Differential Pressure Transmitter Inferential Applications

Differential pressure transmitter for industrial
process control applications.
Image Courtesy Azbil North America

Differential pressure transmitters are utilized in the process control industry to represent the difference between two pressure measurements. One of the ways in which differential pressure (DP) transmitters accomplish this goal of evaluating and communicating differential pressure is by a process called inferential measurement. Inferential measurement calculates the value of a particular process variable through measurement of other variables which may be easier to evaluate. Pressure itself is technically measured inferentially. Thanks to the fact numerous variables can be related to pressure measurements, there are multiple ways for DP transmitters to be useful in processes not solely related to pressure and vacuum.

An example of inferential measurement via DP transmitter is the way in which the height of a vertical liquid column will be proportional to the pressure generated by gravitational force on the vertical column. The differential pressure transmitter measures the pressure exerted by the contained liquid. That pressure is related to the height of the liquid in the vessel and can be used to calculate the liquid depth, mass, and volume. The gravitational constant allows the pressure transmitter to serve as a liquid level sensor for liquids with a known density. A true differential pressure transmitter also enables liquid level calculations in vessels that may be pressurized.

Gas and liquid flow are two common elements maintained and measured in process control. Fluid flow rate through a pipe can be measured with a differential pressure transmitter and the inclusion of a restricting device that creates a change in fluid static pressure. In this case, the pressure in the pipe is directly related to the flow rate when fluid density is constant. A carefully machined metal plate called an orifice plate serves as the restricting device in the pipe. The fluid in the pipe flows through the opening in the orifice plate and experiences an increase in velocity and decrease in pressure. The two input ports of the DP transmitter measure static pressure upstream and downstream of the orifice plate. The change in pressure across the orifice plate, combined with other fluid characteristics, can be used to calculate the flow rate.

Process environments use pressure measurement to inferentially determine level, volume, mass, and flow rate. Using one measurable element as a surrogate for another is a useful application, so long as the relationship between the measured property (differential pressure) and the inferred measurement (flow rate, liquid level) is not disrupted by changes in process conditions or by unmeasured disturbances. Industries with suitably stable processes – food and beverage, chemical, water treatment – are able to apply inferential measurement related to pressure and a variable such as flow rate with no detectable impact on the ability to measure important process variables.

Share your process measurement challenges with instrumentation specialists, leveraging your own process knowledge and experience with their product application expertise to develop an effective solution.

Industrial Transmitters for Pressure and Differential Pressure from CTi Controltech

Advanced Pressure Transmitters for Process Measurement

Direct mount pressure transmitter
Courtesy Azbil

The measurement of pressure is a common task throughout many industrial spheres. Depending on the application, a wide range of process or machinery operation status can be derived from a pressure reading. Accuracy, ruggedness, and flexibility in application are hallmarks of a useful pressure transmitter.

Azbil North America advanced pressure transmitters offer a combination of features that can make them an advantageous selection for almost any application.

• Stability of +/-0.1% for 10 years
• Little to no downtime for calibration
• Sensor technology that provides day-one accuracy for the life of the transmitter
• Customizable display
• Alarm outputs
• Fast response
• International standard certifications

The advanced pressure transmitter is available in variety of mounting configurations to suit most applications. More information is contained in the document included below. Share your process measurement challenges with application experts, combining your own process knowledge and experience with their product application expertise to develop effective solutions.

Advanced Direct Mount Pressure Transmitter from CTi Controltech

Vortex Gas Flowmeter With Built-in Pressure and Temperature Compensation

Azbil MVF Series Vortex Gas Flowmeter

Industrial process control operations require measurement, lots of measurement. Whether it be temperature, level, flow, pressure, or some other attribute of the process, manufacturers have always responded to customer demand with a broad array of engineered products geared toward higher performance and lower cost.

Combining related measuring functions or capabilities into a single device can result in space and cost savings, as well as simplifying maintenance operations. Azbil NA does just that with their MVF Series of vortex flowmeters for gases.

