What type of liquid is to be measured?
This will help select a device with the most compatible construction materials. Submersible pressure sensors are comprised of sensing module, body, cable and seals, all of which must be compatible with the substance in which the sensor will be immersed.
What is the specific gravity or density of the liquid compared to freshwater?
The hydrostatic pressure generated by a fixed height of liquid is proportional to the specific gravity. 10 metres of water is a very close approximation to a 1 bar hydrostatic pressure, but if the specific gravity is say 0.9 of water (fresh water has a specific gravity of 1.0) a 10 metre head of this liquid would generate a 0.9 bar hydrostatic pressure instead. The output of some submersible pressure sensors can be scaled to correct for specific gravity without the need for the installer to compensate for it within the signal conditioning instrumentation.
How viscous is the liquid media?
This will help in determining whether a flush profile is required for the sensing element. High viscosity liquids can choke the protective caps and pressure ports that are fitted to the end of the sensor.
What is the length of cable required from the deepest transducer position to the termination point?
This may seem like an obvious question, but it is not always possible to terminate the sensor cable at the point where the sensor is suspended from and instead it may need to be connected some distance away from the drop point. When extending a submersible cable it is important to ensure the vented reference is kept dry and open to ambient air pressure. The cable will need to be long enough to reach a dry vented enclosure such as a termination junction box.
What is the maximum and minimum level of liquid that will be measured from the intended transducer location?
The total depth of liquid is not necessarily the required hydrostatic range. To improve accuracy of measurement the sensor range should be specified for the change in level. For example a 30 metre deep reservoir may only vary in level by 5 metres throughout the year, so only a 5 metre range is necessary if it is possible to suspend the sensor 5 metres below the maximum water level.
Is there any restriction on the diameter of the transducer that can be used?
For some installations such as hydrological surveys a small diameter borehole is drilled which precludes the use of many submersible pressure sensor probes which are typically more than 1 inch (25.4mm) in diameter. Also, since many submersible sensors are relatively long compared to the diameter they can become snagged in the hole if there is not sufficient room to allow free movement when installing the probe. Where the liquid level is very dynamic the sensor can also restrict the rise and fall time of liquid in the borehole if the probe has only a slight clearance tolerance with the borehole.
Is the air above the liquid surface open to atmosphere or sealed to a fixed or varying gas pressure?
On occasions it is necessary to cap the top of a well or tank to control the pressure or type of gas present directly above the liquid. In these applications the liquid level reading should be compensated by using an additional pressure sensor to monitor the gas pressure on top of the liquid so it can be subtracted from the level reading to obtain the true reading.
What will be the highest and lowest temperature of the liquid?
All submersible sensors have an operating temperature range which should cover the variation on temperatures for most applications. In more extreme environmental conditions or where there are high process generated temperatures, the operating limits of the sensor maybe exceeded. The electronics housing and cable are completely immersed in the liquid and since they tend to be the lowest rated components for temperature, they should be checked for a compatible temperature rating. Although it is less likely to occur there may also be situations where the temperature is too low for the sensor, which may cause damage to the electronics or cable due to thermal contraction or brittleness brought on by the low temepratures.
What signal output is required by the monitoring instrumentation?
4 to 20mA current loop is the most robust signal and can be used over long distances without degradation from wire losses. O to 10 or 5 volts dc is convenient for use with data acquisition cards and low millivolt signals are best for remote sites with limited battery power. SDI and Modbus are standard digital protocols used mostly by hydrologists for high precision data collection.
Will the transducer be located in an area classed as a hazardous zone?
In Europe many sites such as refineries, waste water treatment plants and landfill sites are required by health and safety authorities to ensure that installations are ATEX approved to reduce risks of explosions from flammable gases.
Is the region where the installation is going to be located prone to lightning strikes?
Installations outdoors will always be at risk at some time or another from lightning strikes, but some regions are at more risk than others due to regional geography and weather characteristics. In these areas where strikes on instrumentation are numerous, some form of lightning protection maybe necessary to reduce the risk of failures. Although nothing is likely to withstand a direct lightning strike, many failures are caused by voltage surges from local strikes rather than direct hits, so high voltage surge protection circuitry can reduce the likelihood of failure significantly.
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