Sensor Conditioning

Select products with a conductivity input capability.

• Electrolytic conductivity is defined as the electrical conductance of a unit cube of solution as measured between opposite faces. Like its reciprocal quantity resistivity it is a measure of the bulk qualities of a material.
• The unit of conductance is the Siemens ( S ), which is the reciprocal of resistance (Ω).
• The unit of conductivity is commonly expressed as micro Siemens per centimetre (μS/cm). Note that conductivity introduces the dimensional factor of 1/cm.
• The dimensional factor in conductivity measurement is determined by the cell constant or probe K factor.

Conductivity of various solutions
Ultra pure water	0.1 to 0.05 μS/cm (10 to 18MΩcm)
Tap Water	200μS/cm

The relationship can be expressed as : Conductivity = Conductance x K factor.

K factor can vary from 0.01 to 100. Most commonly available probes are between 0.1 and 10. They are usually scaled in factors of 10 to make the job converting the conductance measured to conductivity reading easy.

Selecting the correct K factor for a particular installation maximises the accuracy of the measuring instrument.

Note that the constant for a given probe can vary from stated because of factors such as poor insulation, caking of solids on the electrode surface, gas bubbles, and erosion of the probe surfaces.

As stated in the introduction, measuring conductivity is useful because it is a good indicator of the concentration of ions in a solution. Unfortunately the conductivity of an aqueous solution varies with temperature so large errors may occur.

Temperature variation of the conductivity of a solution can typically vary from 0 to 5%/°C depending on what chemicals are in the solution and their proportion. Many substances the conductivity vary almost linearly with temperature. Neutral salts are typically 2%/°C so a 25°C change in temperature will result in a 50% change in conductivity reading.

Clearly some form of temperature compensation must be used for most applications if the conductivity reading is to be of any use.

Temperature compensation can be done in a number of ways. One is to draw off a sample of solution and maintain it at a fixed temperature to measure the conductivity. The more common practice with modern process instrumentation is to measure the solution temperature and to automatically scale the reading. This is all done internally within the instrument. The temperature compensated conductivity probe usually has a temperature sensor such as a thermistor built in. For accurate temperature compensation the instrument user must fully specify the temperature characteristic of the solution. The more practical method is to set the instrument to compensate at 2%/°C .

Used for the control or alarm of the level of conductive liquids. The tip of a sensing electrode is placed at the level which the liquid is the be controlled. When the level of the liquid touches it, an electrical circuit is completed through the liquid to earth - the current that flows is used to trigger a relay.

The degree to which water ionises to form (H+) ions varies with temperature so that the pH reading always returns to the same value at some given temperature. However, weak acids, bases and some salts ionise only partially in solution. The amount of ionisation is temperature dependant. If these ions affect pH, this will cause a pH change with temperature.

Oxidising or reducing properties of a chemical reaction. In this application, the term oxidation is used in its electrochemical sense and applies to any material which loses electrons in a chemical reaction. By definition, a reduction is the opposite of oxidation, or the gaining of external electrons. There can be no oxidation without an attending reduction.

For example, a ferrous ion may lose an electron and become a ferric ion (gaining increased positive charge) if a reduction, say, of stannic to stannous ions (which is the reverse of this operation) occurs at the same time.

This measurement uses electrodes similar to those in pH measurement (except that metal is used instead of glass), but the two types of measurement should not be confused. The measurement depends on the oxidising and reducing chemical properties of reactants (not necessarily oxygen). The inert metal electrode versus the reference electrode will produce a voltage that is related to the ratio of oxidised to reduced ions in solution.

This measurement is similar to that of pH in the requirements that it places on the voltmeter used with it. It is useful for determinations in waste treatment, bleach production, pulp and paper bleaching, and others.

Measuring pH in an industrial situation is useful because it is a good indicator of the concentration of ions in a solution.

The pH measurement head is designed to house all combinations of Electrodes together with auxiliaries in the measurement Electrode, Reference Electrode, Temperature Compensation and perhaps an Ultrasonic Cleaning Head.

There are three main types of heads;

• Total Immersion type can be totally or partially Immersed in the process and has the advantage of being cheap but is easily damaged.
• Flow type is a modification of immersion type, being completely enclosed but having an entry and exit port. There are some of this type filled with baffles to protect the glass electrodes from the flow through the load.
• In-Line type is housed between two flanges and is essentially a piece of stainless steel pipe into which the measurement reference and temperature electrodes are fitted. Pipe diameters of up to 2" (50mm) can be accommodated with this type of In-Line unit.

Amplifiers (Signal conditioners) connecting to these probes should have an input impedance of 100M? or better to prevent loading and early ageing of the electrode.

An LVDT is an electromechanical transducer that produces an electrical output proportional to the displacement of a separate movable core.

The LVDT has many commendable features that make it useful for a wide variety of applications.

• Friction-less Measurement, there is no Physical contact between the movable core and coil structure, which means that the LVDT is a friction-less device. This permits its use in critical measurements that can tolerate the addition of the low-mass core, but cannot tolerate friction loading.

