Conductivity Analyzer Sensors

One application for a conductivity detector is when bleach is used to clean a process and then flushed with water. The water can be tested for conductivity and when the conductivity is really low it means that all or most of the bleach has been flushed away.

In solutions, electric current is carried by moving particles (ions). In solids, electrons are directly transferred from atom to atom. The mobility of the ions is directly proportional to temperature. If the temperature of a solution increases, the conductivity Increases. Therefore, the temperature of the solution must be measured, and a correction factor must be included in the circuitry for temperature changes.

As temperature increases in an aqueous solution, conductivity increases. This will cause the molarity to appear to be higher than it is. This principle is important with regards to the 3081C transmitter because this transmitter will automatically compensate for temperature differences and calculate what the output would be at 25 deg C.

Alternating current is used rather than direct current, with respect to a contact conductivity detector, so that polarization does not occur between the electrodes. The ions are kept mobile between the electrodes while running at typically 1000Hz per cycle. This mobility allows for proper conductivity detection.

The amount of current flow between the electrodes depends on 3 variables. See figure below.

• The solution conductivity
• Plate separation
• Surface area of the plates
  
  

If one was to test the conductivity of distilled water, one would find very little conductivity in the distilled water and could increase the conductivity by adding salt. The level of conductivity will rise if a packet of salt is dumped into the solution being analyzed because salt has electrolytes and electrolytes are directly related to conductivity. Even one single grain of salt will affect the solution. Conductivity is measured in S/cm or Siemens per centimetre. A Siemens is another unit for 1/ohm or a mho.

In order to calibrate a sensor, a solution with a known coductivity, known as standerdization solution, must be used. The conductance of a known standard solution across a 1 cm cube of a material is called specific conductivity.

Potassium Chloride (KCl) is a salt that is easily dissolved in water. It is the most common standardization solution because of its relative harmlessness. Most importantly though, regardless of concentration % By Weight, a known value will be obtained. That is to say that KCl, cannot reach past a known value of µS at 25°C, regardless of the amount added to the solution.

Up to a certain level, the conductivity of a solution increases with concentration. Some solutions such as acids , however, reach a conductivity peak, after which further increase in concentration results in conductivity deceasing.

The electronics need to “see” a resistance between 500Ω to 10,000Ω so that the resistance of the solution is approximately the same as the other arms of the detector circuit. To accomplish this, for fluid with low conductivities the electrodes can be placed close together, or their area can be increased, so the higher conductance (or the lower resistance) can be obtained.

Sampel calcualtion. Given the following data, calculate the slope, correct slope to 25°C and find the K value.

Given

• C=12,856 µS @ 25°C
  C=11,167 µS @ 18°C
  

Calculation

• Therefore the slope = 12,856 µS - 11,167 µS / 25°C -18°C = 241.29 µS/°C
  Corrected to 25°C (divide by slope at 25° )= 241.29/12856 = 0.019
  K= 100* 0.019 = 1.9%
  

There is a direct relationship between the cell constant in Transmitter Configuration and conductivity reading. Cell constant acts like a multiplier and directly affects the conductivity reading value.

The concentration effects the conductivity of a solution. Up to certain level (Saturation) the conductivity of a solution increases as the concentration increases. However, some solutions will reach a conductivity peak at a specific concentration after which the conductivity may remain the same or decrease with any further increase in concentration.

The formula for the Cell Constant (Ѳ).
Cell Constant Ѳ = k/ L.

Conductance: The ability of a component to conduct electricity, measured in Siemens.
Resistance: A property of a conductor which the passage of current is opposed, measured in ohms.
Conductance = 1 / Resistance: They are inversely related

A conductivity meter is non-ion selective. It measures all of the ions activity in the solution.



Probe constant is the distance between two electrodes divided by the area of each electrode. Therefore two 1cm² electrodes separated by a distance of 1cm, represents a probe constant of 1. High conductivity solutions require a probe constant greater than 1 and low conductivity solutions require a probe constant of less than 1.

The cell constant if the distance between the electrodes is 0.46 cm, and the surface area is 0.82 cm2?

Cell Constant = L/A = 0.46/0.82 = 0.561

A Toroid (Electrodeless) conductivity sensor is used for corrosive, oily and dirty fluids. A disadvantage is it has lower sensitivity than contact sensors.

