Extend edge data gathering with multivariable instruments

A little recognized benefit of advanced process instrumentation is its ability to often measure more than one variable. For example, a pressure transmitter may provide a temperature reading. This is not the case with every type instrument, as some have more capabilities than others. The simpler the instrument, such as those used to measure temperature, the fewer variables. On the other hand, complex instruments such as differential pressure (DP) flow meters often measure or infer multiple process variables.

These secondary, tertiary and greater variables result from a variety of sources. Some are provided simply for compensation of the primary measurement. For example, many sensors used to measure pressure (piezoresistive, capacitive, and others) require compensation for temperature, so the transmitter must include an internal temperature sensor as the adjustment factor.

In other situations, including a different type of sensor extends the capability and versatility of an instrument. Let’s look at these and other scenarios and examine capturing additional information in an Industrial Internet of Things (IIoT) context.

Strictly compensatory

As mentioned, sensing elements can be influenced by operating conditions. For example, a piezoresistive sensor used in a pressure instrument delivers a different signal for a 100-pounds per square inch (psi) reading at 10°C than it will at 80°C. The transmitter must have an internal sensor to determine the pressure sensor’s temperature so the reading can be adjusted appropriately.

A clever engineer may realize there is a provision for capturing the temperature reading. But how can this be realized in practice? First, the instrument maker must provide the compensatory reading for the pressure sensor itself, so the temperature sensor is located to capture that value, but this temperature may not reflect the process. It can in some situations, but the pressure sensor may also be some distance from the process fluid and effectively insulated. The temperature reading may simply monitor the transmitter housing interior. Anyone using the temperature reading must understand where it comes from and what it represents.

Intentionally variable

In some situations, instruments are intended to provide a range of information, and therefore one or multiple sensors are inserted into the process media to attain the highest degree of accuracy. Let’s examine a specific example.

DP flowmeters are the most common technology for flow measurement. The concept is simple: when an obstruction (the primary element) is placed in the path of a fluid flow, there will be a pressure drop proportional to the volume of fluid moving through the primary element. A DP transmitter measures the pressure drop, and its electronics convert the pressure drop into a flow reading. This is the primary variable from the transmitter, but what other measurements are possible?

Multivariable transmitters have additional sensors within a single transmitter. An additional pressure sensor is located within the transmitter module to measure the line pressure. Instead of simply knowing that the differential pressure is 3 psi, this additional measurement allows the pressure on the primary element’s upstream and downstream sides to be known. Additionally, multivariable transmitters can take readings from temperature sensors. These valid process readings can be used as individual values, without need for separate pressure or temperature transmitters on the same line.

Calculating flow only requires knowing the differential pressure. This may be enough but oftentimes a flow measurement point requires full compensation of more than 25 parameters, including density, viscosity, and discharge coefficient. Critical to compensating for these parameters is temperature. Getting accurate process temperature readings requires a temperature sensor in the right location (Figure 1), which under most circumstances means another process penetration. A DP flow meter assembly can incorporate the temperature sensor as part of its installation, ensuring an accurate reading for a reliable mass flow measurement.

Having known characteristics

When it is possible to measure DP, line pressure and fluid temperature, these three values can be combined with measurements fed into the transmitter’s configuration (Figure 2). If plant personnel provide the fluid type, primary element configuration and line size to the transmitter, a range of process measurements can be calculated, such as:

• Mass flow
• Volumetric flow
• Energy flow, and
• Totalized flow.

These measurements can be generated by one instrument with one primary output. If such a sophisticated instrument is installed in a conventional analog I/O environment, the operators will get one variable—flow—and that’s it. An engineer watching the transmitter’s local display may see an indication of what is really going on inside. It will step from variable to variable, showing the flow value, DP reading, temperature and perhaps line pressures depending on how the transmitter has been configured. This represents a missed opportunity.

Capturing supplemental data

A technician trying to interface with existing process instruments in most environments has a challenge. Field instruments communicating with a distributed control system (DCS) via conventional 4-20 mA analog loops do not interface easily with IIoT networking protocols. It may be possible to access a specific instrument through the DCS or historian connected to it. However, if the DCS’s I/O infrastructure is more than a few years old, it may be limited to plain analog loops.

So how does a multivariable instrument, such as the DP flow meter, send its extra data to a host system?

The easiest answer is via a digital process fieldbus protocol, such as Foundationwpe Fieldbus. Protocols can send enormous amounts of information easily, at least in terms of data volumes created by field instruments, which by IT standards is still pretty low. A DP flow meter can send one, two or more variables via the fieldbus signal, and the host system can handle all the data and information. Reaching into the host system using IIoT protocols such as EtherNet/IP should be manageable and straightforward. However, plants using fieldbus I/O are still a minority, although this situation is improving.

A second solution, almost as easy as a fieldbus, depending on the DCS, is using HART-enabled I/O. If the DCS is less than 10 years old, it may be equipped with smart I/O able to read HART information superimposed on the conventional 4-20 mA signal. This HART signal can be detected and decoded by the host system, revealing additional data sent by the instrument. If the plant is progressive and this type of I/O is in use, extracting the extra data is similar to a fieldbus system, although the bandwidth available with HART is lower, so information will not flow as quickly. Unfortunately, however, even some new DCS platforms do not include HART-enabled I/O.

If neither fieldbus or Hart are available, the solutions become more complicated. Some plants overcome the limitations of conventional I/O by adding HART multiplexers. A single device captures HART data from a group of instruments, scanning through them in sequence. Bandwidth limitations make it a slow process, unsuitable for monitoring fast-changing variables.

Using a HART signal converter is a different way to capture the additional readings from a single instrument, such as a DP flow meter. It reads data coming from the transmitter, captures the additional variables and delivers them as 4-20 mA outputs. It’s as if the multivariable instrument was actually a group of separate instruments, each with its own 4-20 mA output.

Simplicity of wireless

Absent one of the sophisticated I/O approaches just mentioned, adding a WirelessHART adapter (or using a native WirelessHART transmitter) is arguably the best interface to get process data, whether simple or multivariable, into an IIoT environment. The adapter (Figure 3) mounts on a transmitter housing and can send data to a WirelessHART gateway without disrupting the established wired connection to the host system. Since WirelessHART is digital, the gateway can convert it to Ethernet or other protocol such as Modbus RTU. The number of plants installing WirelessHART infrastructure has increased rapidly over recent years, so for many it is a simple matter to add one more device to the network. Most HART-enabled instruments can be configured to prioritize variables as needed.

Using WirelessHART solves both multivariable capture and analog-to-IIoT conversion problems, allowing users to maximize the versatility of instrumentation and technology networking capabilities, even in a legacy environment. The result is an installation easily accessible using leading IIoT technologies and protocols.

This article appears in the IIoT for Engineers supplement for Control Engineering and Plant Engineering. See other articles from the supplement below.

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