Measuring Fluctuating Temperature Water Level Using Density

A thermal power plant needed to measure the liquid level in a water storage tank, but due to mounting restrictions and space limitations, they are unable to use traditional liquid level sensing products.

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Another approach for indirectly measuring level of water in a storage tank is by using a differential pressure transmitter to measure pressure, and then calculating liquid level through calculating the density. However, the density of water changes with temperature; when temperature increases the water expands in volume and vice versa. This causes the water level to rise without varying the amount of pressure being applied to the transmitter’s sensors. If only a pressure measuring device is used, the system will lack the temperature data required for correctly adjusting the density calculations. Temperature compensation is needed to accurately determine the level of water within the process.

SOLUTION

An SOR 815DT Smart Differential Transmitter combined with an SOR 1400 Series Temperature Transmitter Assembly gave the plant personnel all the necessary information they required to calculate the density, and therefore, indirectly measure the water level. With the SOR 815DT’s compact design, it was able to be installed into confined areas where other instruments would be too large.

Both transmitters constantly measure the temperature and pressure and the process data is relayed to a digital control system. The temperature transmitter data is used to compensate the level readings accordingly. The combination of transmitters provides the digital control system with the process parameters needed for precisely calculating the density and thus, it is able to accurately determine the water level within the storage tank.

For more information regarding any level, pressure, temperature, or flow requirement, contact Instrument Specialties, Inc. by calling 407-324-7800 or visiting http://isi.group.

Water Quality Monitoring for Environmental Studies and Compliance

The right package of water quality instrument features
enhances effectiveness of field deployment.
Image courtesy In Situ, Inc.

There are many reasons to measure and monitor environmental water quality, chief among them compliance with local, state, or federal requirements. Other applications may call for water quality monitoring for environmental studies of various types. A sonde is an instrument used to collect water quality measurement data in the field. Whatever the issues driving a need for gathering environmental water data, selecting an instrument that combines accuracy, ruggedness, and ease of use can minimize the time and cost involved. There are a number of features to look for.

• Corrosion resistant construction that tolerates a wide range of targeted water sources.
• Minimized and simple setup procedure for quick deployment
• Low maintenance burden
• Smart sensor technology for best accuracy
• Extended measurement range for all parameters to accommodate broad range of water sources
• Low training requirement for technicians to be proficient at deploying sonde in the field
• Minimal sensor drift to increase available time in field and accuracy
• Modest or low frequency requirement for sensor calibration
• Sensor measurement stability. Higher level allows longer field deployment intervals.
• Sensors that are easily changed or replaced without need for high levels of technical training. Also allows for reconfiguring measurement scheme to target different constituents in the water.
• Sensors available for RDO, pH/ORP, turbidity, conductivity, temperature, pressure
• On board diagnostics monitor instrument operation for error
• Active and passive anti-fouling systems to automatically clean sensors, removing foreign matter that can impact measurements
• Extended field deployment time with low power consumption
• Real time data access, as well as on board data storage and other options for flexible data delivery.

These are some of the features that can result in effective data gathering and reduced manpower requirements to accomplish the task at hand. Share your water quality monitoring requirements with instrumentation specialists. Leverage your own knowledge and experience with their product application expertise to develop an effective solution.

Two-Wire vs. Four-Wire Transmitter For Analog Process Signals - What to Consider?

I/O modules are an integral part of process signal connectivity.
Image courtesy of Acromag

Transmitters are everywhere in process control. They take a sensor output signal,amplify and condition it, then send it to monitoring and decision making devices. The most common analog electrical signal used for transmitting process control signals is a 4-20 mA (milliampere) current flow. It has succeeded in its adoption for a number of reasons, not the least of which are its resistance to interference and ability to transmit a signal across a substantial length of cable.

Aside from the sensor connection, there are two basic wiring schemes for these devices. The simplest employs just two conductors to transmit the signal and coincidentally provide operating power for the transmitter electronics. This type of transmitter is commonly referred to as a "loop powered" or "two-wire" device. A DC power supply, typically 24 volts, is wired in series with the 4-20 mA output signal and the transmitter derives its operating power from this source. Loop powered devices generally consume very little power, but process designers must consider the total resistance imposed on the loop by all connected devices. The cable, unless the length is monstrous, poses a measurable but comparatively small resistance. Careful consideration should be given to the resistance imposed by receiving devices, especially if there are several in series, receiving the loop signal. The output voltage of the power supply and the maximum tolerable voltage of the connected devices will serve as limiting factors on loop instrument quantity. Where they can be applied, two-wire transmitters offer a straight forward solution for delivery of analog process measurement signals.