Extremely high conductivity requires a sensor with a probe constant grater than 1.0. However, extremely low conductivity requires less than 1.0. The greater distance between the electrodes is defined by the probe constant, the smaller current signal. So we have to use the appropriate probe constant for a specific solution (extremely high conductivity/ extremely low conductivity) in order to provide the most accurate measurement.

Low conductivity solution can be said to have a range of 0.05 to 200 µS/cm and high conductivity solution in the 10 to 20000 µS/cm.

Conductivity Analyzer 1054

Two advantages that using Toroidal Conductivity Measurement has over using Contacting Conductivity Measurement is lower maintenance due to fouling and corrosion resistance and higher range capabilities. The main drawback of Contact measurement is that the sensor is susceptible to coating and corrosion which drastically lowers the conductivity reading. One drawback of Toriodal measurement is that it lacks the sensitivity of a contacting measurement probe and can not be used for low conductivity solutions.

Conductivity is the ability of a solution to conduct an electric current due to the activity of ions in the solution. Ions in a solution are produced from electrolytes (salts, acids, or bases)

The probe constant is a measure of the current response of a sensor to a conductive solution, due to its dimensions and geometry. Its units are cm-1 (length divided by area). The probe constant varies from 0.01 to 50 cm-1. In general the higher the conductivity of a solution the larger the probe constant will have to be.

AC current used instead of DC current because when DC current is used, the ions migrate to their respective polarity nodes whereas with AC current, the ions stay suspended within the liquid.

Contacting conductivity uses a sensor with two metal or graphite electrodes in contact with an electrolyte solution. An AC voltage is applied to the electrodes by the conductivity analyzer and the resulting AC current flowing between the metal or graphite electrodes in a specific solution is measured.

When using the contact probe it is important to remove the air bubble that becomes trapped within the sensor because the air bubble insulates the sensor and will affect the reading.

A Toroidal probe should be selected for operation when the process is dirty or corrosive.

Conductivity is measured with a Toroidal sensor by an AC current induced by the first coil onto the fluid passing through it. As the fluid passes through the second coil it induces a current on that coil. The amount of current induced on the second coil is proportional to the solution conductivity.

Toroidal sensors can be completely coated by a solid or oily contaminant (up to 1cm of thickness) from the process and still not have (significant) effect on the reading. This characteristic can be an advantage depending on the process.For instance, a high turbidity process.

Contacting Conductivity analyzers use a sensor with two metal electrodes in contact with the solution. We apply AC voltage to the electrodes by the conductivity analyzer. As a result, AC current flowing between the electrodes through a volume of solution is used to measure the conductance or the ability to conduct the current.

Weight Percent is when the molecular weight of a solute is unknown or irrelevant, its concentration may be expressed in terms of its weight relative to that of the solution. (Mass of solute / mass of total solution or mixture) x 100

The conductance decreases when a toriod is placed close to a non-conductive surface (like plastic) because the magnetic field travelling from source toroid to measurement toroid is disrupted (see figure below). The lines of flux travel much slower through plastic because the resistance is higher than the fluid, thus creating less inductance in the measurement toroid. This would work opposite in a metal container because metal has less resistance than the fluid.

The three factors that affect conductance are: Velocity of ions in solution, temperature, concentration of ions.

Galvanic H2S Analyzer

H2S is a colourless, extremely poisonous gas that smells like rotten eggs in low concentrations. Exposure to concentrations of 500 ppm results in loss of balance and ability to reason and can result in death, if first aid is not quickly provided. H2S is also very corrosive. In the presence of small amounts of water and oxygen, it forms sulphuric acid which is very corrosive to any iron based material such as pipelines. For this reason, limitation on H2S concentrations are set to try and keep the work environment safe.

The typical concentration of H2S allowed in gas pipelines varies with the company. Some typical limits are 4.0 ppm (Trans Gas Pipelines) or 16 ppm (NOVA Pipelines). A analyzer is needed to measure and monitor these concentrations. In this case the Galvanic 801 H2S Anlyzer.

The galvanic 801 H2S analyzer measures the rate of reaction of the lead (II) acetate strip with the H2S, rather than measuring the absolute darkness of the stain. The method used to convert the darkness of the stain on the tape to an H2S concentration is Rateometric Colourimetry. A sensor assembly is then used to convert the darkness of the stain on the tape to an electronic signal, which can be used by the microprocessor to calculate an H2S concentration.

