Showing posts with label instrumentation. Show all posts
Showing posts with label instrumentation. Show all posts

Wednesday, October 31, 2018

Understanding HART Communication Protocol

A technological advance introduced in the late 1980’s was HART, an acronym standing for Highway Addressable Remote Transmitter. The purpose of the HART standard was to create a way for instruments to digitally communicate with one another over the same two wires used to convey a 4-20 mA analog instrument signal. In other words, HART is a hybrid communication standard, with one variable (channel) of information communicated by the analog value of a 4-20 mA DC signal, and another channel for digital communication whereby many other variables could be communicated using pulses of current to represent binary bit values of 0 and 1. Those digital current pulses are superimposed upon the analog DC current signal, such that the same two wires carry both analog and digital data simultaneously.

Looking at a standard loop-powered (2-wire) process transmitter circuit, we see the transmitter, a DC power supply (voltage source), and usually a 250 ohm resistor to create a 1 to 5 volt signal readable by any voltage-sensing indicator, controller, or recorder:

HART Communications

The transmitter’s primary function in this circuit is to regulate current to a value representative of the measured process variable (e.g. pressure, temperature, flow, etc.) using a range of 4 to 20 mA, while the DC voltage source provides power for the transmitter to operate. Loop-powered instruments are very common in industrial instrumentation because they allow both power and (analog) data to be conveyed on the same pair of wires.

With the advent of microprocessor-based process transmitters, it became possible for instrument technicians to digitally configure parameters inside the transmitter (e.g. range values, damping values) and also query the transmitter for self-diagnostic alarms. In order to make full use of this digital functionality, though, there had to be some way to communicate digital data to and from the process transmitter over the same two wires used to convey the 4-20 mA analog signal. Otherwise, the only way to access this rich array of digital data inside the transmitter would be to connect a communicator device to some data port located on the transmitter itself, which is inconvenient due to the nature of how these transmitters are used in industry (located in dirty places, often hard to access while carrying a personal computer or other communication device).
HART Transmitter
HART Transmitter
(Siemens)

Thus the HART communication protocol was born to address this need. HART communicates digital data along the loop conductors in the form of AC signals (audio-frequency tones) superimposed on the 4-20 mA DC current signal. A modem built into the smart transmitter translates these AC signals into binary bits, and vice-versa. Now, instrument technicians could “talk” with the new microprocessor-based transmitters simply by connecting a HART communications device at any point along the two-wire cable, even at the far end where the cable terminates at the control system hardware (panel-mounted controller, PLC, DCS, etc.).

Being able to communicate digital data over the same wire pair as the DC power and analog signal opens a whole new range of possibilities. Now, the field-mounted transmitter can communicate self-diagnostic information, status reports, alarms, and even multiple process variables to the control system in addition to the original analog signal representing the (main) process variable. With digital communication, the only data limitation is speed (data rate), not quantity. The control system may even communicate information to the transmitter using the same digital protocol, using this digital data channel to switch between different measurement range sets, activating special features (e.g. square-root characterization, damping, etc.), automatically and remotely.



Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Wednesday, April 18, 2018

Ives Equipment: Growth and Leadership in Automation and Control

For over 60 years, Ives Equipment Corporation has successfully served the industries of eastern and central Pennsylvania, Delaware, Maryland, New Jersey, metro NY, and Virginia with the latest in process control equipment and services. Our business has been built on a foundation of quality people, highly trained and experienced, who take a keen interest in finding the optimum solutions to our customers' control problems.

Thursday, August 31, 2017

Process Instrumentation White Paper: Seven Switch Myths Busted

One Series Pressure and Temperature Transmitter-Switches
One Series Pressure and Temperature
Transmitter-Switches (United Electric)
Summary

With more than 80 years of evolution since its introduction, switch technology as changed significantly enough that some of the common beliefs about switches are no longer true. Seven common myths surrounding switches are analyzed. Recent technology advancements in switch design and how these advancements solve problems in industrial and OEM applications are discussed. Readers will acquire a better understanding of the new technology available to improve control, process efficiency and safety.

1. Blind & Dumb

Prior generations of switches were incapable of displaying process measurements locally, forcing the installation of gauges that created more leak paths and added additional costs. Operators were unaware when installed switches stopped functioning due to welded contacts in the microswitch. Switches required removal from service and manual testing to conform functionality. Often, the control or safety function would go unprotected for days while the switch was in queue to be bench tested, creating an immediate safety concern.

