Dec 28, 2000

Choosing the Right Transducer for the Application


First,a transducer is defined as a device that converts one form of energy to another. Usually that means mechanical, light, heat into electricity, or electricity into mechanical, light, heat, etc.

For water/wastewater applications, we need to consider the former. In this article, we'll talk about pressure transducers that convert mechanical energy into electrical.

Pressure transducers rely on mechanical deflection or stress on a member to sense the magnitude of the mechanical energy. Typically, there is a component or material that changes some electrical parameter in response to the mechanical deflection. The electrical parameter could be resistance, capacitance, inductance or voltage, depending on the technology. These are the basic principles: resistive/ piezoresistive - strain gauge; capacitive - capacitive; and voltage - piezoelectric (which is unsuitable for water/wastewater applications and will not be discussed below).

Piezoresistive or Strain Gauges

The piezoresistive, or strain gauge, uses a basic circuit called a Wheatstone bridge to convert the pressure into electricity. When there is no pressure applied, the electrical output is zero volts, even though there is excitation applied.

In this state, the bridge is said to be balanced. Pressure applied to the diaphragm causes it to deflect slightly. The material used in the strain gauge changes resistance when its length is physically changed. The dimensional changes are small, usually less than 0.001 inch.

The gauges are connected in such a way that one resistor increases in value while the one on the opposite side of the bridge decreases in value. This unbalances the bridge causing voltage to be developed on the output. This voltage will be directly proportional to the deflection and hence the pressure. At this point, it is generally considered that we have a pressure sensor. The output is not calibrated and the device will have significant error due to temperature changes.

Adding circuitry to correct these errors makes the device a pressure transducer. The output could be millivolt, mostly used in laboratory or OEM applications; voltage of 4 to 20 mA is more often used for water/wastewater applications. The higher level signals are usually preferred because wiring is simplified and external amplification is unnecessary.

Capacitive Sensors

The capacitance device uses a variable capacitor that is on the back side of the diaphragm to cause a change in the amplitude or frequency of an oscillator circuit inside the transducer. This change is then converted to a DC voltage or current output.

Application Issues

From the outside, you usually can't tell which technology is being used. The truth is that the "technology" inside is less important than what a given unit will do for you. Manufacturers may tout the features of their particular technology, but these often translate into only marginal benefits. Put another way, how well the manufacturer implements the technology is more important than the technology itself.

Factors that water/wastewater users should consider are performance, cost and packaging with packaging often the hidden determinant in the transducer's cost over its life.

Let's consider some typical applications. Water level is a very common one. This may be a lake, reservoir or tank level. The easiest way to measure water level is with a unit that measures the static head pressure relative to atmospheric pressure (PSIG). The transducer is lowered to the bottom or installed in the tank wall at the bottom. The density of the liquid and its height above the transducer determine the pressure the transducer will see.

Because the measurement is made relative to atmospheric pressure, the backside of the diaphragm must be vented to the atmosphere. This is a problem when the vent hole is in the body of the unit. You cannot immerse it; nor should you use it in a hosedown or humid atmosphere. Water or condensate will find its way in and eventually destroy the device.

An immersible depth sensor will vent through its electrical cable. This is also an excellent solution for hosedown, but it doesn't eliminate the effects of condensation. Humid summer air eventually will get inside the transducer at the bottom of the lake, where it is many degrees cooler.

A desiccant package in the vent tube will prevent damage to the transducer, but the desiccant does need to be changed.

Corrosive Media

Most transducers intended for water/wastewater service will have diaphragms and housings made of stainless steel. Some units have plastic housings for lower cost.

The materials used for the "wetted parts" or "materials in contact with the media" are important even for potable water. For low pressure measurements the diaphragms are thin. Even mildly corrosive fluids can cause pitting in the diaphragm, which can affect performance over time.

For tank level measurement of harsh chemicals, special alloys sometimes are required. Hastalloy and Inconel are available, but will increase the unit's price.

One alternative is to use a unit with a Teflon coating. This is usually less expensive and doesn't affect performance. The pressure media must not contain grit or other sharp particulate because the coating is thin and can be scratched. The coating is applied only to "flush diaphragm" transducers. This flush mounting design also can be useful for viscous media such as level sensing in sludge tanks or in piping where a transducer with a pressure port and orifice could be clogged by sludge or particulates.

Many factors go into selecting a pressure transducer. Ask potential suppliers for their recommendations in solving your problem. Chances are they have seen it before or something similar to it. Their experience can be your best tool in selecting the right transducer.

For more information, contact Data Instruments in Acton, MA - 800-333-3282.

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