To facilitate the H2S and lead (II) acetate reaction, a step is performed before the sample even touches the tape. The tape is first bubbled through a solution of 5% (v/v) acetic acid in distilled water. This action humidifies the gas, which helps to facilitate the H2S and lead (II) acetate reaction as well as eliminate any effects from humidity fluctuations in the sample gas.

The Galvanic 801 is based on the reaction between H2S and Lead (II) Acetate to form lead sulphide and water.

Pb + H2S ----> PbS + H20

Some of the H2S could be absorbed into the water, reducing the sample concentration if the humidifying solution is water.

The 801 H2S Analyzer has a sample chamber to limit the amount of sample gas that will come into contact with the tape. This is achieved with an aperture. Typically a larger aperture will be used when measuring lower H2S concentrations.

The 801 H2S analyzer can also be used to measure total sulphur concentration. An optional total sulphur system can be utilized to allow a standard 801 analyzer to measure total sulphur. This is done by hydrogenation.

• Hydrogenation: The sample is mixed with a hydrogen stream. The sample and hydrogen are heated together at approximately 900°C. At this temperature in the presence of hydrogen all sulphur compounds will be converted to H2S. Also, hydrocarbons heavier than methane will be cracked to methane.
  

The principle of operation of the Galvanic H2S Analyser is (Placed in order of operation in correct sequence):

1. Sample gas containing H2S passes through the tape.


2. The lead acetate is impregnated in low concentration in paper tape.


3. H2S Reacts with lead and forming lead sulphide. The lead sulphide is brown in colour, and the rate of intensity of the colour is proportional to the concentration of H2S in sample gas flowing through.


4. The analyser measures the rate of intensity of the brown stain on tape.

A light source is used to shine through the tape and a photodiode is used to sense the light that gets through the tape. The light source utilized by the 801 H2S Analyzer to illuminate the Lead Acetate Tape is a single (red) light emitting diode (LED).

The Galvanic 801 Analyzer has 2 photodiodes for measurement. One is used to reference the LED intensity while the other measures the darkness of the stain on the tape.

When calibrating the analyzer a calculation must be used to find out the gain that needs to be entered. The calculation to determine the required gain is:

• GAIN = (Value of Calibration Gas / Current Analyzer Reading) * Current GAIN value

Vibration Analysis

In measuring vibration there are three different types of probes (measuring device) used as show below. The device used to read the inputs from the probes and perform the vibration analysis to determine root cause of vibration is the vector analyzer.

• Displacement probe which is used with a Proximetor. The Proximetor converts RF (eddy current) into a voltage output. The gap voltage will represent the distance between the end of the probe and the shaft. Used in the Bentley Nevada process below. Also known as a fixed proximity prode, measures the peak to peak movement of the rotor. It can also be used for axial (back and forth) movement, for thrust measurement of the rotor.
• Velocity probe where the output signal is proportional to the speed of movement. Used in the IRD analyzer process below.
• Acceleration probe where the output signal is proportional rate of change of movement. This signal can be integrated to produce a velocity output.

Vibration Analysis with the Bentley Nevada process analyzer.

We are concerned about vibrations and vibration analysis because vibration causes wear and damage to equipment which in turn causes an inefficiency. Some examples for the cause of vibration are imbalance, misalignment, looseness, unbalanced weight system, wear and tear on equipment, low oil temperature and misalignment of couplings. The most important element to be aware of to optimize and reduce vibration is shaft alignment. Proper alignment saves on coupling elements, bearings and seals. Your equipment will run longer with fewer emergency repairs.

Critical speed is an important factor to look at during design. During this speed the vibrations are amplified due to frequency's being in sync. At a machine's critical speed the vibration is considered to be at far excessive levels, which can cause damage to components very easily. Critical speed is when the frequency or rpm's of the machine become equal to or in sync with the resonant (natural) frequencies of the machine and its system components. ORBIT and DVF (Digital Vector Filter) oscilloscope digital readings are used to determine critical speed.

Different materials are prone to vibration in different ways. This is compensated for within the Bentley Nevada Vibration Analyzer. The gap spacing is changed with the micrometer based on the different materials. Different materials also change the calibration procedures. If a vibration analysis was done on a compressor with a shaft material of type 'x' the target material should also be type 'x". Using different materials will most definitely affect the output and would not be a proper practice.