These industry-wide problems inspired manufactures to innovate the next generation of switches that incorporate liquid crystal displays (LCD), presenting local process variable measurements, and integrated internal diagnostics, monitoring the health of the device. The addition of LCDs and device diagnostics increases up me and improves overall plant safety. Original equipment manufacturers (OEM) benefit from a reduction in installed components and a more dependable turnkey product for their customers.

2. Difficult Adjustments

Set point and deadband adjustments were a nuisance for operators and technicians. The instruments were required to be removed from service and calibrated on a bench in the maintenance shop. Installation instructions were not always available for installed devices, leading to wasted me searching for documentation or requesting additional information from the manufacturer. Delicate adjustments were required to achieve desired set points and deadbands, the dead time where no action happens, varied based on the microswitch inside the control. More often than not, instruments were mis- handled leading to premature failure due to inexperienced technicians.

Today’s generation of switches offer electronic platforms that reduce setup and programming to a ma er of seconds. A user interface on the local LCD provides simple prompts that allow users to program switch set points instantly without the need to remove the instrument from the process. Deadband and set point are now 100 percent adjustable, allowing operators to choose the desired range based on the application requirements. No longer are operators required to order and stock redundant devices in the event one failed in the eld. Users now have the flexibility of programming one switch to match many different process requirements.

3. Unsafe in Critical Applications - Not Appropriate for SIS

Industrial process plants are pushing pressure and temperature limits to new boundaries in an effort to stay competitive in a global market. Many of the systems designed 20 years ago were not intended to run at the current process extremes. It is only a ma er of me before these systems fail. Safety instrumented systems (SIS) are being installed to protect the process, people and the environment. These systems require devices that have been rigorously tested by third party agencies to verify the level of safety performance. Mechanical switches, referred to as sensors in SIS, are one of the most common components to fail in these systems. Users and designers require a switch that matches their required system performance level while also being fault tolerant.

Based on the strict performance requirements of SIS, newly introduced hybrid switches integrate the functionality of a switch and a transmitter. The switch portion of the device provides a direct digital output (relay output) to a final element that will instantly bring a process to a safe state in the event of a critically abnormal situation. The analog transmitter signal can be used for trending to determine the health of the device and the process. These new transmitter-switches and recently SIL 2 and 3 exida-certified devices (One Series Safety Transmitter) offer operators a simple and safe product that matches the demanding performance requirements of safety instrumented systems.

4. Problematic in Tough Environments

Whether installed on plant rotating equipment, such as turbines, or on demanding OEM auxiliary equipment, such as pumps or compressors, switches are required to function in tough environments that include shock, vibration, heat and pressure. Vibration is one of the leading causes of electro- mechanical switch failure. Most switches are mechanical in design and utilize a plunger to activate a microswitch. In areas of high shock and vibration, the plunger position can fluctuate and lead to false trips.

New solid-state, electronic switches provide a solution to the common problems with mechanical switches installed in high vibration applications. Because they have no moving parts, these switches can be mounted directly to the equipment or process without connecting impulse lines to keep them isolated from vibration. Industry leading turbine manufacturers and end users operating large compressors in petrochemical plants are experiencing much more reliability and fewer false trips with these new electronic switches, compared to the old mechanical designs.

5. Deploy Electromechanical Designs When Line Power is Unavailable

Most pressure switches sold over the past 80 years were designed to operate without electric power by incorpora ng a sensor that measures pressure by placing force on a plunger that would actuate a microswitch.

The first genera on of digital switches required line power to operate and were not adopted due the unavailability of line power and the cost of wiring. The new genera on of switches operates from leakage current in the circuit when connected to a host device, such as a Programmable Logic Controller (PLC), allowing electronic switches to be drop-in replacements for the old mechanical switches. Today, we have the ability to replace a blind and dumb mechanical switch with a new solid-state, electronic switch that offers a digital gauge, switch and transmitter in one instrument without adding any wiring or hardware.

