DAVID W SPITZER'S E-ZINE (December 2025)

Bridging across the probes can occur and cause a measurement that is higher than the actual level in the vessel. When accumulations over time are normal for the process, routine maintenance may be required to keep the system operating. Careful analysis of the sensor design with regard to its ongoing maintenance should be performed.

The material itself can cause the intensity of the reflected radar signal to degrade when the material exhibits poor reflective qualities on the surface of the material such as when contaminants on the surface of the material cause the energy to reflect poorly. Notwithstanding other problems, radar contact level measurement is typically limited to materials with a dielectric constant greater than approximately 1.3 to 1.7. Some users report that they do not use these devices in applications where the dielectric constant is less than 1.9. Liquids with lower dielectric constants generally do not reflect well and may cause the level measurement to be erratic or fail to operate. That said, at least one supplier claims to be able measure the level of material with a dielectric constant of 1.10.

Radar contact level instruments are designed to measure the material level. However, foams that have a relatively large dielectric constant will conduct a significant amount of radar energy. This can cause a reflection that essentially causes the instrument to measure the level of the foam rather than the actual desired measurement surface.

In addition, accuracy can be degraded based upon the surface on which the energy is reflected. For example, level measurement may typically measure the top of a layer of foam due to its ability to absorb radar energy. However, the characteristics of the foam, such as its composition, density, bubble size, and the like, can cause the foam to not absorb the radar energy and measure the level of the material. At other times, the foam might absorb radar energy and measure a point interior to the foam layer. In this application, varying amounts and consistency of foam can cause the measurement to become erratic.

Excerpted from The Consumer Guide to Capacitance and Radar Level Gauges

One of the gases was fed from high pressure gas cylinders, while the other gas was generated in a process unit located approximately 100 meters from the flowmeters. Both flowmeters had sufficient straight run and were installed properly. There were pressure and temperature measurements downstream and upstream of the orifice plate respectively. Flow controllers were used to control the gas flows, and I seem to recall that a multiplier was used to ratio the flows.

Observation of the operation and its recordings revealed that the controlled flow rates of both gases were both controlling at set point, but the process gas flow was less steady than the cylinder gas flow. Both gas temperatures were steady. The pressure of the cylinder gas was steady and at its design pressure. However, the pressure of the gas from the other process unit would vary somewhat near its design pressure and occasionally drop off and recover a few minutes later.

As in many applications, the problem lies in more than one location. First and foremost, these flow measurement systems were not compensated for pressure and temperature variations.

The pressure of the gas from the cylinders should be compensated because even though it could easily be controlled, the pressure gauge used to control its pressure could be in error (or be read incorrectly) and cause significant flow error. The temperature of the gas varied with ambient conditions, so flow errors of a few percent were likely.

The most ominous problem was that of the process gas where the pressure at the flowmeter could vary between 60 to 120 percent of its design pressure and cause significant flow measurement errors. Even when steady, the operating pressure introduced errors because the pressure was only close to its design condition. Temperature variations were less dramatic than the cylinder gas, but could still produce measurement error. Pressure and temperature compensating this gas flow would mitigate the problem and likely eliminate the symptoms.

Pressure and temperature instruments for most flowmeters locate these measurements upstream and downstream of the flowmeter respectively. Note that the designer of this installation located the pressure transmitter downstream on the downstream tap of the orifice plate and the temperature transmitter upstream of the orifice plate. This did not cause problems with this particular installation (but it could with other installations) because orifice calculations were available for the downstream pressure tap location. The temperature transmitter location did not cause a problem because it was installed far enough upstream to not distort the velocity profile in the flowmeter and the temperature difference between the upstream and downstream locations was small.

It is apparent that the person(s) who designed this installation likely did not consider the need for pressure and temperature compensation, and if they did, they did not understand where the transmitters are commonly located. Recognizing that a temperature variation of 3 degrees Celsius will alter the volume of a gas by approximately 1 percent at near atmospheric conditions, temperature compensation should be considered in virtually all gas applications. Similarly, relatively small pressure variations can cause significant volume changes, so pressure compensation should be considered in virtually all gas applications.

Even if it is decided not to pressure and/or temperature compensate the flowmeter, properly locate and install taps for a pressure and temperature transmitters. They are relatively inexpensive... and you never know when you will need them. 

 This article originally appeared in Flow Control magazine.

Note that a change has occurred in only one parameter, yet the effects of this single change can affect more than one aspect of the flow measurement. In this application, the temperature has changed by only a few degrees.  It would seem that this would not pose a problem, but...

First, increasing the gas temperature will also increase the temperature of the pipe wall and cause the pipe to expand. This will produce a corresponding increase in the pipe cross-sectional area. Similarly, the flowmeter itself will increase in size. Because flowmeter performance is predicated on the geometry of the flowmeter in the piping, a change in gas operating temperature can adversely affect the flow measurement.

Second, gas density decreases as its temperature increases. In this application (per Boyle's Law), a gas temperature increase of 3 degrees Celsius decreases the gas density by approximately 1 percent. Therefore, the 10 degrees Celsius temperature increase will decrease the gas density by approximately 3 percent. This change will affect the measurements from linear flowmeters by approximately 3 percent, and differential pressure producers by approximately 1.5 percent.

Third, increasing gas temperature typically increases the viscosity of the gas.

Fourth, at the same flow rate, increasing pipe size, increasing viscosity, and decreasing density tend to decrease Reynolds number. Reynolds number changes can also affect the flow measurement.

The effect of a temperature change on the flow measurement is the combination of the effects of changing geometry, changing fluid density, changing fluid viscosity, and changing operating Reynolds number. Detailed analysis may be necessary to quantify their effects on the flow measurement.

Additional Complicating Factors

The 10 degrees Celsius temperature change cited in this application might be considered to be relatively small, especially in an outdoor location. Significantly larger temperature fluctuations routinely occur in many processes that can result in correspondingly larger effects on the flow measurement.

This article originally appeared in Flow Control magazine.