Vortex flowmeters are based upon the recognized principle of vortices being generated downstream of a blunt obstruction at a frequency dependent upon the velocity of the fluid (von Kármán effect). The fluid velocity can be combined with temperature and pressure measurements to produce a calculated mass flow. Azbil, in their MVF Series, provides built in compensation for temperature and pressure, eliminating the need for additional instrumentation. The vortex gas flowmeter provides an output signal that is proportional to the mass flow, as well as a pulse signal. The flowmeter is suitable for use with air, nitrogen, argon, oxygen, carbon dioxide, city gas (13A), methane, propane, butane, and other inert gases or mixtures outside the explosion limit range.

See the data sheet below for more detail. You can also contact product application specialists for any assistance needed to select the right configuration for your application. Share your process measurement and control challenges with product experts. Combine your process expertise with their product application knowledge to produce the most effective solutions.

Vortex Gas Flowmeter for Process Measurement from CTi Controltech

Understand These Areas of Process Instrument Protection

Properly protect process measurement instrumentation
to prevent mishap 

The performance of every process is critical to something or someone. Keeping a process operating within specification requires measurement, and it requires some element of control. The devices we use to measure process variables, while necessary and critical in their own right, are also a possible source of failure for the process itself. Lose the output of your process instrumentation and you can incur substantial consequences ranging from minor to near catastrophic.

Just as your PLC or other master control system emulates decision patterns regarding the process, the measurement instrumentation functions as the sensory input array to that decision making device. Careful consideration when designing the instrumentation layout, as well as reviewing these five common sense recommendations will help you avoid instrument and process downtime.

Process generated extremes can make your device fail.

Search and plan for potential vibration, shock, temperature, pressure, or other excursions from the normal operating range that might result from normal or unexpected operation of the process equipment. Develop knowledge about what the possible process conditions might be, given the capabilities of the installed process machinery. Consult with instrument vendors about protective devices that can be installed to provide additional layers of protection for valuable instruments. Often, the protective devices are simple and relatively inexpensive.

Don't forget about the weather.

Certainly, if you have any part of the process installed outdoors, you need to be familiar with the range of possible weather conditions. Weather data is available for almost anywhere in the world, certainly in the developed world. Find out what the most extreme conditions have been at the installation site....ever. Planning and designing for improbable conditions, even adding a little headroom, can keep your process up when others may be down.

Keep in mind, also, that outdoor conditions can impact indoor conditions in buildings without climate control systems that maintain a steady state. This can be especially important when considering moisture content of the indoor air and potential for condensate to accumulate on instrument housings and electrical components. Extreme conditions of condensing atmospheric moisture can produce dripping water.

Know the security exposure of your devices.

With the prevalence of networked devices, consideration of who might commit acts of malice against the process or its stakeholders, and how they might go about it, should be an element of all project designs. A real or virtual intruder's ability to impact process operation through its measuring devices should be well understood. With that understanding, barriers can be put in place to detect or prevent any occurrences.

Physical contact hazards

Strike a balance between convenience and safety for measurement instrumentation. Access for calibration, maintenance, or observation are needed, but avoiding placement of devices in areas of human traffic can deliver good returns by reducing the probability of damage to the instruments. Everybody is trained, everybody is careful, but uncontrolled carts, dropped tools and boxes, and a host of other unexpected mishaps do happen from time to time, with the power to inject disorder into your world. Consider guards and physical barriers as additional layers of insurance.

Know moisture.

Electronics must be protected from harmful effects of moisture. Where there is air, there is usually moisture. Certain conditions related to weather or process operation may result in moisture laden air that can enter device enclosures. Guarding against the formation of condensate on electronics, and providing for the automatic discharge of any accumulated liquid is essential to avoiding failure. Many instrument enclosures are provided with a means to discharge moisture. Make sure installation instructions are followed and alterations are not made that inadvertently disable these functions.

Developing a thoughtful installation plan, along with reasonable maintenance, will result in an industrial process that is hardened against a long list of potential malfunctions. Discuss your application concerns with your instrument sales engineer. Their exposure to many different installations and applications, combined with your knowledge of the process and local conditions, will produce a positive outcome.

Dual Input Temperature Transmitter Has Some Useful Applications

Advanced Temperature Transmitter
Courtesy Azbil N.A.