6. Antiquated Technology


Today’s process plants run their processes faster and ho er than they were originally designed. Ultimately, these plants will have to ini ate modernization projects to support the new demands of the process. Old switches provided users with digital, on-off signals that were either wired to control a piece of equipment directly or sent to a PLC for alarm functionality. As plants go through modernization projects, they restructure control system input/outputs (I/O) to support more analog signals than the digital signals used in the past. Transmitters are commonly chosen and recommended over switches in these new projects, but transmitters do not provide the internal control functionality found in switches.

These modernization projects are costly requiring new equipment, updated wiring, expanded I/O, extensive engineering resources, and costly down me. Users are diligently exploring new ways to reduce overall project costs. The average process transmitter can cost upwards of $2,000 compared to the average process switch costing around $500. Process plants often have 100 to 1,000 switches installed. To upgrade all switches to transmitters could cost a plant up to $1.5 million. Consequently, switch manufacturers researched and developed new electronic switches that are capable of producing both digital and analog signals required by these new modernization projects, while keeping a similar price point to the original mechanical switches installed.

This dramatic savings allows plants to reduce the overall modernization project costs by upgrading the 2nd most likely component (sensor) to fail in a tradional safety system, without upgrading the rest of the safety system and reducing the down me needed to complete the project during a short shut- down turn- around project.

7. The Speed of Response of Transmitters is Faster than Switches


Without question, electromechanical switches are faster than any pressure transmitter on the market. With transmitters, huge amounts of conversions, computations, compensation, and other work must be done to get an accurate signal. Even using today’s high-speed processors, they cannot match the speed of the instantaneous reaction of a mechanical device. The fastest of these devices can be be er than 5 milliseconds while process transmitters can range from 300-500 milliseconds or more. Purpose built transmitters for safety applications designed for speed of response in lieu of accuracy (not needed in safety applications) can be as fast as 250 milliseconds. New solid state transmitter-switches can react in 100 milliseconds or less in the switch mode. If your application requires fast response such as in positive displacement (PD) pumps and turbine trip for over-speed protection, consider new solid-state transmitter-switches over process transmitters.

Recommendations


United Electric Controls has recognized the challenges faced by users and developed new products to match their growing needs. In an effort to reduce plant project costs and help OEMs design and build affordable and reliable equipment for the industrial sector, we have developed a new line of electronic switches that provide drop-in replacement of old mechanical switches. These new switches reduce the costs of plant modernizations. Built-in digital and analog communication provides users the op on of control- ling a piece of equipment locally or sending information back to a central control system for process trending and health, or both.

About this white paper:

UE ViewPoint white papers provide Executive, Business and Technical Briefs written by product, application and industry subject matter experts employed by United Electric Controls.  For more UE ViewPoint papers, visit this link.

Sunday, July 23, 2017

Ives Equipment Business Groups

Ives Equipment organizes its extensive product line into four distinct groups:

Ives Equipment and Controls, providing instrumentation and control products to the chemical, petro-chemical, refining, bulk storage, primary metals, pulp & paper, powergen, gas & oil distribution and OEM markets.

Pharmaceutical, Bio-pharm, and Sanitary, providing hygienic, ultra-pure and sanitary instruments, connectors, fittings, tubing and gaskets to the pharma, bio-pharm, food and beverage, life-science and labortory industries.

Analytical Instruments, used to analyze process material samples and record the data for quality, conformance and compliance.

Water and Wastewater Treatment, providing instruments, analyzers, valves and controls for the transfer, storage, analysis, treatment, and logging of municipal and industrial water treatment systems.

Sunday, April 30, 2017

Vibrating Point Level Switch Operating Principles and Use

vibrating point level switch
Vibrating point level switches (SIEMENS)
When asked the primary reason to remember the year 1711, the event probably not on the minds of many is the invention of a device called the tuning fork. The tuning fork has been used as an source of resonating pitch for over three hundred years, and is still used to tune musical instruments today. While the tuning fork was initially applied to tune musical instruments, the concept of resonant frequency of a material or object has been utilized in numerous commercial, scientific, and industrial applications to provide feedback or insight into a process or operation. The vibrating fork level switch is one such industrial application where resonant frequency is used to deliver a data point or provide a control output for process operation.