A "four-wire" transmitter gets its name from, you guessed it, the two pairs of wires used to provide operating power and a signal transmission path. Provided with a separate power source, possibly even 120 volts AC, this transmitter type will often be found in applications where the sensor may have power requirements that cannot be met with the limitations inherent in the loop powered device. While it may seem that the separate power supply negates the need to consider total resistance load on the signal loop, this is not the case. The signal loop still will be limited by the DC power supply that serves as the driving force of the loop.

In many cases, the question of "two-wire or four-wire" will be answered by the transmitter manufacturer. Since the two-wire scheme is a less burdensome installation, it may be the only product offering when a suitable device can be designed for an application. That said, a diligent search will probably find two and four-wire versions of transmitters for almost every application.

What are some decision making guidelines?

• Some types of transmitters have sufficiently high power requirements that they cannot be loop powered. In this case, four-wire may be the only option.
• For low resistance loads, use 2 wire transmitters for a simpler installation.
• Allow some headroom in the loop resistance to accommodate at least one added receiving device in the future. For example, a temperature signal may serve as an input to a controller now, but need to service a recording device potentially added in the future.
• Distance should not be mindlessly overlooked, but is generally not a limiting factor, as most installations would be compatible with the distance limitations for two- or four-wire device output signals.
• When signal transmission distances become unwieldy, due to cabling costs or other factors, consider a wireless transmitter instead of a wired device.

An important aspect of applying 4-20 mA signal loops is to maintain the capability to add another receiving device to the circuit. The use of information in the form of process signals has been growing for a long time and is likely to continue. It is certainly easier to wire an additional device into an existing loop, than to install an additional sensor, transmitter, power supply, and cabling to accommodate the additional device.

Share your process measurement requirements and challenges with process instrumentation experts, leveraging your own process knowledge and experience with their product application expertise to develop complete and effective solutions.


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Make Good Use of Technical Sales Representatives

Technical sales representatives bring outside expertise

Process and control equipment is most often sold with the support of sales engineers working for the local distributor or representative. Realizing what these specialists have to contribute, taking advantage of their knowledge and talent, will help save time and cost, contributing to a better project outcome.

Consider these contributions:

Product Knowledge: Sales engineers, by the nature of their job, are current on new products, their capabilities and their proper application. Unlike information available on the Web, sales engineers get advanced notice of product obsolescence and replacement. Also, because they are exposed to so many different types of applications and situations, sales engineers are a wealth of tacit knowledge that they readily share with their customers.

Experience: As a project engineer, you may be treading on fresh ground regarding some aspects of your current assignment. You may not have a full grasp on how to handle a particular challenge presented by a project. Call in the local sales person - there can be real benefit in connecting to a source with past exposure to your current issue.

Access: Through a technical sales engineer, you may be able to look “behind the scenes” with a particular manufacturer and garner important information not publicly available. Sales reps deal with people, making connections between customers and manufacturer's support personnel that may not normally be public facing. They make it their business to know what’s going on with products, companies, and industries.

Of course, sales engineers will be biased. Any solutions proposed are likely to be based upon the products sold by the representative. But the best sales people will share the virtues of their products openly and honestly, and even admit when they don’t have the right product. This is where the discussion, consideration and evaluation of several solutions become part of achieving the best project outcome.

As an engineer who designs or manufactures a product or process, it's highly recommended you develop a professional, mutually beneficial relationship with a technical sales expert, a problem solve. Look at a relationship with the local sales engineer as symbiotic. Their success, and your success, go hand-in-hand.

Achieving Close Control of Process Temperature

Temperature sensor type, construction, and
location contribute to system performance
Courtesy Krohne

Temperature control is a common operation in the industrial arena. Its application can range across solids, liquids, and gases. The dynamics of a particular operation will influence the selection of instruments and equipment to meet the project requirements. In addition to general performance requirements, safety should always be a consideration in the design of a temperature control system involving enough energy to damage the system or create a hazardous condition.