• Infinite Resolution, the inductor principle by which the LVDT functions gives a truly infinite resolution. This means that the LVDT can respond to the most minute movement of the core and produce an output.

• Null Repeatability, The null position of an LVDT is extremely stable and repeatable. Thus the LVDT can be used as an excellent null position indicator in high-gain closed-loop control systems. It is also useful in ratio systems where the resultant output is proportional to two independent variables at null.

• Cross-Axis Rejection, An LVDT is sensitive to axial core motion and relatively insensitive to radial core motion. This means the LVDT can be used in applications where the core does not move in a straight line.

• Core and Coil Separation, permits the isolation of media such as pressurised, corrosive, or caustic fluids from the coil assembly by a non-magnetic barrier interposed between the core and the inside of the coil. It also makes the hermetic sealing of the coil assembly possible and eliminates the need for a dynamic seal on the moving member.

• Environmental Compatibility, An LVDT is one of the few transducers that can operate in a variety of hostile environments. For example, a sealed LVDT is constructed of materials such as stainless steel that can be exposed to corrosive liquids or vapours.

When the core is moved from the null position, the induced voltage in the coil toward which the core is moved increases, while the induced voltage in the opposite coil decreases. This action produces a differential voltage output that varies linearly with changes in core position. The phase of this output voltage changes abruptly by 180° as the core is moved from one side of null to the other.

Pot input usually have zero and span adjustments that cover 20% of full scale.

The reference voltage generated by the transmitter will have a minimum pot value specified.

Modules with a Slidewire input have a non-interacting offset up to 100% of slidewire range enabling use on applications with sectional slidewires. Minimum span is 20% of slidewire value.

Slidewire Calibration Example

• 1000 Ohm slidewire.
• 4mA output at 500 Ohm.
• 20mA output at 750 Ohm.

Offset in this case is 50% (of 1k Ohm). Span is 250 Ohm = 25% of range.

Because of the large amount of applications, a wide range of different pressure measurement techniques are available to the instrument engineer. Included are a range of mechanical measurement techniques incorporating similar principles and the popular advancing electromechanical techniques.

The physical principle incorporated in the design of a pressure transducer depends mainly on the range of pressures to be measured. The majority of pressure transducers consist of a force summing device which converts pressure to displacement or force, and an electrical element (e.g. resistive, capacitive, inductive, piezoelectric) which respond to displacement or force with an electrical output.

Solid state pressure sensors typically consist of a piezoresistive strain gauge in a Wheatstone bridge configuration which is integrally formed on a diaphragm (typically silicon). The application of a constant current source to the sensor produces a voltage output that is linearly proportional to the input pressure or strain on the piezo-resistive bridge network.

The Bourdon Tube in its simplest form and consists of a tube of oval section bent in a circular arc. One end is sealed and attached through a link to a pointer, the other end is fixed with an opening to accept pressure. When pressure is applied the tube tends to return to its circular profile and straighten out. The change in profile is transferred to the pointer which indicates a proportional pressure reading.

Resistance is a measure of electrical conductivity. Its units of measurement is the OHM (Ω).

When a voltage V is connected to a load resistor R a current I will flow according to OHM's law;

R = V ÷ I

Example Resistance Transmitter Ordering Detail

• Input: 3-wire connection
• Cal: 10-50 ohms
• Output: 4-20mA.

Select products that designed to accept pulses and produce a standard process signal or an alarm or a re-powered isolated pulse output.

Pulses from a sensing probe can define from

• magnetic sensors with a sine wave output
• flow metes this a square wave output
• proximity sensors with a square wave output.

The signal conditioning module may supply power for the sensing probe to operate. Engineering outputs representing flow, velocity and speed may be derived from a pulse input.

Important parameters to consider when ordering pulse input signal conditioners are pulse amplitude and frequency.

A Load Cell will be specified with the following parameters;

• Capacity or maximum load that can be applied.
• Sensitivity typically specified in mV/V (millivolts output / Excitation Applied) at maximum load.
• Recommended or maximum Excitation voltage to be applied.

Using the equation Range = (Actual Load ÷ Capacity) × Sensitivity &times Excitation

When connecting to a transmitter the Range, Excitation and Tare must be set.

If a Load cell of 1000kg capacity, with 2mV/V sensitivity is connected with 10Vdc excitation and the Actual Load is 500kg max. then:

• Range = (500kg ÷ 1000kg) × 2mV × 10 = 10mV

If the same 1000 kg load cell has a 100kg tare (tare = empty tank for example), with a 800 kg live load then:

• Range = (800kg ÷ 1000kg) × 2mV × 10 = 16mV
• Tare = (100kg ÷ 1000kg) × 2mV × 10 = 2mV

The Tare adjustment is the cancelling out of the load effect of the material holding container or vessel. An example would be the common metal tray used in grocery shopping, its influence (weight) is removed before the weight of the groceries is taken.