However, many level measurement systems inherently do not measure level --- they infer level. Most notable is hydrostatic level measurement that infers level using fluid density. Whereas level measurement is (theoretically) supposed to determine the location of an interface, hydrostatic measurement measures the effective mass of the fluid and infers the location of the interface. Therefore, the (inferred) level measurement will vary when the density of the material in the vessel changes.

Other examples of inferred level measurement include ultrasonic and radar level measurement systems where the measured distance from the sensor to the material is used to infer the level of the material.

The most common application of level measurement systems is to determine the inventory of a contained material. Although this may be desirable for the efficient operation and safety of the process, level measurements are also used for accounting purposes.

Excerpted from The Consumer Guide to Non-Contact Level Gauges.

Removing a differential pressure transmitter from service in a "normal" installation is relatively straightforward.  You should start by opening the bypass valve to create a hydraulic jumper across the transmitter that protects the transmitter from pressure transients that might create a large differential pressure across the transmitter.  The high-side and low-side shutoff valves can then be closed to isolate the transmitter.  Putting the transmitter back in service entails ensuring that the bypass valve is open (creating the hydraulic jumper) before opening the high-side and low-side shutoff valves. 

My student's objection was that using this procedure in high-pressure steam service could cause injury to the instrument technician and any people who happened to be nearby.  In steam service, the impulse tubing is designed to create condensate seal legs that isolate the transmitter from live steam.  Opening the bypass valve first will cause flow to occur in the impulse tubing because the upstream pressure tap is at a higher pressure than the downstream pressure tap.  This flow will cause the condensate seal to be removed from the impulse tubing and expose the transmitter and its associated impulse tubing to live steam.  Aside from potentially damaging the transmitter, this sudden thermal shock could cause the normally cold impulse tubing to fail and potentially cause injury. 

To remove a transmitter from steam service, the student recommended first simultaneously closing the high-side and low-side shutoff valves before opening the bypass valve.  This procedure effectively maintains the condensate seals so live steam does not reach the transmitter or most of the impulse tubing.  Similarly, returning the transmitter to service should be performed by first closing the bypass valve and then simultaneously opening the high-side and low-side shutoff valves. 

This procedure can allow the transmitter to be exposed to an excessive differential pressure and potentially damage the transmitter.  However, it also reduces the potential for injury --- which makes perfect sense. 

This article originally appeared in Flow Control magazine. 

1. Decrease proportional to the square root of steam density
2. Decrease proportional to the steam density
3. Increase proportional to the square root of steam density
4. Increase proportional to the steam density
   

Differential pressure flow measurement devices are commonly applied to measure steam flow.  In addition, pressure and temperature measurements are typically implemented using individual or multivariable transmitters.  The compensated steam mass flow rate can be calculated using flow computer functionality that can be implemented in a flow computer or other computational device. 

For differential pressure flowmeters, the volumetric flow (Q) is proportional to the square root of the ratio of the differential pressure (?P) divided by the fluid density (?). However, the mass flow (W) is the product of the volumetric flow and density. Therefore, for a given raw differential pressure measurement, the compensated mass flow of steam is proportional to the square root of the steam density.  Answer C is correct.

Additional Complicating Factors

Flow computers typically use steam tables to determine the steam density.  Using other devices with more rudimentary density algorithms (such as the gas laws) may result in significant mass flow measurement error.

This article originally appeared in Flow Control magazine.