Trends in Electronic
Flow Computers
Advances in electronic design, speed, ease of use, I/O options, and
software have increased the usefulness of these economical devices.
Bob Carrell,
Hoffer Flow
Controls, Inc..
The earliest electronic flow computers were derived from industrial electronic counter technology.
They can trace their roots to purely mechanical and electromechanical devices that date back to the 1950s, when they were little more than pulse accumulators and frequency integration devices.
Today, electronic flow computers are available in a variety of configurations, from simple to sophisticated. In this article, we will review the advances these devices have made, with particular focus on units priced between $700 and $2500.
But first, we should consider why the flow computer is so widely used in light of all the controller choices available today. With advances in PLC, DCS, and PC-based control systems, it would seem that the flow computer should be destined for extinction. Certainly there are cases where these other control systems have replaced the need for flow computers, but flow computers still fill a large niche.
The latest advances in flow computers have actually increased the range of applications they can serve. The two primary factors that drive the decision to use a flow computer are ease of use and economics.
Ease of Use
The flow computers of today are designed to interface with various flow measurement sensors. The computers include software and features to optimize flow sensor performance. An operator can readily use these capabilities, as well as reconfigure the flow computer for new applications with minimal training.
Economics
In many cases, economics is the deciding factor when it comes time to choose between a flow computer and another primary control device. Flow computers typically generate the best return on investment when each unit is used with a limited number of flowmeters (one computer with one to eight flowmeters). Most flow computers, however, are designed and sold for use with a single flowmeter. Flow computers used with two or more flowmeters are most commonly used in specialized applications, such as custody transfer of refined petroleum products, net flow or energy measurement between two or more flowmeters, and ratio control of two or more flow streams.
In general, flow computers also provide the best economic benefit when local operator control, interface, and display are required. They are particularly effective in manual or semi-automated applications in which flow is the primary variable to be controlled or monitored.
It would not be possible in the space of this article to fully encompass the variety of flow computers and their applications. Instead we'll focus on the most broadly applied versions and on design trends. This discussion applies to three general classes:
- Rate/Totalizers. Remote-mounted devices that, at a minimum, calculate and display an accumulated total and instantaneous rate.
- Batch Controllers. Typically remote mounted, these devices are normally operated manually and dispense a predetermined amount of fluid. They interface with the flowmeter input and provide one or more control outputs to open and close flow control valves or to start and stop pumps.
- Mass Flow Computers. Typically used with volumetric or velocity-based flowmeters, as well as temperature and pressure sensors, these flow computers are used to derive flows of liquids, gases, and steam in terms of mass units.
Virtually all flow computers fall under one or more of these three broad classes. Certain flow computers have the attributes of one of these classes but have been designed for specific applications. An example of this would be a flow computer used on a steam line or a heated or chilled water line to calculate energy gains or losses to the fluid stream.
| Plug and Flow with Nova Flow |
| Hoffer Flow Controls of Elizabeth City, North Carolina, is introducing the Nova Flow this month as the first plug-and-flow computer. Designed by Hoffer in conjunction with Chronotek of Houston, Texas, Nova Flow is configurable as a rate/totalizer, batch controller, or mass flow computer through the use of a system architecture previously seen only on more sophisticated and costly control devices.
For more advanced applications and options, Nova Flow has eight expansion slots to accommodate electronic modules, which provide I/O and communications options (see Figure 1, page 60). You can choose from several optional modules to best suit a particular application. Configuration of the basic unit and the modules is implemented using Windows-based software, which is included with the unit. The basic unit performs rate/totalization and batch control for a single flowmeter input for uncompensated volumetric measurement applications. One of the expansion modules lets you configure the unit for temperature and pressure or direct density input corrected mass flow on liquids or gases. The flowmeter input can be a magnetic coil, RF, TTL, open collector, or dry contact. You select the flowmeter input type through jumpers without having to change rear terminal wiring connections. The basic unit includes eight additional lines of digital I/O that are configured via software. One optically isolated digital output comes standard with the computer. Nova Flow supports several digital formats, including 0–5, 0–10, or open collector up to 30 VDC and 250 mA. Optional modules currently available include:
With this choice of modules, you can specify what is required and still retain flexibility for future upgrades. Hoffer estimates that with the basic unit and add-on modules, the computer can be configured more than 50,000 distinct ways. The Nova Flow is designed to meet intrinsically safe design ratings for Class I, Div. 2, Groups A, B, C, and D, as well as CE requirements for EMI/RFI emission limits and immunity. In addition, the device includes or supports AGA 8/API 14.2, AGA 7, OIML Tc8 Sc7, R117, R118, and ISO 6551 and 7637 for custody transfer applications. Self-diagnostics for internal functions, programming setup, and external inputs are also included. |
Displays
One of the first things you notice about a flow computer is the display. Traditionally, the displays have been red, single-line light-emitting diodes (LEDs) or liquid crystal displays (LCDs), with the most advanced having a backlight feature either standard or as an option. Today, the choice of displays has increased significantly. LEDs are still available but have been surpassed by LCDs with multiple lines and longer character strings, as well as by vacuum fluorescent displays (VFDs) and full graphical LCDs.