The operating principle of the vibrating fork is based on the oscillating fork resonating at a known frequency in air when it is set in motion. Upon contacting a medium other than air, the resonant frequency is shifted, depending on the medium contacting or immersing the fork. Typically, fork-type level switches are installed on either the side or the top of a liquid process tank. An exciter keeps the fork oscillating, and a detector circuit monitors fork vibrating frequency, providing a change in the output signal when the frequency changes. Contact or immersion of the fork in liquid will change the fork vibrating frequency sufficiently to produce a change in output signal. Depending on the configuration of the level switch, it can function as a liquid level alarm, or provide a control output for a pump, valve, or other device. Sensor response, the change in fork vibration frequency, is a function of liquid density. Liquids with greater density will generally produce a larger frequency shift in the vibrating fork.

The wide use of vibrating level switches across various process industries is a testament to the reliability of the technology. The devices protect against overfill, indicate high and low points inside tanks, and are useful over a wide range of temperatures. A sturdy design, coupled with product variants that include a variety of sensor materials, selectable probe length, and specialized output features make vibrating fork switches applicable in many operations where level indication is needed. Chemical processing, mining, food and beverage, plastics, and other industries utilize the switches, thanks to their customizable designs and consistent performance. An advantage offered by vibrating fork level switches is a resistance to factors that sometimes confound other technologies employed for level indication. The devices will reliably function despite flow, bubbles, foam, vibration, and coating complexities related to the subject liquids. Additionally, vibrating fork switches are reliable in both high level and low level indication scenarios.

Highly viscous liquids are generally not good candidates for the application of a vibrating fork level switch. Some liquids present the potential for material accumulation between the forks, possibly resulting in poor performance. Both of these limitations are addressed by various design features incorporated by different manufacturers.

The SIEMENS SITRANS LVS200 is a vibrating point level switch for high or low levels of bulk solids. The standard LVS200 detects high, low or demand levels of dry bulk solids in bins, silos or hoppers. The liquid/solid interface version can also detect settled solids within liquids or solids within confined spaces such as feed pipes. It is designed to ignore liquids in order to detect the interface between a solid and a liquid. Additionally, the SITRANS LVS200 has an optional 4 to 20 mA output for monitoring buildup on the fork to determine when preventative maintenance should be performed in sticky applications.

For more information on any level sensing application, contact Ives Equipment by visiting http://www.ivesequipment.com of calling 877-768-1600.

Wednesday, March 22, 2017

New Ives Equipment Video

Ives Equipment, founded in 1954, provides a diverse range of process control equipment, including valves, regulators, wireless products, flow products, pressure gauges, control products, level instrumentation, sanitary products, temperature instruments, analytical products, electric heat trace and bio-pharmaceutical products.

For more than 60 years, Ives Equipment Corporation has successfully served the industries of eastern and central Pennsylvania, Delaware, Maryland, metro NY, New Jersey, Virginia and Washington DC with the latest in process control equipment and services.

The Ives business is built on a foundation of quality people, highly trained and experienced, who take a keen interest in finding the optimum solutions to customers' control problems.

Sunday, November 6, 2016

Your Plant's Partners Make A Huge Difference in Performance and Profitability

The business relationships you make and the partnerships you choose have a dramatic impact on your plant operations. Choosing the right instrumentation and process equipment partner will save you time, money, and make your plant safer. 

For more than 60 years, Ives Equipment Corporation has successfully served the industries of eastern and central Pennsylvania, Delaware, Maryland, metro NY, New Jersey, Virginia and Washington DC with the latest in process control equipment and services.

The Ives business is built on a foundation of quality people, highly trained and experienced, who take a keen interest in finding the optimum solutions to customers' control problems. If you need a proven, experienced, and reliable business partner, choose Ives.

Friday, April 8, 2016

Veterans in Automation

Hire Vets
Veterans are excellent candidates for
Automation careers
Reprinted with permission from the Automation Federation (www.automationfederation.org)
Note: At time of writing, both Preston Mihalko and Nick Abbenante were employees of Ives Equipment.





Automation jobs require a combination of technical know-how combined with interpersonal skills, making them a great fit for veterans.

It’s always difficult to find suitable candidates for openings in industrial automation as most of these jobs require both technical and interpersonal skills, often with a bit of management expertise added to the mix. Universities graduate hundreds of thousands of mostly young men and women every year with business and liberal arts degrees, and many of these grads have excellent interpersonal skills, but generally aren’t very technical.