A probe is used to measure the vibration using distance. It's a gap to voltage transducer to measure vibration. Also, a key phasor can be used to determine rotor RPM and the location of the maximum peak movement. The relationship between the sensor gap and the resulting voltage is as the gap increases the voltage increases. The output from a vibration probe is not completely linear. The probes output will be linear for part of the operational output range but not at the very low or very high ends of the range. The gap voltage for the X and Y probes on the rotor kit should be approximately 9.5 volts for the zero setting.

The calculations used for this process are:

• If the time measured from the oscilloscope waveform is 26 ms, the Speed (RPM) would be calculated as follows:
  Frequency = 1/ time = 1/26ms = 38.46 Hz
  Speed (RPM) = Frequency (Hz) x 60 = 38.46 x 60 = 2307.6 RPM
• A mil is equal to 1/1000 th of an inch
• When using a digital voltmeter (DVM) to determine the peak to peak voltage of the proximetor reading, the formula that must be applied to the signal is:
  (Measured Voltage / .707) * 2 = Peak to peak voltage

Vibration Analysis with IRD

The IRD analyzer is used for spot measurement. This means data may be collected daily or weekly depending on the procedure developed for preventative maintenance to observe any variations on the concerned equipment. The IRD analyzer uses a velocity probe.

It important to monitor vibration because servicing is performed only when needed. Use of this analysis method prevents catastrophic failures and allows more effective planning and scheduling of maintenance work. These analyses' often can tell when, for example, bearings need to be replaced. Because of these early warnings, repairs can be scheduled before the pump completely fails.

When measuring and producing vibration signatures the IRD "amplitude range" should be checked and set before recording any vibration signature. It important to turn the frequency knob slowly when a vibration peak is detected to ensure that the maximum peak value is recorded by the X-Y plotter. A filter circuit is designed to pass or reject a specific frequency band.

When troubleshooting, the strobe light can be used to determine balancing, verifying alignment, checking for loose bearings and other connections. The most likely cause when frequency is in terms of RPM is 1x RPM, 2 x RPM and 3 x RPM

• 1x RPM: unbalance
• 2 x RPM: mechanical looseness
• 3 x RPM: misalignment

It is desired to have the shaft appear as though it is standing still while the strobe light is shining on it. If an image seems to appear more that once, but also seems to be standing still the shaft is not spinning at the same frequency as the strobe light is shining. This produces a false positive and is not an accurate reading of RPM. If two images appear, while there should only be one, generally it means that the shaft is spinning twice as fast as the strobe light is flashing.When the strobe light frequency and the shaft rotational speed are the same the shaft will appear to stand still (And be a clear image). At this point the light frequency equals the RPM.

The relationship between unbalance and amplitude is proportion. The most common cause of vibration is unbalance. Amplitude is proportional to unbalance, largest in radial direction.

LEL/Toxic Gas Analysis

Explosive and toxic gases are a hazard to any job site. This is why analyzing and monitoring the atmosphere is important.

Monitoring hazard es gases for safety, process control and air quality is a fairly complex topic. Gas measurement is more complicated than measuring other elements such as voltage, humidity and temperature. For, there are literally hundreds of different types of gases and each application has a unique requirement.

For example, some applications may require the detection of a specific gas and have to eliminate the measurement of other surrounding gases. In other words, in a methane and propane environment methane may want to be detected and surrounding propane eliminated (separate detection). It may be desirable in some cases to measure a quantitative value off all gases present.

Most sensors are not specific to one gas but rather to a group or family of the gas. In order to select a proper detector for optimum results one must understand the types of sensors available and there response characteristics to the appropriate gas.

Job site air quality detectors must be durable, weather resistant, dust proof and explosion proof. Also, they should be inexpensive, have long life expectancy and be suitable for multi sensor systems. A minimally skilled person should be able to operate and maintain it. So, it would be a good idea to choose a simple system.

There are 2 main categories of work area sensors:

• Toxic monitoring for human health. This requires the use of a sensor that is sensitive to low concentrations of gases.
  
• Combustibility monitoring for safety. This requires a sensor that can monitor high concentrations of gas.

The Occupational Health and Safety Organization's (OH&S) permissible exposure limit (PEL) in a 8 hour work day for carbon monoxide (CO) is 50 ppm. However, the lower explosive limit (LEL) is 12.5% by volume concentration. So, considering this gas is toxic and combustible, 2 sensors need to be used to monitor concentration because they are widely disparate.

