Question

Critically explain the effects of temperature (both when temperature increases and temperature decreases) on level measurement with suitable expression.   

Answer 1

Level measurement systems that use differential pressure ΔP as the sensing method, are by their very nature affected by temperature and pressure.Level is measured at the position of the interface between phases, where the phases are liquid/gas, solid/gas, or immiscible liquid/liquid. Level is simply a measure of height. It defines the position of the interface, that is, the surface where the two phases meet with respect to a reference point. This measurement is often converted to a volumetric or gravimetric quantity.

Integral to process control in many industries, level measurement sensors fall into two main types. Point level measurement sensors are used to mark a single discrete liquid height–a preset level condition. Generally, this type of sensor functions as a high alarm, signaling an overfill condition, or as a marker for a low alarm condition. Continuous level sensors are more sophisticated and can provide level monitoring of an entire system. They measure fluid level within a range, rather than at a one point, producing an analog output that directly correlates to the level in the vessel. To create a level management system, the output signal is linked to a process control loop and to a visual indicator.

Temperature can have a negative effect on the accuracy of the level measurement. This article will cover the why temperature has the effect and how this effect can be reduced.

Density (mass per unit volume) of a liquid or gas is inversely proportional to its temperature.

ρ α 1/T

Thus, for any given amount of liquid in a container, the pressure P exerted at the base will remain constant, but the height will vary directly with the
temperature.

H α T

Consider  temperature changes around a liquid storage tank with a wet leg. As temperature falls and the wet leg cools off, the density of the liquid inside it increases, while the temperature in the tank remains practically unchanged (because of a much bigger volume and connection to the process).

False high level indication can be caused by an increased wet leg temperature, gas or vapour bubbles or a drained wet leg.

A high measured tank level, with the real level being dangerously low, may prevent the actuation of a safety system on a low value of the trip parameter.

Consider the following scenario. A given amount of liquid in a container [figure (a)] is exposed to higher process temperatures [figure (b)].

As the amount (mass) of liquid does not change from figure (a) to (b), the pressure exerted on the base of the container has not changed and the indicated height of the liquid does not change. However, the volume occupied by the liquid has increased and thus the actual height has increased.