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.


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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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