Temperature measurement is a mainstay of many commercial and industrial processes and operations. Much can be deduced from an accurate measure of process temperature, and a large segment of the industrial control products marketplace is devoted to reliable and accurate measurement of temperature

There are a variety of instruments and methodologies for measuring temperature, the most common of which is probably direct contact between the substance under scrutiny and a temperature sensor of some type. In industrial process applications, contact sensors are often thermocouples or resistance temperature detectors (RTD), chosen for their cost, accuracy, and flexibility of installation.

Every operating process is "critical" to some group of stakeholders. The process may be of great importance for a number of reasons:

• The process output may serve as an input to another process with great value.
• The process output may be of great direct value to the stakeholders.
• The process may have significant levels of hazard associated with improper or out of control operation.
• Out of control operation may result in substantial financial loss to the stakeholders.

When temperature is an important indicator of process function, whether for financial or safety reasons, the operator cannot tolerate a loss of the temperature signal. One manufacturer has an advanced solution in the form of a dual input temperature transmitter with built in functions that:

• Switch to the backup sensor if the primary has a failure indication.
• Alert the operator if the deviation between the two sensor readings indicates sensor drift. 
• In wide range temperature applications, switch between sensors with differing measurement ranges for better accuracy.

Along with HART communications and other useful features, these advanced temperature transmitters can help reduce risk and increase performance and safety. Assess how these advanced devices can enhance your process performance. A product data sheet is included below. Product specialists can help with product configuration and selection, along with any application concerns you may have.

Advanced Industrial Temperature Transmitter from CTi Controltech

Innovative Magnetic Level Gauge Provides Accurate Indication Under Adverse Conditions

Magnetic Level Gauge on Tank
Courtesy Jerguson

We, as engineers, industrial process operators and stakeholders, recognize the necessity and value of a continuous stream of accurate and timely information about our processes. Our experience has also taught us that the environment and activities surrounding our installations can have a significant impact upon our ability to continually gather accurate process measurements. Some of our concerns include:

• Weather - An element whose impact cannot be understated....or easily predicted.
• Physical Contact - Equipment and measurement devices must be protected from damaging impact.
• Security - Vandalism, cyber invasion, and other external threats are possible

Our responsibility, as operators of machines and handlers of materials that can produce hazardous or life threatening conditions in the case of failure or error, is to foresee every reasonably probable event that could adversely impact the safe and proper operation of our industrial processes.

One manufacturer has developed an innovative solution to a potential problem in the application of magnetic level controls.

The short video below outlines the source of the potential failure and the way in which the product design change successfully overcomes a potentially adverse impact on process measurement. Invest less than three minutes of your time to watch the video and build your application knowledge by learning from the experience of others. Do not hesitate to contact a product application specialist for more detail, or to discuss your process measurement needs.

Multivariable Vortex Flowmeter

AX Series Flowmeter
Courtesy Azbil North America

Azbil North America recently added the AX series of multivariable vortex flowmeters for industrial process measurement and control to their product offering. The instrument combines temperature, pressure and velocity measurement in a single instrument to provide accurate mass flow measurement of gases, liquids and steam. Incorporating several variable measurements into a single package reduces potential leakage points, installation complexity, and space requirements compared to arrangements with discrete instruments for each variable. Configurations for in-line or insertion installation are available for line sizes ranging from ½” to 8” for in-line and 2” and greater for insertion. A sizing guide, application and product configuration assistance is available from a product specialist.

Multivariable Vortex Flowmeter from CTi Controltech

Types of Level Instruments in Process Control

Tuning fork level controls
(form of echo level sensing)

In many industrial processes, the measurement of level is critical. Depending on the nature of the material being measured, this can be a simple or complex task. Several different technologies for sensing level are briefly explained here.




Sight Glasses or Sight Gauges
A very simple type of level measurement device for direct visual measurement of level in a tank or vessel. A sight gauge is usually a clear glass or plastic tube connected to the a vessel at the top and bottom of the level range needing to be viewed. As long as interface (different fluids with different densities floating on each other) the fluid level inside the vessel will be at the same hight as the level in the tube.