Mounting
Each technology has its own mounting concerns.
Obviously, head pressure is mounted on the bottom end, whereas ultrasonic is mounted on the top end. So access for either has to be available. The cost of making access, like dealing with a sterile tank in a sanitary application might outweigh the minor benefits of an alternative technology.
Head pressure can be sensed with a tiny impulse tube (although 3" flanges are not uncommon) but whatever means are used to transfer the liquid head pressure is subject to
- forces of fluid motion
- venturi effect in a drain line
- agitator developing a force against the sensing element
- fill fluid dropped from a height
- blockage. Sludge can build up and if it hardens, prevent the transfer of head pressure to the sensing element.
- head pressure can not 'see' head pressure below the elevation point of the tap.
Ultrasonic beams are somewhat conical in shape, expanding outward from the diaphragm. Ultrasonic transducers have a "beam angle" spec, lke 10°, that defines how widely the beam 'spreads' as it moves out from the transducer. The implication of beam angle is that generally ultrasonic needs to be mounted some distance away from the side wall to enable the conical shaped energy pulse to expand without smacking a side wall too soon which would provide a strong false echo.
The stand-offs that are on the tops of some large tanks present specific problems for ultrasonics if the diameter to height ratio is not sufficient, because the beam hits the lower edge of the tank ceiling.
Anomalies
Ultrasonic doesn't deal well with round bottom tanks when the level drops really low. Usually the setup is to do a cutoff at a low level where ultrasonic confidence level drops off. Head pressure can measure this same scenario, but needs some form of characterization to get volume from a round bottom dish.
Ultrasonic has to deal with the interior of the tank, as others have mentioned: agitators, ladders, baffles, manways. Signal processing tweaking is sometimes necessary in which the skill to tweak is either learned or purchased as a service.
Medium
Each technology has peculiarities about the medium it works in.
Ultrasonic, being non-contact, does well in acid tank applications that require very expensive alloys for pressure transmitter wetted parts.
Ultrasonic transducers work at different ultrasonic frequencies. I discovered that one particular frequency was totally absorbed in the CO2 vapor blanket. No echo at all.
The organic materials used for ultrasonic transducers are temperature limited. Whereas a 1/2" steel impulse line on a pressure transmitter will drop 100°F per foot of tubing/pipe, to allow a pressure transmitter to remain cooler than the process, the ultrasonic sees the temperature above the process in a tank. The flip side is that head pressure transmitters used for water tanks might need to be heat traced so that the impulse tubing doesn't freeze in cold climate winters.
Head pressure is the clear winner for pressurized vessels. The speed of sound varies with gas blanket density, so ultrasonic has to be tuned somehow to compensate for gas density changes. Most ultrasonic have fairly low pressure ratings (like atmosphere), whereas pressure transmitters can take very high DP pressures. All modern ultrasonics, to my knowledge use temperature measurement at the transducer to compensate for air density changes. The key is that it's 'air' density compensation, which might or might not correspond to the speed of sound in a chemical vapor blanket changing with temperature. Radar is much better top-mount non-contact for these applications.
Ultrasonic is not great at shooting through steam. Radar is much better.
Condensation on ultrasonic transducers generally absorbs much of the signal if the transducer can not 'blow' the condensation droplets off the transducer when it pings.
Turbulence jitter has to be dealt with by both technologies, generally by averaging readings.
Hazardous area
Head pressure transmitters can be bought as either hazardous area explosion proof or intrinsically safe. All the ultrasonic I am aware of are EXP only, since the ping pulse exceeds IS energy levels.
Rangeability
All ultrasonic transducers have a dead zone from the tip of the transducer extending some distance, typically about a foot (~300mm). Pressure has no deadband to deal with.
Pressure transmitters typically have much wider rangeability than ultrasonic. The range of typical pressure transmitter can be 0 to 30" (<1m) or up to 0 to 400 or 500 in. w.c. (~12m water) Because the energy needed to shoot longer distances increases so dramatically, long shot ultrasonic transducers are large and bulky, whereas short shot transducers are small. But each has a limited range, for instance a short shot might be 1' to 20' (0.3m top ~6m). This means that tanks can not be filled full, without loosing level measurement within the upper dead zone.
Power
Wiring and power play a role, too. If a 66' (20m) tank already has a twisted pair for 2 wire loop powered DP cell, consider the cost of running wire for AC power needed to shoot 66' with an ultrasonic. In some cases it might pay, in others, the AC wiring cost would be prohibitive.
2 wire loop power ultrasonic is for short to medium shots, not long shots.
Viewing the tank level
If you have ultrasonic 2 wire loop power, it's display is up on top of the tank. Who's going to see it up there? Does that mean you need to put a loop powered indicator somewhere so the operator on the ground knows the level? Low mounted DP has the same problem, sometimes the access point is not terrible convenient for the use of the transmitter's indicator. The ultrasonics that use a remote transducer and a electronic indicator/transmitter box are useful for putting the level reading at eye height, but need to be in non-hazardous areas and require AC power.
Submersible head pressure
Submersible pressure transmitters are used for wells and their limitations are general purpose cable and vent materials suitable for water (not most chemicals) and the need to prevent condensation buildup in the vent tube. The temperature in wells is frequently below the dewpoint of humid summer air and moisture in the vent will condense, drop to the bottom and gradually offset the reference side reading (which should be atmosphere).
Solids measurement
Head pressure cannot do this at all. The advances in ultraonic signal processing, which is now in its fifth generation can provide fairly accurate measurements of solids.
Open Channel Flow
Flow measurement by level measurement behind a weir or open channel restrictive flow element has a long history of being done well with ultrasonics. Why mess with success?
Dan