Engineering and other STEM graduates have the required technical background, but are always in short supply and command very high salaries. And many of these grads are a bit lacking in interpersonal skills as they have spent their formative years relentlessly hitting the books, not developing relationships.

As far as management skills, these are best gained through on-the-job training, where actual supervision takes place. Unfortunately, not many candidates for entry and mid-level automation jobs have this type of experience.

But veterans naturally combine all three of these skills, as just about every position in each branch of the military requires training and hands-on experience in one or more technical areas. Interpersonal skills are developed by forced close cooperation among those serving, and many relatively young veterans have impressive experience managing teams as commissioned and non-commissioned officers (NCOs). If fact, many would say NCOs, in particular, make great job candidates because they have risen through the ranks to become the backbone of the Army and Navy as sergeants and petty officers, respectively.

Preston Mihalko and Nick Abbenante are two great examples of veterans who have made the successful transition from the military to industrial automation, and their stories illustrate how their skills seamlessly transferred from the military to the civilian sector.

Navy vet finds success in sales

Mihalko spent three years as a service warfare officer in the Navy. He was onboard a ship for much of this time as his specialty was anti- submarine warfare. Technical training was part of daily life as he was required to understand ship operations, as well as the specialized equipment used to detect submarines.

In many ways, a ship is a small city at sea, with larger vessels crewed by thousands, and even smaller ships crewed by hundreds. As such, there are onboard systems for everything from propulsion to electrical generation to water treatment. And, of course, these systems need to be monitored and controlled around the clock, with very stringent requirements for reliability and uptime.

The mechanisms of a nuclear submarine or large surface ship rival any industrial installation, so the parallels between onboard and industrial automation systems are obvious. It’s virtually impossible to serve aboard a ship for any length of time without absorbing extensive technical knowledge and gaining lots of hands-on experience.

As far as interpersonal skills, these are naturally gained by living in very close quarters with hundreds or even thousands of sailors. These colleagues span the range from front-line workers to supervisors, starting with newly enrolled sailors and advancing to experienced ship captains. Close cooperation is forced, and virtually no one leaves the Navy without knowing how to develop and maintain work relationships with people from every walk of life and exhibiting most every temperament.

If one is an officer, like Preston, then these work relationships contain a supervisory component. The management training given to officers in the Navy and other branches of the service has been honed for over a hundred years, and it consists of both book learning and hands-on experience. Those who move up the ranks exhibit a strong sense of personal responsibility and judgment, giving them the ability to lead and work effectively with others.

Upon his honorable discharge, Preston was looking for a career where he could successfully combine his interpersonal and technical skills. “I was interested in sales and not operations because I like being in front of customers, and because I enjoy face-to-face technical interactions. I first went into medical device sales, then from there to industrial automation sales,” says Mihalko.

Successful industrial salespeople must work with application- driven products, as does Mihalko, and therefore need to exhibit a combination of capabilities. Technical skills are needed so salespeople can understand the products and how they t customers’ needs, and interpersonal skills are required so salespeople can properly communicate how products satisfy customer needs and expectations.

Mihalko found the specific skills he learned in the Navy transferrable to the field of industrial automation. “I was able to apply shipboard systems and concepts to understand industrial automation issues. Interaction with equipment on a daily basis helped me to gain hands-on experience and understanding, which I found applied both on board a ship and on land. In terms of intangible benefits, I gained the ability to network with co-workers through my interactions with other Navy personnel,” he notes.

Fittingly, Mihalko uses nautical terms to describe the position he’s held for the last three years working in outside sales for an automation distributor based in the Maryland/Washington, DC area. “I’m the captain of my own destiny in outside sales, and I enjoy marketing myself along with the brands I represent. The challenge of the uncertainty and having to figure out solutions is also appealing,” he indicates.

Among ‘a few good men’

As the iconic ad intones, the Marines are “looking for a few good men.” Nick Abbenante thought he might t the bill, and he ended up serving for four years as a Marine Corp First Commander Infantry Officer. In this position, he worked as a military occupational specialist in the areas of combat and weapons. Abbenante’s plans weren’t as specific as Mihalko’s, but were similar in many ways. “I wanted direct interaction with customers, and knew I wanted to be in outside sales,” he says. As a member of one of the most tightly woven cadres in any branch of service in any country, Abbenante knew what it meant to work closely with and depend on others. These interpersonal skills and deep relationships were something he could transfer directly to a sales position.