Let's narrow the application range to non-flammable flowing fluids that require elevated temperatures. In the interest of clarity, this illustration is presented without any complicating factors that may be encountered in actual practice. Much of what is presented here, however, will apply universally to other scenarios.

What are the considerations for specifying the right equipment?

KNOW YOUR FLOW

First and foremost, you must have complete understanding of certain characteristics of the fluid.

• Specific Heat - The amount of heat input required to increase the temperature of a mass unit of the media by one degree.
• Minimum Inlet Temperature - The lowest media temperature entering the process and requiring heating to a setpoint. Use the worst (coldest) case anticipated.
• Mass Flow Rate - An element in the calculation for total heat requirement. If the flow rate will vary, use the maximum anticipated flow.
• Maximum Required Outlet Temperature - Used with minimum inlet temperature in the calculation of the maximum heat input required.

MATCH SYSTEM COMPONENT PERFORMANCE WITH APPLICATION

• Heat Source - If temperature control with little deviation from a setpoint is your goal, electric heat will likely be your heating source of choice. It responds quickly to changes in a control signal and the output can be adjusted in very small increments to achieve a close balance between process heat requirement and actual heat input.
• Sensor - Sensor selection is critical to attaining close temperature control. There are many factors to consider, well beyond the scope of this article, but the ability of the sensor to rapidly detect small changes in media temperature is a key element of a successful project. Attention should be given to the sensor containment, or sheath, the mass of the materials surrounding the sensor that are part of the assembly, along with the accuracy of the sensor.
• Sensor Location - The location of the temperature sensor will be a key factor in control system performance. The sensing element should be placed where it will be exposed to the genuine process condition, avoiding effects of recently heated fluid that may have not completely mixed with the balance of the media. Locate too close to the heater and there may be anomalies caused by the heater. A sensor installed too distant from the heater may respond too slowly. Remember that the heating assembly, in whatever form it may take, is a source of disturbance to the process. It is important to detect the impact of the disturbance as early and accurately as possible.
• Controller - The controller should provide an output that is compatible with the heater power controller and have the capability to provide a continuously varying signal or one that can be very rapidly cycled. There are many other features that can be incorporated into the controller for alarms, display, and other useful functions. These have little bearing on the actual control of the process, but can provide useful information to the opeartor.
• Power Controller - A great advantage of electric heaters is their compatibility with very rapid cycling or other adjustments to their input power. A power controller that varies the total power to the heater in very small increments will allow for fine tuning the heat input to the process.
• Performance Monitoring - Depending upon the critical nature of the heating activity to overall process performance, it may be useful to monitor not only the media temperature, but aspects of heater or controller performance that indicate the devices are working. Knowing something is not working sooner, rather than later, is generally beneficial. Controllers usually have some sort of sensor failure notification built in. Heater operation can be monitored my measurement of the circuit current.

SAFETY CONSIDERATIONS

Any industrial heater assembly is capable of producing surface temperatures hot enough to cause trouble. Monitoring process and heater performance and operation, providing backup safety controls, is necessary to reduce the probability of damage or catastrophe.

• High Fluid Temperature - An independent sensor can monitor process fluid temperature, with instrumentation providing an alert and limit controllers taking action if unexpected limits are reached.
• Heater Temperature - Monitoring the heater sheath temperature can provide warning of a number of failure conditions, such as low fluid flow, no fluid present, or power controller failure. A proper response activity should be automatically executed when unsafe or unanticipated conditions occur.
• Media Present - There are a number of ways to directly or indirectly determine whether media is present. The media, whether gaseous or liquid, is necessary to maintain an operational connection between the heater assembly and the sensor.
• Flow Present - Whether gaseous or liquid media, flow is necessary to keep most industrial heaters from burning out. Understand the limitations and operating requirements of the heating assembly employed and make sure those conditions are maintained.
• Heater Immersion - Heaters intended for immersion in liquid may have watt density ratings that will produce excessive or damaging element temperatures if operated in air. Strategic location of a temperature sensor may be sufficient to detect whether a portion of the heater assembly is operating in air. An automatic protective response should be provided in the control scheme for this condition.

Each of the items mentioned above is due careful consideration for an industrial fluid heating application. Your particular process will present its own set of specific temperature sensing challenges with respect to performance and safety. Share your requirements with temperature measurement and control experts, combining your process knowledge with their expertise to develop safe and effective solutions.