Thermal Lockout for Hermetic Pressure Switches

Thermal Lockout for Hermetic Pressure Switches Infographic

 

In hydraulic systems, cold start-ups often cause pressure switching errors that disappear once the system reaches its normal operating temperature. The culprit behind these false high pressure signals is increased fluid viscosity, which temporarily increases the differential pressure across the diaphragm or pressure sensing device. A thermal lockout mechanism based on a snap-acting, temperature-sensitive bimetallic disc can prevent this problem.

Check out our infographic to see how it works.

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Off-Road Vehicle Sees Success In Pressure Switch Signal Delay

If your hydraulic systems suffer from pressure spikes, your pressure switches may not work as expected. The spikes can trigger the switch prematurely, even though continuous pressures remain below the set point. The result is a hard-to-control hydraulic system plagued by false switching signals.

Fortunately, there’s a simple solution to these spike-induced switching errors: Just add a time delay to the switch. That’s exactly what we did in a recent application.

Off-Road Vehicle Experiences Pressure Transients. In an off-road vehicle, a pressure switch was used as part of a hydraulic monitoring system, which was designed to trigger a warning light in the cabin if it experienced sustained excess pressure. But driving over rough terrain was causing unintended pressure spikes, causing the annunciator light in the crew compartment to constantly switch on and off.

Our engineers incorporated a PC board into the pressure switch that was programmed to delay signal output based on certain adjustable parameters, including the amount of time—down to the millisecond—and whether the signal occurred on the rising or falling pressure. In this case, engineers set the time delay at 3.2 seconds. As a result, the warning light remained off unless a pressure change lasted longer than that.

742S

Pressure Switches For Tough Environments. In addition to integrating time delays, these pressure switches had to be ruggedized for use in an off-road application and are designed to withstand pressure spikes, leaks, temperature extremes, moisture, chemical exposure, vibration and shock loads. These ruggedized features include:

  • Proof pressures up to 12,000 psig
  • Maximum operating pressures up to 7,500 psig
  • Wide temperature range of -40 to 250°F
  • High overpressure capability
  • High shock and vibration survivability
  • Snap-action electrical switch tested to 1 million cycles
  • IP67 compliance

To learn more about pressure switch design, download our latest white paper.

Download the white paper


 

Video: Bellows for Temperature Switching

Temperature switching is another common use for metal bellows. The bellows are typically attached to a fluid-filled capillary. Changes in temperature cause the fluid to expand or contract, causing the bellows to move and actuate a switch, a gauge, or a valve. Bellows are found across many industries and are used extensively in thermostats, self acting temperature valves, and other industrial control systems. Watch our latest video to learn more.

 

New Diaphragm-Operated Pressure Switches For Tough Environments

With the release of our 745 and 785 Ruggedized Pressure Switches, we’re expanding our product line to help you better address low pressure applications that require high-sensitivity.

745 Pressure Switch. Our diaphragm-operated Model 745 Fixed Setting Pressure Switch is designed for low-pressure operations that require a compact form factor and high degree of sensitivity. Typical application examples include off-road or industrial OEM equipment, train braking systems, compactors, compressors and fuel systems.

785 Pressure Switch. Equipped with either a 2- or 3-pin Deutsch connector, this miniature, diaphragm-operated pressure switch is designed for high-sensitivity, low-pressure applications. Like the 745 model, this switch is IP67-compliant and has a NEMA Type 4 rating for a high degree of protection in rugged indoor and outdoor applications.

Both pressure switches are built to thrive in tough environments, providing pressures between 7 and 160 PSIG. They can withstand oil, fuel, water, dust, vibration and shock and feature a wide temperature range of -40 to +250°F.

745_785

Depending on the application, you can choose from several standard electrical and pressure port configurations. You can also customize your model with special items, including wire, cable, seal materials and weather-proof connectors.

For more information on the 745 Series, download the 745 Series Datasheet.

For more information on the 785 Series, download the 785 Series Datasheet.

 


Video: Bellows Sealing Applications

Of all the uses for metal bellows, the most common ones take advantage of their flexibility in coupling or sealing applications. Watch our latest video to learn why bellows work so well in sealing applications.

 

 

Bellows For Pressure Sensing

One of the most common uses for metal bellows is in pressure sensing devices, including all kinds of industrial pressure gauges and switches.  Watch our latest video to learn why bellows work so well in pressure sensing applications. 

 




Maximize Aneroid Performance

Altitude-sensing aneroids play a crucial role in flight systems. But keep in mind that the aneroid will only be as good as the metal bellows or diaphragm capsule that serves as the pressure sensing element. Here's a brief introduction to what you need to know about aneroid performance.

 




New 2-Pin Deutsch Pressure Switch For Tough Environments

Our 784 Series of ruggedized pressure switches can now address more applications than ever, thanks to the addition of a compact 2-pin Deutsch option. Other than the change in connector, the pressure switch offers all the same performance characteristics as the original 3-pin Deutsch version.