Sales, like many other areas of industrial automation, requires organizational skills, which Abbenante felt he gained in the service. “The Marines taught me the value of prioritizing, multi-tasking, attention to detail, taking initiative and persistence. And, like any branch of service, not being afraid to get involved,” he explains. Abbenante combined his skills and training to find the type of job he wanted in industrial automation, where he’s worked for the last two years as an outside sales rep with responsibility for the company’s Maryland territory. He works very closely with process plant personnel in this position to help them analyze applications, and specify the proper equipment to solve problems and optimize processes.

“Being a salesperson is like having my own little enterprise. One must show ownership and take stock in the success of your own business territory, which contributes to the success of the larger enterprise. Every day brings new challenges at different plants,” he notes.

Both Mihalko and Abbenante have some advice for other veterans looking to make the transition from the military to industrial automation.

Advice to other veterans and potential employers

Some people look at military life and don’t immediately see the connections to the private sector. To an employer considering a recently discharged candidate, or even a new veteran moving back into civilian life, there may seem to be a gap. Abbenante thinks both sides are looking at it incorrectly, and need to reconsider how to match military skills and experiences with job requirements. “Don’t look for an exact match to your skills, but instead look for positions with requirements complimentary to your knowledge, and then build upon that,” he suggests.

Mihalko chimes in with his advice: “You have an open-ended career choice because you’ve gained many different skills in the service. Sales is one area, but there are many other avenues which might match up your skills and abilities,” he points out.

And both Abbenante and Mihalko encourage disabled veterans to look for positions in sales and other areas where heavy lifting and other physical rigors aren’t involved. “Sales gives you the opportunity to solve problems and think on your feet, but not necessarily in a physical way,” Mihalko emphasizes.

Mihalko used the services of the military to find private-sector work. “I went through a transitioning veterans’ organization to find a position. I attended a career event with interviews set up based on my resume’s match with available jobs,” he says. Abbenante took a different route than Mihalko. “I worked with headhunters I met at a conference, and they assisted in setting up interviews,” Abbenante notes.

Although Abbenante and Mihalko served in different roles in the military and followed their own unique paths to find positions in the automation eld, both are proud to be automation industry professionals.

Monday, March 21, 2016

The Ten Things Everyone Should Know about pH and ORP

pH ORP probe
pH ORP probe
(courtesy of AquaMetrix)
Reprinted with permission from AquaMetrix

1. pH measurements are only good to 0.1 pH units

Electrodes are funny things. They are the only electronic components that don’t even have specifications listed in their data sheets. One major figure of merit, the impedance of the glass electrode, is on the order of megahoms and can vary by a factor of two. Cross sensitivity to other ions (e.g. sodium), response time and differences between any two electrodes limit the accuracy of measurement. Expecting accuracy of greater than 0.1 pH units is unrealistic.

2. Speaking of accuracy... It is not the same as precision.

For a consistent process a pH probe can achieve precision of results to within 0.02 units but it’s accuracy will always be limited by variables such as calibration accuracy, high sodium content or Careful routine calibration, however, will narrow the gap between the accuracy of readings closer to the lower level of precision.

3. ORP measurements are only good to ± 20 mV. 

Once again the measurement of ORP might be characterized by a high precision but the accuracy of the reading is constrained by the difficulty of calibration, as explained in point 6, and the non-buffered calibration solutions that allow the ORP value of the calibration solutions to change over time. Whereas the buffered composition of pH calibration solutions insures that they will change minimally an ORP calibrations solution is a mixture of Fe2+ and Fe3+ salts. Just the addition of air to the mixture will increase the ORP of the mixture. So don’t look for “NIST traceable” on the label of an ORP calibration solution.

4. ORP measurements are relative.


The process electrode is nothing more than a platinum (or gold) band upon which oxidation (reduction) reactions take place. To complete the circuit, as in all potentiometric devices, is a reference electrode. Usually that is the same Ag/AgCl electrode used in a pH probe so the REDOX potential that you read is the difference between the Pt band process electrode and the arbitrarily chosen reference electrode. What matters most with an ORP measurement is its change to an agreed upon standard.

5. pH calibration requires two points.


