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KPSI Level & Pressure Transducers: Application NotesSealed-Vented-Absolute: How To Know Which Transducer to Specify |
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Challenge: "A job to design a liquid level measurement system has just come in. The client requires a submersible hydrostatic pressure transducer in the system, but these transducers can be built in three different pressure formats-psig (vented gage); psis (sealed gage); or psia (absolute). Which format is best for this job? The most important factor in deciding which transducer to use is to make sure the difference between the three pressure formats is understood." The Differences Between Pressure Transducers Determining Which Transducer to Use  Now
that the differences in construction between the different pressure formats have
been identified, it is time to consider other factors to help determine which
will be right for the job. An
absolute pressure transducer is best suited for jobs that involve a system or
tank that is under pressure and not open to the atmosphere.
If it were to be used to measure a liquid level where the liquid is
exposed to atmospheric pressure, a secondary barometric pressure measurement
would have to be made so that it could be subtracted from the absolute
measurement, leaving only the pressure exerted on the transducer by the liquid
being measured.
Vented
gage is the most common format used for a hydrostatic level measurement because
of the open reference side of the sensor. Since
atmospheric pressure is also acting on the surface of the liquid being measured,
the effects of changes in atmospheric pressure on the measurement are negated.
The end result is that the measurement is the most accurate possible
reflection of the level of liquid above the pressure transducer.
The one drawback to vented gage, however, is that the vent tube provides
a possible path for moisture from the atmosphere (i.e. humidity or rain) to
enter the pressure transducer. The
accurate performance of a vented gage transducer depends upon keeping moisture
out of the vent tube since the weight of the accumulated liquid will cause level
readings that are lower than the actual liquid level.
It is therefore necessary to use a moisture protection device to prevent
moisture-laden air from entering the transducer.
Extending the Life of
the TransducerThe most common method for preventing moisture incursion is the attachment of a desiccant-filled cartridge to the vent tube at the cable's electrical termination end. This will allow air to pass through the desiccant, which absorbs the moisture in the air, as the barometric pressure changes. Some manufacturers use an indicating desiccant that changes colors as it becomes saturated so that cartridge replacement is apparent upon inspection. Another method to prevent moisture from entering the transducer is to attach an aneroid bellows to the vent tube. The bellows, a closed system that prevents moist air from entering the vent tube, expands and contracts with changes in the barometric pressure, thereby equalizing the pressure in the vent tube with the current barometric pressure. Either the bellows or the desiccant cartridge can be mounted in a junction box or panel near the electrical termination of the transducer cable. The longevity of the sensor also depends upon keeping moisture out of the vent tube. Gold bonding wires internal to the sensor are only a few mils in diameter; direct exposure to water will cause their rapid corrosion and result in failure of the pressure transducer. A couple of in-the-field remedies can be attempted if moisture does get into the vent tube and pressure transducer. The transducer cable can be coiled and the cable and transducer placed in a pan. The pan should be placed in an oven at 50°C for two hours to dry the transducer and its components. It is important that the temperature does not exceed 50°C or damage to the transducer and cable may occur. Alternatively, the cable and transducer can be suspended in a vertical position (transducer end up) overnight to allow the water to drain out. There may be cases where it is impractical to use either the desiccant cartridge or the aneroid bellows. The job may be in a remote location where it is not possible to periodically check the desiccant and see if it needs replacement or there may not be a suitable location to mount the aneroid bellows. Also, the level to be measured may be so deep that the effects of daily atmospheric pressure changes will have a negligible effect on the measurement. In instances such as these, a sealed gage pressure transducer is the best option. Additional Considerations Conclusion |
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In
summary, the most accurate pressure format to specify for any open-atmosphere
system is vented gage. Whether used
in a lake, river, reservoir, well, or sewage lift station, a vented gage
transducer will negate the fluctuations in atmospheric pressure and ensure the
most precise indication possible of the actual level of the liquid being
measured. If it is not practical
from a maintenance point of view to use a vented gage, then sealed gage is the
next best option. Absolute
transducers are best suited for closed systems that are under pressure or vacuum
and not subject to the effects of atmospheric pressure.
