Calibrating correctly for remote locations

Monitoring water depth in high altitudes can pose problems if a transducer has been calibrated for operation at sea level. Chris Lilly of Virginia-based Pressure Systems Inc. explains how to make the right choice of instrument.

When a new contract comes in to design a liquid level measurement system for a client requiring a submersible hydrostatic pressure transducer, which one will be best for the job?

By far, the most common format used in making a hydrostatic level measurement is the vented gauge, constructed so the reference side of the actual pressure sensor in the transducer is open to the atmosphere. This is accomplished via a vent tube integral to the transducer cable. When the cable enters the body of the transducer, the tube is connected to a nipple which enters the reference side of the internal pressure sensor. In this configuration, the sensor reference and the atmospheric pressure acting on the surface of the liquid being measured is exactly the same pressure.

Unlike vented gauge, sealed gauge and absolute pressure transducers are identical in their physical construction - i.e. they don't have a vent into the reference cavity of the sensor. The sensor is sealed with a vacuum on the reference side. The absolute pressure transducer's zero output reading is relative to the vacuum. A sealed gauge transducer has its zero output reading electronically elevated to simulate a reference to one nominal atmosphere and is suitable for applications where the effects of daily fluctuations in the atmospheric pressure will not induce errors in measurement levels.

Other factors to consider
An absolute pressure transducer is best suited for jobs that involve a system or tank which is under pressure and not open to the atmosphere. If it is to be used to measure a liquid level where the liquid is exposed to atmospheric pressure, a secondary barometric pressure measurement will have to be made so that it can be subtracted from the absolute measurement, leaving only the pressure exerted on the transducer by the liquid being measured.

Vented gauge 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 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 gauge, is that the vent tube provides a possible path for moisture from the atmosphere to enter the pressure transducer. The accurate performance of a vented gauge 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. Therefore a moisture protection device is necessary.

The 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 allows air to pass through the desiccant, which absorbs the moisture as the barometric pressure changes.

Another method 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 these barometric changes, thereby equalising the pressure in the vent tube. 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 mm in diameter; direct exposure to water will cause their rapid corrosion and result in failure of the transducer.
In-the-field remedies can be attempted if moisture does get in. The transducer cable can be coiled and the cable and transducer placed in a pan 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 the transducer and cable may be damaged. Alternatively, the cable and transducer can be suspended in a vertical position (transducer end up) overnight to drain out the water.

Sealed gauge applications
There are cases where it is not practical to use either the desiccant cartridge or the aneroid bellows. The job may be in a remote area 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 gauge pressure transducer is the best option.

Since a sealed gauge transducer is electronically set to simulate the effects of one nominal atmosphere, it is important to take into account the elevation where the transducer will ultimately be installed. Normally, these transducers are calibrated with the zero output compensated for one nominal atmosphere at sea level.

However, if the installation is to be made in a remote mountainous area or in a town that is well above sea level, it is imperative that the manufacturer of the transducer is aware of the elevation at the installation site. Using the elevation information from the customer, the manufacturer can calculate the nominal atmospheric pressure at the installation location and will set the zero output for that pressure.
In summary, the most accurate pressure format to specify for any open-atmosphere system is vented gauge. 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 gauge, a sealed gauge is the next best option. Absolute transducers are best suited for closed systems under pressure or vacuum and not subject to the effects of atmospheric pressure.


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