Water Level Sensing Using the Hydrostatic Method
By Mark A. Miller
The hydrostatic technique of water level sensing has proven to be one of the most desirable methods available for municipal, environmental and process applications. For most applications, the hydrostatic method satisfies the specifying engineer's concerns regarding reliability, cost, accuracy and maintenance requirements.
The Theory
Hydrostatic pressure may be used to indicate water level by the following relationship:
Given: The density of pure water at 60°F at sea level is 62.37 pounds per cubic foot.
Therefore: The pressure exerted by a water column one foot in height is:
Or, another way to look at it is, what height of water column is required to generate a pressure of one pound per square inch?
This example illustrates the fact that the pressure exerted by a column of water is dependent only on the height of the column and not on the affected area.
Barometric Compensation
When designing a system for determination of water level using the hydrostatic method, one must remember that the pressure sensed by the level transmitter is the total of the hydrostatic pressure plus the local barometric pressure acting on the surface of the liquid.
Thus, in an uncompensated design, such as an absolute or sealed gauge design, a change in local barometric pressure will cause a change in transmitter output. For example, when calculated as a percentage of full scale of 10 feet of water column, a change in the barometric pressure of just 0.1 inches of mercury will cause an error of greater than 1 percent. If this error is not acceptable, a separate measurement of barometric pressure must be made, and this value must be subtracted from the pressure indication given by the level transmitter.
Because of the expense and complexity involved in such a process, many standard level transmitters are equipped to compensate automatically for barometric pressure changes. The simplest way to accomplish this is to vent the reference side of the sensor to local barometric pressure. In this way, the same barometric pressure acts on both sides of the sensing diaphragm and thus effectively cancels itself out.
The Technology
The design of a hydrostatic pressure transmitter begins with the selection of the basic pressure sensor. Numerous sensing technologies are available, but the piezoresistive silicon technology is most often chosen due to its inherent low cost and high performance.
Silicon is ideal as a base material because of its near-perfect elasticity and low costs for raw materials and production. In production, boron atoms are ion-implanted into the crystal lattice of the silicon to create a classic 4-arm Wheatstone Bridge. The silicon is then back-etched to create a diaphragm in the correct orientation to the strain resistors. Then, as pressure is applied, the diaphragm flexes. The strain is sensed by the resistors, causing a bridge imbalance and an electrical output proportional to the applied pressure. In applications where the media to be measured is a clean, dry, non-corrosive gas, it is only necessary to mount the diaphragm to a substrate and then to package it properly for electrical and pneumatic connection. However, for many other types of gases and for nearly all liquids (including water), the sensor must be isolated from the media.
A common media isolation technique is the incorporation of a metal isolation diaphragm typically made of 316 stainless steel or other material, if required. The cavity behind the diaphragm is filled with a non-compressible fluid usually silicone oil. Perfected through over 30 years of experimentation and production, media-isolated silicon pressure sensors can be designed to function accurately and reliably at up to 15,000 psi, and at temperatures of up to 150°C.
The Variables
Signal conditioning The vast majority of level monitoring and control systems are configured to accept a 2-wire, 4-20 mA transmitter output. A smaller number use transducers with an output of 0-5 vDC (3 wire), and there are a few that will accept only millivolt (4 wire) inputs. Therefore, a signal conditioning circuit is combined with the sensor to produce the desired output format. This circuit typically:
(1) regulates the incoming supply voltage; (2) provides precision sensor excitation; (3) contains an instrumentation amplifier to condition the millivolt output of the sensor to the desired format; and (4) incorporates null and gain adjustments for setting zero pressure and full scale outputs to the desired setpoints. While some manufacturers make the null and gain adjustments user-accessible, others believe that a stable sensor combined with factory-set null and gain and a hermetically sealed enclosure result in better long-term reliability.
Packaging Packaging is especially critical in the case of submersible transducers and transmitters. It must ensure that the signal conditioning circuit is adequately protected from the media while still being able to vent the sensor to the local barometric pressure. Many transducers and transmitters are housed in stainless steel enclosures, with a smaller number using different materials chosen to suit specific applications. The specifying engineer must therefore be careful to recommend a level transducer or transmitter that is available in a material that is compatible with the anticipated environment.
Cable Vent Tube As noted previously, automatic compensation for barometric pressure variation is accomplished by venting the reference side of the sensor to atmosphere. This is typically achieved via a hollow tube contained within the electrical cable.
Of course, in this case, the issue of installation technique must be considered early on to prevent the hollow tube from allowing troublesome foreign material such as water to enter the transducer enclosure. Installation of a dessicant-filled vent filter at the top end of the cable vent tube is a common, inexpensive approach to preventing moisture (water vapor) migration into the vent tube.
The specifying engineer should note that dessicant filters require periodic replacement as they become ineffective due to moisture saturation.
In applications where regular dessicant filter replacement is not feasible, the installation of an aneroid bellows is a viable option. Patterned after the bellows used in mechanical barometers, the aneroid bellows responds to variations in barometric pressure, relaying the variation down the vent tube to the sensor. Since the system is "closed," the need for periodic maintenance is virtually eliminated.
Lightning and Other Transient Voltages Like any other object in contact with groundwater, the water level transmitter or transducer is subject to transient voltages of all types the most common of which is caused by lightning. There is nothing that can protect equipment of this type from a direct lightning strike. However, for a transient voltage such as that caused by a remote strike, there are a number of things that can be done to provide some measure of protection.
First, the electrical cable should be shielded with braided or wrapped copper. Then, if the shield at the termination end is connected to a good earth ground, low level transient voltages can usually be diverted to ground and away from the transmitter electronics.
The next level of protection is accomplished through the use of line protectors. These must be installed in the correct locations, and they must be highly responsive and capable of handling high current. It is a common practice to install line protectors only at the power supply end of the transmitter cable.
Unfortunately, this practice protects only the power supply and readouts; it affords no protection to the submersible level transmitter. However, transmitter protection may be accomplished by placing an additional signal line protector between the transmitter electronics and the cable.
As of this writing, only one manufacturer, Pressure Systems, Inc., is known to provide such a protective device, accompanied by a warranty that extends through the life of the transmitter to which it is installed.
The Results
Piezoresistive silicon-based water level transmitters and transducers have gained wide acceptance for the advantages they offer over other level-sensing devices. They provide the most precise, continuous-reading technology available at reasonable costs; and, when properly installed and maintained, they deliver exceptionally long service life.
About the Author: This article was condensed from a technical paper by Mark A. Miller, Senior Product Application Specialist for Pressure Systems, Inc./KPSI Products.
