Tuesday, February 28, 2017

Showing the Difference in Function Between High and Low Pressure Ports on a Differential Pressure Transmitter

The following video demonstrates the different responses of a differential pressure transmitter to both positive and negative pressures applied to its high and low pressure ports. The response of pressure and vacuum to the "high" port is opposite the effect of pressure and vacuum (respectively) applied to the "low" port.

Monday, February 27, 2017

Industrial Temperature Sensors and Indicators

Variety of electrical temperature sensors
Variety of electrical temperature sensors (Pyromation)
Temperature sensors are vital to everyday products and in the manufacturing of just about every product we use. Household ovens, refrigerators, and air conditioners all depend on temperature control to function properly and temperature control is essential in all process industries such as chemical, power, refinery, food processing, telecom, cement, fertilizer, pull & paper, plastics and petro-chemical.

Temperature sensors are devices which are used to temperature measurement of a medium (i.e. liquid, solid or gas). The sensor detects change in the temperature, and accordingly, change its physical or electrical property in a manner that can be measured. These sensors come in many different forms and are used for a wide variety of applications.

We as human’s simply sense temperature as hot or cold, but in process control, precise measurement of temperature is required in order to control a process efficiently. Accordingly, the correct temperature sensing device needs to be properly selected.

Types of Temperature Sensors

vapor actuated temperature indicator
Vapor actuated temperature indicator (REOTEMP)
Mechanical temperature sensors and indicators:

Devices that use the physical expansion and contraction of materials (like non-compressable fluids,
vapors, or differential metals) to mechanically open or close a set of contacts.

  • Examples of mechanical temperature sensors are bulb and capillary thermostats, thermometers, mechanical temperature switches, and bi-metallic thermostats.
Electrical temperature sensors:

These sensors undergo a measurable electrical change such as resistance, voltage, or current proportional to a given change in temperature.
Temperature Probe Assembly
Temperature Probe Assembly

  • Examples are thermocouples which generate a micro-voltage based upon a temperature differential between two dissimilar metals; RTDs that increase resistance as the sensing temperature increases; and thermistors,  which dramatically decrease resistance as temperature increases. 
Electrical temperature sensors can be housed in a wide variety of sheaths and/or wells for protection, and come in varying tolerances and accuracies to best suit their intended use.

For more information on temperature sensors, contact Power Specialties at (816) 353-6550 or visit http://www.powerspecialties.com.

Basics of Rotameters (Variable Area Flowmeters)

Rotameter by Yokogawa
Rotameter (Yokogawa)
A rotameter is a flow measuring device that belongs to a group of instruments called variable area flowmeters. For decades variable area flowmeters have become established in industrial measurement technology with an economical, mature measurement principle.

The variable area flowmeter is an instrument for metering the flowrate of liquids and gases in a pipeline. It includes a vertically oriented conical tube, whose diameter is larger at the top than at the bottom, through which the fluid flows upward and in which a vertically moving float is positioned. The height of the float in the tube increases as the flowrate increases in such a manner that the resistance to the flow is always balanced by the weight of the float and remains constant regardless of the flowrate. The height of the float in the tube is a measure of the flowrate.

The value of the flowrate can be read from a scale. Variable area flowmeters are the most cost effective solution for almost all applications involving the measurement of industrial process liquids, gases or steam. They meet the application requirements by featuring a wide range of design varieties and sizes.

Rotameters offer long life and high reproducibility and are excellent mechanical back-up meters because no external power supply is needed. For more information on rotameters (variable area flowmeters) visit http://www.powerspecialties.com or call Power Specialties at (816) 353-6550.

The video below does a great job introducing the various types and uses for rotameters.

Sunday, February 26, 2017

The Bubbler Method of Liquid Level Measurement

Bubbler Level Measurement
Bubbler Level Measurement
(courtesy of Yokogawa)
Remotely utilized measurements of tank liquid level are common throughout the fluid processing industries. Various means of inferential or direct measurement are available, each with its own set of performance, maintenance, and cost attributes that may make it the preferred choice for a particular application.

Determining liquid level using a hydrostatic pressure measurement is a simple, easy to implement strategy for delivering a continuous level signal to the process operating and control system.

There can be situations where mounting the measuring gear near the bottom of a tank or other vessel may be impractical or undesirable. A pressure transmitter mounted low on a tank may expose it to damage from plant traffic or other physical hazards. It is also possible that the tank may not have a suitable fitting located low enough to provide the needed measuring range. Having a potential leakage point at a fitting low on the tank may also be undesirable. Another, and certainly not final, scenario would be an application involving a corrosive liquid which must not come in contact with the pressure sensor.

The bubbler method of liquid level measurement employs an arrangement that overcomes all of the previously mentioned impediments. It can utilize connections at the top of the tank, above the liquid level. The way in which the method works will keep the pressure sensor out of contact with the process liquid, providing isolation from potential corrosive effects.

The apparatus for level measurement using the bubbler method employs a simple dip tube that extends from the pressure sensor or transmitter to nearly the bottom of the tank or vessel. A small amount of purge air or gas continually flows through the dip tube and will bubble out the bottom of the tube. This dip tube arrangement essentially transfers the hydrostatic pressure at the bottom opening of the tube to the pressure transmitter, while the purge gas keeps the liquid from advancing up into the pipe. The bubbler can be used on atmospheric or pressurized tanks with a properly configured pressure or differential pressure transmitter.

Probably the most significant application point of the bubbler method that will distinguish its use from many other level measurement methods is the importance of maintaining air or gas flow in the dip tube. The flow provides isolation of the sensor, but the flow must also be set to a level that will not impact the pressure measurement in the tube itself. If the flow is excessive, back-pressure in the tube can be offset from the level imparted by the tank contents, with the result being an incorrect measurement.

Below is an application note from Yokogawa, showing how their pressure transmitters and rotameters can be used to create the setup. Share your process measurement challenges and requirements with instrumentation experts, combining your own process knowledge with their product application expertise to produce effective solutions.