithm adds or subtracts the Deadband value from the Set Point value to determine when to reset the switch. If the Deadband value is set to zero, the switch will not reset and will remain in the tripped position. Per this definition, a Deadband with a value of zero is undefined and invalid.

    Any value other than zero is acceptable by the current version of the firmware.

  4. #6 by Rick Frauton on January 13, 2012 - 1:29 pm

    Great article and blog. Just stumbled onto this. Nice work!

    • #7 by danstips on January 13, 2012 - 1:47 pm

      Thanks, Rick, for your compliment and for the additional info on the One Series.

Care to Comment?

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: