Process measurement / transducers---part 5: Level transmitters

There are numerous ways to measure level that require differing technologies and to encompass all the various units of measurement.

• • Ultrasonic, transit time
• • Pulse echo
• • Pulse radar
• • Pressure, hydrostatic
• • Weight, strain gauge
• • Conductivity
• • Capacitive

For continuous measurement, the level is detected and converted into a signal that is proportional to the level. Microprocessor-based devices can indicate level or volume.

Different techniques also have different requirements. For example, when detecting the level from the top of a tank, the shape of the tank is required to deduce volume.

When using hydrostatic means, which detects the pressure from the bottom of the tank, the density is to be known and remains constant. Level sensing is a simpler concept than most other process variables and allows a very simple form of control. The sensors can be roughly grouped into categories according to the primary level sensing principle involved. The signals produced by these means must then be converted into a signal suitable for process control applications, such as an electrical, pneumatic or digital signal.

Installation considerations

The following are outlines of the more important considerations that need to be considered when installing either atmospheric or pressurized vessels.

Atmospheric vessels:

Most instruments involved with level detection can be easily removed from the vessel.

Top mounting of the sensing device also eliminates the possibility of process fluid entering the transducer or sensor housing should be the nozzle or probe corrode or breaks off. Many level measurement devices have the added advantage that they can be manually gauged. This provides two important factors:

1. Measurements are still possible in the event of equipment failure

2. Calibration and point checks can provide vital operational information.

One common installation criteria for point detection devices is that they be mounted at the actuation level, which may present accessibility problems.

Pressurized vessels:

Two main considerations apply with level measurement devices in pressurized vessels:

1. Facilities for removal and installation while the vessel is pressurized

2. The pressure rating of the equipment for the service.

Pressurized vessels can also be used to prevent fugitive emissions, where an inert gas such as hydrogen can be used to pressurize the process materials. Compensation within the level device needs also to be accounted for as the head pressure changes.

The accuracy of the measuring device may be dependent on the following:

• Gravity variations

• Temperature effects

• Dielectric constant.

Also the presence of foam, vapor or accumulated scum on the transducer affects the performance.

Impact on the overall control loop:

Level sensing equipment is generally fast responding, and in terms of automated continuous control, does not add much of a lag to the system.

It’s good practice though, to include any high and low switch limits into the control system. If the instrumentation does fail or goes out of calibration, then the process information can be acquired from the high and low limits. Apart from the hard-wired safety circuits, it’s good practice to incorporate this information into the control system.

Future technologies:

The cost of sensing equipment is not a major consideration compared with the economics of controlling the process. There is therefore a growing demand for accuracy in level measuring equipment.

Newer models incorporate better means of compensation, but not necessarily new technologies. Incorporating a temperature compensation detector in the pressure-sensing diaphragm provides compensation and also an alternative to remote pressure seals and ensures the accuracy and stability of the measurement.

Greater demands in plant efficiency may require an improved accuracy of a device, not just for the actual measurement, but also to increase the range of operation. If the safety limits were set at 90% due to inaccuracies with the sensing device, then an increased range could be achieved by using more accurate equipment.

Demands are also imposed on processes to conform to environmental regulations.

Accurate accounting of materials assist in achieving this. Such technologies as RF admittance or ultrasonic minimize the expense of this environmental compliance. Problems occur in trying to sense level in existing vessels that may be non-metallic. RF flexible cable sensors have an integral ground element which eliminates the need for an external ground reference when using the sensor to measure the level of process materials in non-metallic vessels.

User models in measuring transducers

As an example of the extremes that can occur between the same type of measuring transducer, consider the case of the thermocouple. Firstly in its most simple form, it consists of two dissimilar metal wires joined together to form a loop consisting of two junctions or connections. The Seebeck effect (If the temperature of the two junctions is different, a current will flow in the loop.) then comes into play. Looked at in practice, a thermocouple-measuring circuit actually measures the difference between the two junctions forming the circuit. Unfortunately three major problems occur with this form of temperature measurement.

Voltage generation of a thermocouple

Only very low emfs are generated, typically around 1.8 to 6.0 × 10^-12 V per 0 °C; so electrically induced noise, either as the normal or common mode type, can become a problem. Normal mode noise being the more difficult one to remove from a system, this usually being achieved by the introduction of guard lead wires, or careful cable screening.

Thermocouple linearity

The output of any type of thermocouple is not linear relative to the applied or measured temperature range, This can cause linearity, scaling, ranging and calibration problems.

Cold junction compensation

To complete any electrical circuit, requires the formation of a loop, so in the case of the thermocouple, as second junction has to exist to achieve this. This is called the 'cold junction' and usually sits at ambient temperature, which of course varies and introduces measurement errors which can be extremely large, especially if measurement of the physical quantity is close to the ambient or cold junction temperature.

The simplest form of thermocouple application is in the form of a galvanometer which has the sensitivity to measure the low voltages involved. This is equipped with a temperature-sensitive compensating resistor, located next to the input terminals where the measuring circuit's cold junction is. This resistor forms part of that measuring circuit and corrects the effect, by changing its resistance and hence the current flow in the circuit, of ambient temperature changes.

The problem with this arrangement is that it’s direct reading, and hence does not easily lend itself to inclusion in process control systems, and the physical circuit from the indicator to the thermocouple measuring tip has to be 'tuned' to a specific resistance for the cold junction compensator to be accurate. To overcome the non-linearity problem, the scale of the instrument is scaled to the 'profile' of the related response curve of the thermocouple type being used.

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