Calibration measures the response of an instrument as one changes the measurement variable in a known way. For pH measurements that measurement variable is the concentration of hydrogen ions. One calibrates a pH probe by drawing a line through points representing the response of a pH probe to more than one H+ ion concentrations (or pH values). Therefore calibration requires at least two points.

6. ORP calibration can only realistically be done with one point.

This sounds like a reversal of point 4 but it’s not. ORP is not a measure of any one species (e.g. H+ ions or oxygen molecules). It measures the collective REDOX potential of everything in the water. Furthermore calibration solutions, e.g. 200 mV Light’s solution and 600 mV Zobell’s solution are two completely different mixtures of reagents. Therefore all we can is choose one calibration solution and calibrate for it.

7. ORP measurements can be slow.

Stick an ORP probe in a calibration solution and you will get a steady reading with- in half a minute. Take the same probe and stick it in a glass of tap water and it might take 20 minutes for the read-

ing to settle to the 200-300 mV that is typical of tap water. The response of the process electrodes to the REDOX reactions that take place on the surface of a Pt electrode depends on the speed of the many reactions that give the potential and the rate at which molecules diffuse through the water. The Fe2+ and Fe 3+ ions that comprise most of the ORP value in calibration solutions react very quickly with the Pt but the Cl- and dissolved oxygen that make up tap water react much more slowly. So the key to successful ORP measurement is patience.

8. pH measurements must be temperature compensated to be accurate.


A pH measurement is the determination of H+ ions in solution. Higher temperature causes the chemical activity to increase and the pH reading to increase accordingly. So we must remove the temperature effect by measuring it and using the well known Nernst equation to correct it for the reading at 250C. (The correction is quite simple. The pH value is proportional to temperature when the latter is an absolute value (i.e. in Kelvins).

9. ORP measurements are affected by temperature but are NOT corrected for it.

An ORP value simply reflects the ability of whatever is in the water to oxidize whatever contaminants are in the water. Of course oxidation speeds up at higher temperatures. But since ORP measures the rate of chemical reactions and not any one chemical species there is no need to correct it. However we can convert the temperature reading to the ORP that we would measure at 250 C so that we have a basis for comparing the chemistry of the process. That’s why we provide a temperature sensing thermistor or RTD with our differential ORP probes.

10. A differential probe properly cared for will last a long time but it won’t last forever.

Over time chemicals in the process make their way through the junction or salt bridge and into the pH 7 buffer that bathes the reference electrode. Manufacturers go to great length to minimize this contamination but they can only slow it down. Aquametrix differential probes allow the user to cheaply and quickly replenish both the pH 7 solution and the salt bridge so that our probes our industry leaders when it comes to probe lifetime. Nonetheless electrodes themselves lose their efficiency as the glass becomes contaminated and/or eroded by the process. However the good news that, with routine calibration and maintenance a differential probe can last for years in most environments.

Thursday, September 17, 2015

Simplifying Plant Safety instrumentation

White paper courtesy of United Electric Controls

Safety implementation typically is done by a group that includes plant instrument engineers and technicians, who are charged with finding simple and reliable solutions. Often, these situations involve the question of when to shut a process down. Such decisions frequently hinge on key process variables such as flow, level, temperature and pressure. these must be in a specified range at various locations within chemical and petrochemical plants, refineries and power plants, including everything from critical process vessels to eye wash stations.

For such point safety applications, a properly designed and implemented digital switch with self-diagnostics can be an important part of the answer. As an element of a multiple technology solution, a digital switch-based approach can help eliminate common-mode failures, significantly improve response time, achieve needed safety integrity levels (SILs), and simplify plant safety instrumentation.

To read the entire white paper, see the embedded document below:

Monday, July 27, 2015

Upgrading a Mechanical Pressure Switch to an Electronic (Solid State) Version

This video demonstrates how to upgrade from a traditional mechanical pressure switch to a solid state pressure switch.

The example here uses the United Electric Controls One Series as the example.

This type of product (One Series) allows you to choose from explosion-proof, intrinsically safe and energy limited models that monitor gauge pressure, differential pressure or temperature. With up to two fully adjustable set points and deadbands, available 4-20 mA analog output, and absolutely no moving parts. They are used in a wide variety of applications where mechanical switches are not considered.