In general, you should consider LEDs for the most basic applications or for situations where the background light is not conducive to LCDs. For example, direct, bright sunlight is often a problem for LCD and VFD units. Certain highly specialized applications require only red or green illumination of displays, and in those cases, the LED is often the best choice. Examples of these applications include avionics and marine applications, where red and green are more conducive to maintaining night vision or for use around light-sensitive materials. For most other applications, the LCD and VFD are preferred for more sophisticated flow computers. These displays provide greater flexibility in the type, variety, and volume of information displayed.
 |
| Photo 1. With a new plug-and-flow system architecture, Hoffer's Nova Flow flow computer provides more than 50,000 application configurations. With optional electronic modules, you can specify digital and analog I/Os for flow, temperature, pressure, and density sensors; control outputs; configurable data logging; and up to three serial and network communications channels. You configure the Nova Flow via Windows-based software. The unit also features a full graphical display that can be customized to display selected parameters. |
Hoffer Flow Controls' Nova Flow (see sidebar "Plug and Flow with Nova Flow" and Photo 1, page 59) and the Kessler-Ellis's SuperTrol (see Photo 2, page 59) take advantage of this display technology. The Nova Flow comes standard with a full graphical LCD. The SuperTrol uses a two-line LCD or VFD as its standard screen. These screens are larger than the displays available one or two years ago, and they have features that were previously available only on specialized flow computers. The features include adjustable backlighting, customization of the information to be displayed, and the ability to program the unit to manually or automatically cycle through a series of displays. The last two features are particularly useful when the flow computer handles multiple meters or other sensor inputs.
Application Flexibility
Traditionally, flow computers have been application specific. A volumetric rate/totalizer could not be readily adapted to use as a volumetric batch controller. Flow computers used on liquids could rarely be used for gas measurement. Today, these limitations are disappearing due in large part to greatly increased microprocessor and memory capacity at lower costs.
Photo 2. Kessler-Ellis's SuperTrol I helped introduce the concept of a user-configurable, multifunction flow computer. The unit can be used for rate, total, and batch control operations and includes an audit trail function for many custody transfer applications. You set up the flow computer using Windows-based software. The SuperTrol I offers a variety of display options to adapt to ambient light conditions.
New flow computers, such as the Nova Flow and SuperTrol I, can be configured and programmed to accomplish tasks that used to require specialized flow computers. Either of these computers can be used as a rate/totalizer or batch controller and can perform basic computations (e.g., input signal scaling) or more advanced computations (e.g., temperature compensation, net flow between two flowmeters, and custody transfer). The Nova Flow can also be configured as a mass flow computer for use with gases and cryogenic liquids requiring temperature, pressure, and compressibility compensation.
This flexibility reduces training for operators, who now have one platform on which to run different applications. For the manufacturer, it reduces the number and types of hardware sets that must be built for each application. This results in savings reflected in the selling prices of flow computers. Prices have remained steady or gone down while capabilities and options have expanded at an almost exponential rate.
A Better Human-Machine Interface
Manufacturers are now incorporating active screens and soft keys for programming and configuration into the newest generation of advanced flow computers. (Earlier flow computers relied far more on dedicated control panel keys for these tasks.) The move toward flow computers capable of handling a greater variety of applications has driven the need to provide this feature to keep the packaging of the units to a manageable size and to make them ergonomic.
 |
| Figure 1. The rear panel of the Nova Flow incorporates connectors for the basic unit and eight slots that accommodate user-selected electronic modules for a variety of I/O functions. The modular architecture allows for mass customization by providing more than 50,000 I/O, control, data logging, and communications combinations to choose from. |
The Nova Flow and SuperTrol highlight another human-machine interface improvement: the control panel button. Early model flow computers used mechanical pushbuttons similar in size and feel to the keys on a PC keyboard. Although these devices provided excellent tactile feedback, they were less than reliable in the typically harsh industrial environments in which flow computers are installed. One of the first improvements was to incorporate flat-panel microswitch technology. Although this helped reduce the mechanical failure rate of the keypads, it provided no tactile feedback to the operator.
