Archive for category Pressure Switches

February 2: A day to look back

Yesterday and today, people all over Chicago and the Midwest were looking at pictures from last year. We had 22″ of snowfall in one day. The roads were closed. The airports were closed. Even the Lesman offices were closed. And today’s weather forecast? 45°… in Chicago… in February.

This morning in Pennsylvania, a groundhog named Phil came out, looked back, and saw… his shadow.

All this looking back made me a little reflective myself. I’ve been writing this blog for about 6 months now. So I thought I’d take a minute and recap the articles people keep coming back to read:

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Using digital switches for critical safety systems

The following is an excerpt from “Digital switches with self-diagnostics can improve results and ease the implementation of safety systems” by Rick Frauton of United Electric, originally published in the September 2011 issue Pumps & Systems magazine. Rick is the product manager for UE’s One Series line of electronic pressure and temperature switches, and has been working with customers to identify applications where these switches can improve application and plant safety. Thanks, Rick, for being our guest blogger this week.

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Years ago, most switches were blind mechanical devices actuated electromechanically or by pneumatics. They offered no indication of reliability, such as success or failure in response to a command. This lack of feedback was particularly worrisome in safety applications. The result could be catastrophic, should a malfunction occur in place of the proper response to a tripped pressure or temperature alarm.

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Should I use a chemical seal with my pressure instruments?

Diaphragm seal mounts directly into the process linke to reduce damage to the Bourdon tube.Pressure instruments in contact with the process can take a real beating. Process fluids can corrode the wetted parts and destroy the sensing element.  Media the solidifies can clog the pressure-sensing port.  Or, an installed instrument can affect the process by providing a spot for media remnants to remain after cleaning and purging. Specialized chemical seals deal with these pressure sensing issues.

Here’s a list of questions to ask about your application that will determine if you need a chemical seal with your gauge, switch, or transmitter.

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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?

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