Currently, the 5 most common air quality and safety monitoring devices are:

• Electrochemical sensor is a oxidation reduction sensor (redox). This sensor consists of a sensing electrode and counter electrode. Or in other words, a anode and cathode see picture below. These 2 electrodes are separated by a thin electrolyte. Any gas that enters the sensor reacts with the anode at the surface and involves either a oxidation or reduction reaction. The anode materials catalyzes the reaction and needs to specifically developed for a given gas. A electric current is generated proportional to the gas concentration. This sensor consumes little power.This makes it ideal for PEL applications because it can be a portable battery operated unit. It is gas selective and not suitable for combustible gas detection and has a 2-3 year life expectancy. Some specify a exposure dosage for maximum concentration. For example, if a sensor has a rate of 4000 ppm/hour and is exposed to 40 ppm constantly the sensor would last 100 hours. These sensors can effectively measure around 20 gases in low ppm ranges like CO, H2S, SO2, NO2 and Chlorine.



• Catalytic combustible sensor ignites the surrounding gas molecules when they come into contact with the coil. The coil is a platinum wire coated with catalytic ally treated metal oxide and is used within a Wheatstone bridge as seen in picture below. A combustible gas mixture will only burn when it reaches ignition temperature. In the presents of catalytic materials the ignition temperature is reduced, so the gas mixture will ignite at a lower temperature. When the gas burns on the surface of the coil the temperature of the platinum wire is increased and the wire's electric resistance is decreased, thus creating the input to output relationship. This detector is good for general purpose and suitable for both portable and stationary continues measurement. It can be used for most hydrocarbon gases, but most common is methane. Other gases need a correction factor implemented for proper and accurate measurement. It is important to know that this sensor can be poisoned. Certain chemicals like silicon compounds, sulfur compounds and chlorine will deactivate the catalyst and make the sensor unresponsive. This phenomenon is known as catalytic poisoning.



• Solid state sensor uses the transfer of electrons. The sensor consists of transition metal oxides. These metal oxide are prepared structured to form a bead or thin layer chip sensor. When gas comes into contact with the metal oxide the gas dissociates into charged ions or complexes. A heater within the sensor keeps optimal gas detection temperature see picture below. Two biases electrodes are embedded into the metal oxide to measure the change in conductivity. Probably one of the most versatile, this sensor can be used for low concentration toxic gas or high range combustible gases. The detection response characteristics can be changed by using different types of metal oxides, changing processing techniques or changing the process temperature. It is constructed in a simple manner, thus giving a problem free life from 10-25 years. The sensor is robust, so vibration and shock do not affect the operation of the sensor. However, selectivity is limited. Background gases could trigger false alarms.



• Infrared (IR) sensor measures the temperature of the gas molecules in relation to IR absorption. The absorbed energy from the IR cause the gas to increase in temperature which relates to concentration. The temperature change is sensed by the pyroelectric crystal which creates a current flow proportional to temperature see picture below. Also, the wavelengths of the light can be measured for a qualitative analysis. Dissimilar gases absorb light at different wavelengths which means every gas has a finger print. These finger prints are archived for identification purposes. IR sensors are widely used in analyzers and monitors. A certain group of sensors are simple, rugged and suitable for high hydrocarbon and CO2 concentration measurement. Sensor poisoning, burnout and sensor fatigue due to high concentrations is non existent. For, the sensor does not contact the gas. It is protected by optical devices. If the zero gas calibration is maintained the response and span remain acceptably accurate. Any signal loss will result in a alarm. Good choice for high concentration measurement.
  


• Photoionization detector (PIDs) uses ultraviolet (UV) light to ionize gas molecules. This sensor can detect volatile organic compounds (VOCs). UV radiation is generated by a special lamp to generate energy to ionize the gas molecules. The resulting free electrons collect at the surface of the electrodes and create a current flow. The current flow is proportional to the gas concentration. The lamp's energy is in electron volts (eV). Standard lamp's have energy levels from 8.4, 9.6, 10.6, and 11.7 eV. The energy magnitude can be determined by the lamp used. 11.7 eV lamp, for instance, uses lithium fluoride. The ionization potential of the gas has to below the output of the lamp. If benzene was being detected, with has a potential of 9.24 eV, a 9.6, 10.6, or 11.7 eV would need to be used. This detector can detect many VOCs at low concentrations and any gas with a energy potential lower than the lamp. The lamp comes into contact with the gas, so needs frequent cleaning. This makes it undesirable for continues or online monitoring. This sensor is good for portable use and periodic readings.