The solution to this problem has been the development of microswitch panel buttons that use various mechanical memory materials to provide a pushbutton-like feel to the control button. The front panel overlays are made from polymeric materials that provide a good measure of protection from splash, washdown, dust, and dirt. Beneath the overlay are additional layers of polymeric material and a component that provides resistance to movement of the button by the operator. This element is often shaped like a spring washer and can be made from a polymeric or stainless steel material. The microswitches are typically incorporated into one of the polymeric layers. The result is a button that provides good tactile feel to the operator, even one wearing gloves, while delivering the superior reliability, durability, and economy of microswitches.
Finally, the latest generation of flow computers can display screens in a foreign language. This option reflects the increased international demand for flow computers.
System Architecture and Configuration
The Nova Flow and SuperTrol, though similar in capabilities and price/performance, represent new and different architectural concepts that have been adapted from more advanced control systems.
In the case of the SuperTrol, virtually all the hardware needed to use the device in any application is incorporated in the basic unit. You decide how the unit will be used and configured through a Windows setup program. The hardware set is universal except for mass flow applications in which a different version is required.
The Nova Flow uses a modular architecture for its hardware set. A series of electronic modules is available to configure the unit for specific applications, though the basic unit can perform many flow computations without additional modules. The modules plug into the rear of the unit and provide options related to multiple flowmeter inputs (see Figure 1, page 60); multiple pressure, temperature, and density inputs; analog I/Os; and advanced communications. The Nova Flow also uses a Windows-based configuration program that lets you configure the unit for a specific application (see Figure 2, page 62). The new flow computers also let you software-select features that previously were hardware-dependent. This includes analog output type, operating input voltage, and output power supply voltage.
| Where Advances Have Been Made |
The basic functionality of flow computers tends to remain consistent from generation to generation. The areas in which significant advances have been made in recent years include:
|
Computing Horsepower
Like many high-tech products, the industrial flow computer owes much of its improvement and market growth to the availability of faster microprocessors and larger memory components at ever-decreasing prices. Today, 16-bit CPUs running at clock speeds in the 40 MHz range are the norm rather than the exception for these devices. Onboard flash memory for the latest generation of flow computers is now measured in megabytes rather than kilobytes, as it was only a few years ago. Real-time clocks and calendars are standard, and the computers are Y2K compliant. It's the basic increase in computing power and speed and memory capacity that's led to the development of the multipurpose flow computer.
These advances let you expand the basic design of the computer and extend its life. By selecting microprocessors that have the flexibility and added capacity, you can more easily adapt the basic flow computer to new applications as they arise. It's now feasible to think in terms of customized flow computers produced in smaller quantities for a specific user or market than has traditionally been possible.
I/O Options
As recently as the mid-1990s, any discussion of I/O options for flow computers would have been very brief. You could count the number and type of I/O signals that most flow computers supported on one hand. Today, there are still basic batch-control and mass-flow computers available with limited I/O options, but the trend is toward far more options. Most new flow computers now support a variety of direct inputs from flow sensors, including DC pulse, TTL, open collector, magnetic pickup, dry contact, and analog. Direct RTD inputs are standard features for temperature compensating units, as are analog signals for pressure and density inputs. In many cases, setup software lets you assign and configure I/Os without having to change the hardware.
 |
| Figure 2. By using highly developed display menus and setup software, the complexities of the Nova Flow and SuperTrol are readily managed by the user. The manufacturers have reduced the number of program menu levels that must be navigated. Improvements in display resolution and size have also reduced the use of cryptic terminology in the menu screens, improving readability and ease of use. |
Some flow computers can now take and calculate flows from more than one flowmeter. For example, the Nova Flow can accept and process two flowmeter inputs and two sets of pressure and temperature inputs to derive compensated mass flow for two different flow streams. This capability has previously been available only in the most sophisticated flow computers, costing two to four times as much as the newer systems.
