CONDUCTIVITY MEASUREMENT AND ITS CALIBRATION Leif Jensen, Insatech A/S
Agenda • Conductivity – what is it ? how/why it is measured • Conductivity – why is it important ? performance and regulatory requirements • Conductivity – the importance of temperature and its compensation • Conductivity – how to calibrate • Quality assurance februar 2017 2
What is Conductivity in Liquids? • Conductive liquids are called electrolytes • Electrolytes have positively and negatively charged particles called ions. • Positive charged Ions are Cations and Negatively charged ions are Anions februar 2017 3
Conductivity – What is it? • Conductivity is a measurement of how well electrical signals pass through a liquid februar 2017 4
Conductivity – what is it? • Conductivity is a measurement of how well electrical signals pass through a liquid • Used to measure Chemical concentration Control of water purification plant – Water for injection and purified water systems Concentration of dissolved solids Interface detection – is it clean or dirty Heat exchanger breakthrough detection The SI unit of conductivity is S/m Defined as reciprocal of resistance, and, unless otherwise qualified, it Unit of measurement - Siemens – refers to 25 °C (standard nS,uS,mS nano,micro,milli temperature) februar 2017 5
Cell Constant • To make a measurement of current flow and the unit siemens useful, it is necessary to define what is being measured. • The unit Specific Conductivity defined as conductivity of a liquid column with a length of 1cm and a cross section of 1cm S/cm = S * Length cm / Area cm2 • All sensors are supplied with a cell constant, a factoring number derived from the relationship between the size of the contacts and their distance apart. CC = Length cm / Area cm2 • The SI unit of conductivity is S/m and, unless otherwise qualified, it refers to 25 °C (standard temperature) februar 2017 6
Conductivity – How is it Measured? • It is measured with a transmitter and a sensor that has a defined cell Constant! Or an Installation Factor • The sensor and transmitter will have built in temperature compensation – incredibly important due to the temperature effect on Conductivity measurement februar 2017 7
Conductivity – How is it Measured? • Two methods of measurement 1 - Contacting- specific closed cell design & 4 electrode OPEN cells. 2 - Inductive – Toroidal. In a Pharmaceutical manufacturing environment both methods are used Typically polished sanitary sensors - pure waters and CIP Closed cell design for water purity /steam condensate – WFI systems –UP systems Range is usually 0.056 uS to 100 uS New Open sensor design – wide range from 100uS to 200mS Toroidal/open cell for concentration - Typically CIP only Usually CIP/SIP skids and dilution applications – washers Range is typically 1000uS (1mS) to 200mS februar 2017 8
MEASURING PRONCIPLE vs. CONDUCTIVITY RANGE How each Measuring Principle is applied currently februar 2017 9
Conductivity – Why is it important? WFI UPW For purified Water and Water for Injection (WFI), the USP defines the following conductivity requirements: • The USP <645> calibration and performance requirements • Meter reports uncompensated conductivity or uncompensated resistivity. • The display resolution is 0.1 μS/cm or better. • The meter reads accurately to ±0.1 μS/cm when a 0.1% precision resistor replaces the sensor (to calibrate/verify the meter). • The sensor cell constant is calibrated/verified to ±2% • Temperature accurate to 2°C (effective USP 28) • Appropriate dynamic range to meet the above requirements. There are no specific requirements for the dynamic range of the conductivity system, but the conductivity system operate in pharmaceutical waters typically 0.2 to 4 μS/cm and in the fluid that the sensor is calibrated in (ASTM D1125 solution D, 146.9 µS/cm) februar 2017 10
Conductivity – Why is it important? CIP For CIP Clean In Place - it is imperative that the correct cleaning is carried out using the correct conductivity measurement • Correct measurement leads to correct concentrations at high temperature - 1% Caustic at 80 degC for example • The incorrect Temperature Compensation will typically cause 20%-30% more Chemical use • For higher Concentration Aggressive washes – 4% Caustic – 3% Nitric , you will damage your plant – Diaphragms seal, Valves etc by constantly being at high temperature and larger concentrations than you think . • To ensure the Final rinse has been successful februar 2017 11
Sensor Geometry – specific or contact sensors In practice sensors do not use flat plate construction • Rod within tube • Rings • Facing electrodes • Polished sanitary type 316L februar 2017 12
Open Sensors • Measuring range from 0.1 to 1,000,000µS/cm • Suitable for pure water and chemical concentration • Easy clean construction • Polished surface finish • However – very, very challenging to calibrate! 4 pole cell Field februar 2017 13
INDUCTIVE – PRINCIPLE OF OPERATION februar 2017 14
What is the transmitter doing? DC Voltage Excitation • If a dc excitation voltage is used then the electrically charged ions will gather at the oppositely charged plate forming a non conductive layer. • This is called polarisation. • Electrochemical reactions can also take place (eg electro plating is carried out in this way). februar 2017 15
Ac Voltage Excitation • When an ac voltage is used the ions move alternately towards first one plate and then the other. • Therefore min. polarization occurs (assuming the correct frequency is used). • All manufacturers uses square signals with changing polarity in complex ways. februar 2017 16
Temperature Effect • Temperature has a very pronounced effect on the conductivity of a solution. The magnitude of this effect is variable with:- Solution type Solution concentration Temperature Temperature change • Temperature effect is typically non linear – this makes complex matrices or calibration correction quite challenging. • Temperature effect can be as large as 7-10% per Degree C – especially in purer solutions! • The SI unit of conductivity is S/m and, unless otherwise qualified, it refers to 25 °C (standard temperature) februar 2017 17
Cause of temperature dependence Z i · e • n i = Speed of ionic travel · E 6 · π · h · r i • Dissociation. Variable up to 10%/ ° C Water = H2O non conductive Dissociated H+ OH- conductive • At higher concentrations is the effect of “speed of ionic travel” is much greater than the dissociation effect, but in pure water dissociation is the dominant effect • NOTE: Generally a 10°C temperature increase will improve • cleaning efficiency by 50% (above 30°C) – this is why CIP is usually at 80 degC februar 2017 18
Review of importance of temperature • If temperature compensation is turned on , temperature sensors should be calibrated within the conductivity sensor • If temperature compensation is used , choose the correct algorithm for compensation – it could save you 30% on your chemical costs , and save your plant • If Temperature compensation is Turned OFF , you must report the temperature with the conductivity value • Do an audit of all instruments before commissioning februar 2017 19
Look at the varying temperatures in a cycle februar 2017 20
Temperature Compensation UPS februar 2017 21
NaOH matrix februar 2017 22
If you only have 1 transmitter doing two jobs which curve? Sample % per °C from 25C • Salt solution (sodium chloride) 2.12% • 5% NaOH (sodium hydroxide) 1.72% • Dilute ammonia solution 1.88% • 10% HC1 (hydrochloric acid) 1.32% • 5% sulfuric acid 0.96% • 98% sulfuric acid 2.84% • Sugar syrup 5.64% • 10% KC1 (potassium chloride) 1.88% februar 2017 23
Incorrect Compensation effects – NAOH 1% Incorrect Compensation effects – NAOH 1% With NaC1 comp v NaOH or base comp februar 2017 24
Incorrect Compensation effects – Phosphoric acid1% Incorrect Compensation effects – Phosphoric1% With NaC1 comp v Matrix or Acid comp februar 2017 25
Conductivity Calibration TWO METHODS ARE TYPICALLY USED IN CONDUCTIVITY CALIBRATION • Comparison to another instrument (calibration reference system-Insacal) usually accredited • Use a known value standard solution (standard solution) usually traceable februar 2017 26
Recommend
More recommend