IP67-Compliant Pressure Switch. Whether 2- or 3-pin, the 784 pressure switch complies with IP67 standards for maximum environmental protection. This model thrives in tough environments containing oil, fuel, water, dust, vibration or shock.

 

784 2-Pin Deutsch

With a temperature range from -40 to 250°F, the 784 pressure switch comes with a piston, fixed pressure, integral pressure port snubbing and an 8 to 6,000 psig range. Some common applications include:

  • Off-road equipment
  • Industrial OEM equipment
  • Train braking systems
  • Packaging equipment
  • Compactors
  • Compressors

Other Features. This customizable pressure switch also features a snap-action electrical switch tested to one million cycles, gold contact switches for dry circuit applications, high overpressure capabilities and custom configurations. In addition, this model meets UL safety standards and NEMA type 4 outdoor applications and instances where water is present.

You can learn more about our design approach to ruggedized sensors in our latest white paper.

Download the white paper


Understanding Metal Bellows Manufacturing Methods

You can’t begin to talk about metal bellows performance without considering manufacturing methods. All key performance parameters depend on the way the bellows is formed or welded. Let’s take a look at two of the most common manufacturing methods: seamless hydroformed bellows and seam-welded formed bellows.

Seamless hydroformed bellows. As their name suggests, seamless metal bellows have no welded joints. Instead, they’re produced in a multi-step deep drawing process. A secondary hydroforming step forms the convolutions. The deep drawing and hydroforming processes can hold consistent dimensional tolerances. For example, wall thickness variation from a well-controlled deep drawing process is within ± 0.0001 inches.

In high-volume applications requiring a small package size and a precise spring rate, seamless bellows are by far the most popular choice. These applications include many kinds of appliance, HVAC and industrial controls and sensors.



Hydroformed Seamless Bellows

Seamless bellows do have some package size limitations. Their sweet spot for OD lies between 0.25 and 3 inches. Seamless bellows also have a higher initial tooling cost than seam-welded bellows. At production volumes, however, the tooling costs of bellows all but disappear.

Seam-welded formed bellows. Made from rolled, welded and formed sheet metal stock, seam-welding excels at producing large diameter bellows cost effectively. Diameters up to eight inches are not uncommon and would be cost prohibitive with seamless deep draw production methods. Seam-welded bellows can also be fabricated to any required length. And they have low initial tooling costs.

So seam-welded bellows are perfect for applications that need a large, cost-effective bellows, such as expansion joints or other connections that compensate for the movement of mating components.

Seam-welding bellows, however, do not lend themselves to applications that require a very precise spring rate. The reason is that their wall thickness tolerances are determined by the sheet metal stock, not the precision deep drawing process. Typical wall thickness variation for seam-welded models is ± 0.0005 inches, or about five times greater than a seamless bellows.

With wall thickness tolerances influencing spring rate and mean effective area, seam-welded bellows inherently have a less precise response to pressure and applied forces, which rules out many sensing and control applications.

 

Check out our white paper

 

 


Video: Ruggedized Switches Withstand Pressure Spikes

Ruggedized pressure switches have a stable switching response and a long lifecycle in the face of temperature extremes, moisture, chemicals, vibration, shock and pressure spikes—the biggest threat of all. Our high quality switches have a number of design features that help withstand these spikes. Check out this video now.

 

 




How To Make Pressure Switches That Last

Simple design principles for harsh operating conditions

Ruggedized pressure switches play a critical role in failure-intolerant applications. At Sigma-Netics, we define rugged as a stable switching response and long lifecycle in the face of seven failure modes—pressure spikes, leaks, temperature extremes, moisture, chemical exposure, vibration and shock loads.

Switches that can withstand these life-ending failure modes have three design principles in common. They tend to have a piston rather than diaphragm design. They have carefully designed pressure management features and seals. And they use materials and construction methods that mitigate chemical and moisture exposures, as well as thermal expansion mismatches.

Pressure Switch White Paper

In our latest white paper, we'll take a closer look at some design principles of ruggedized pressure switches.

 

Download the white paper

 

 


Designing With Metal Bellows

Metal bellows have long been a key component in demanding sensing and sealing applications. In fact, they’ve become so familiar that many engineers now think of bellows as generic items: Just specify a few key dimensions, pick a metal alloy and you’re good to go.

The reality, however, is that not all bellows technologies are created equal. Different manufacturing methods and material grades can have profound effects on how metal bellows will perform in terms of their stroke, pressure capabilities, spring rate and temperature response.

Manufacturing methods and materials will also determine how long your bellows will last in the field. The wrong bellows may work for a short time but not achieve your expected lifecycle.

Designing with Metal Bellows White Paper

Manufacturing methods and materials will also determine how long your bellows will last in the field. The wrong bellows may work for a short time but not achieve your expected lifecycle.

Our latest white paper takes a close look at the different aspects of metal bellows–including design considerations, manufacturing methods and materials–to help you pick the right one for your application.