Fully isolated I/Os are also offered for many of the latest generation of flow computers. The KEP SuperTrol offers both an analog and a pulse output, which are isolated and assignable to one of four calculated parameters. The Nova Flow offers two optically isolated analog outputs that are software selectable. The availability of the isolated outputs provides for cleaner and more secure data transmission to remote control or monitoring systems by reducing or eliminating the risk of ground loops, RFI, or EMI. It also reduces emitted noise and radiation, critical for many certification standards.
Digital Communications
Digital communications for flow computers has historically been limited to either RS-232 or RS-485, and both were just options. Today, you should expect one of these to be a standard feature of any system you buy. The SuperTrol has RS-232 and RS-485 as standard features, and the Nova Flow supplies both, with an option for a third RS-232 or RS-485 channel. Also, new flow computers can now interface with standard industrial networks. To date, the most commonly interfaced network is Modbus, but several manufacturers are developing support for Profibus, DeviceNet, and Foundation Fieldbus. Manufacturers have also started preliminary work to support Ethernet systems. These new options represent a quantum leap in the capability and interoperability of flow computers.
In addition to traditional hard-wire communication options, a growing trend is the availability of IR communication links for remote operation and configuration of flow computers. The links are available on a limited basis but will become more widely available in the next year or two. The most commonly used protocol for IR transmission is IrDA, developed by the Infrared Device Association. This is the same protocol used in most consumer electronics. The typical application for the technology in the industrial market has been in flammable or hazardous areas. A handheld IR communicator is used to link to a rate/totalizer, batch controller, or flow computer mounted in an explosionproof enclosure. The flow computer has a built-in IR window in its front panel through which it receives and transmits to the handheld device.
Another option offered with many of the newer flow computers is telephone modems for remote dial-up via landlines, cellular, or radio links. Typically, these modems are offered as external units. This configuration is driven by economics for the most part. It's far less costly to provide a standard external modem available from one of several manufacturers rather than custom build a unit to fit within the confines of a flow computer. The need to meet EMI and RFI limits for certain standards, as well as intrinsic safety ratings, are driving this trend.
| Acronyms |
| The acronyms for quality, interoperability, and safety standards have become a part of the process control industry lexicon.
CE--European Conformity CSA--Canadian Standards Association CENELEC--European Committee for Electrotechnical Standardization BASEEFA--British Approvals Service for Electrical Equipment in Flammable Atmosphere UL--Underwriters Laboratories FM--Factory Mutual |
Data Acquisition/Logging
Thanks in large part to the reduced cost of computer memory, many flow computer manufacturers are now offering basic data logging as either a standard or optional feature. Data logging memory usually falls in the 256–516 KB range. Although small in comparison to dedicated industrial data loggers, this capacity is normally adequate for most flow computer applications. The data can be downloaded on a timely basis to a larger memory and control system when linked via one of the communications options.
Configuration of the data logging function is accomplished via the setup software provided with the computer. Most inputs, outputs, or calculated parameters can be logged, with the frequency being determined by the user. Special functions (e.g., logging peak values) are available with most of the data logging systems.
Safety and Regulatory Conformance
Since their introduction in the 1980s, use of flow computers has expanded into every market on the globe. With this growth has come the need to meet various international, regional, and local standards and practices. These standards and practices encompass product quality, interoperability, and product safety. Acronyms such as CE, CSA, CENELEC, BASEEFA, UL, and FM have become a common part of the process control industry lexicon (see sidebar "Acronyms," page 63). In terms of flow computers, no single product can be expected to meet all likely standards used in every corner of the world. It is, however, becoming common to see flow computers certified for multiple standards. Advances in electronic hardware design have made these multiple certifications possible for many flow computers. Review the manufacturers' specifications carefully when it comes to conformance issues.
A Bigger Niche
From its rather humble beginnings as a simple mechanical counter, the modern flow computer has grown over the last two decades to occupy a significant application niche in flow measurement. Today, you can choose from a variety of products, from basic to sophisticated.
These devices can perform advanced calculations that were once the domain of far more costly machines. Advances in software and electronic design have greatly increased the flexibility, simplicity of use, and interconnectivity of the flow computer.
Bob Carrell is the National Sales Manager of Hoffer Flow Controls, Inc., PO Box 2145, Elizabeth City, NC 27906-2145; 252-331-1997, fax 252-331-2886, bcarrell@hofferflow .com
