Showing posts with label pressure switch. Show all posts
Showing posts with label pressure switch. Show all posts

Wednesday, May 9, 2018

Ashcroft Materials Compatibility and Corrosion Guide

Ashcroft products
Ashcroft products.
Below is a very good materials compatibility and corrosion guide courtesy of Ashcroft.

The reference is intended to serve solely as a general guide in the recommendation of materials for corrosive services and must be regarded as indicative only and not as any guarantee for a specific service.  There are many conditions which cannot be covered by a simple tabulation such as this, which is based on uncontaminated chemicals, not mixtures.

Many of the chemicals listed are dangerous or toxic.  No material recommendation should be made when there is insufficient information, a high degree of risk, or an extremely dangerous chemical.  The end user is responsible for testing materials in his own application, or for securing the services of a qualified engineer to recommend materials.

The end user is responsible for the choice of product(s) in his own application, based upon his own determination of the materials, chemical, and corrosion factors involved. THIS GUIDE AND ITS CONTENT ARE PROVIDED ON AN “AS IS" BASIS WITHOUT WARRANTY OF ANY KIND.

You can refer to the embedded document below, or you can download your Ashcroft Corrosion Guide PDF from this link.

Monday, May 8, 2017

Industrial Pressure Switches

Industrial Pressure Switch
Industrial Pressure Switch (Ashcroft)
A pressure switch is a device that detects the presence of fluid pressure. Pressure switches use a variety of sensing elements such as diaphragms, bellows, bourdon tubes, or pistons. The movement of these sensors, caused by pressure fluctuation, is transferred to a set of electrical contacts to open or close a circuit.

Normal status of a switch is the resting state with stimulation. A pressure switch will be in its “normal” status when it senses low or minimum pressure. For a pressure switch, “normal” status is any fluid pressure below the trip threshold of the switch.

One of the earliest and most common designs of pressure switch was the bourdon tube pressure sensor with mercury switch. When pressure is applied, the bourdon tube flex's enough to tilt the glass bulb of the mercury switch so that the mercury flows over the electrical contacts, thus completing the circuit. the glass bulb tilts far enough to cause the mercury to fall against a pair of electrodes, thus completing an electrical circuit. Many of these pressure switches were sold on steam boilers. While they became a de facto standard, they were sensitive to vibration and breakage of the mercury bulb.
Pressure Switch Symbols
Pressure Switch Symbols

Pressure switches using micro type electrical switches and force-balanced pressure sensors is another common design.  The force provided by the pressure-sensing element against a mechanical spring is balanced until one overcomes the other. The tension on the spring may be adjusted to set the tripping point, thus providing an adjustable setpoint.

One of the criteria of any pressure switch is the deadband or (reset pressure differential). This setting determines the amount of pressure change required to re-set the switch to its normal state after it has tripped.  The “differential” pressure of a pressure switch should not to be confused with differential pressure switch, which actually measures the difference in pressure between two separate pressure ports.

When selecting pressure switches you must consider the electrical requirements (volts, amps, AC or DC), the area classification (hazardous, non-hazardous, general purpose, water-tight), pressure sensing range, body materials that will be exposed to ambient contaminants, and wetted materials (parts that are exposed to the process media).