VHP Vaporized Hydrogen Peroxide monitoring GAS DETCTION SYSTENS. Dräger. Technology for Life

Transcription

1 VHP Vaporized Hydrogen Peroxide monitoring GAS DETCTION SYSTENS Dräger. Technology for Life

2 Valid for DrägerSensor H 2 O 2 LC DrägerSensor H 2 O 2 HC in Transmitter Dräger Polytron 7000 (Dräger Polytron 2 discontinued) Items: Calibration Performance Linearity 2-Point calibration Residual effects Acceptable deviations during calibration Calibrated shelf life Calibration period Expected Life Time, End of Life Sensitivity Ratio H2O2 / SO2 Flow Sensitivity Latest Specification Cross-sensitivities Sampling Applications Recovery time Calibration gas concentration 2

3 This small compendium is about to explain some properties and features of the electrochemical hydrogen peroxide sensor from Dräger. New sensors from Dräger are factory calibrated with H 2 O 2 and have attached a calibration certificate. Calibration All sensors show an unpredictable drift of the gas sensitivity over time. That means an applied known concentration will not be displayed as such. The magnitude of the deviation is a result of two effects. One is the intrinsic instability of the sensing technology and the other is the impact of the application. The deviation can be compensated by regular calibration. Calibration is applying a defined known gas concentration and adjusting the electronic amplification until the display reads the expected value. Calibration restores the optimum measurement performance for the time being. Unfortunately hydrogen peroxide gas (H 2 O 2 ) as test gas is readily not available in cylinders with a certificate. Therefore the gas has to be generated onsite for performing the calibration. Three different technics of generating H 2 O 2 are currently practised at Dräger. Depending on the target concentration for a specific sensor, a designated technic is been chosen. Techniques The international guidelines of metrology describe that the methods of calibration should be traceable to appropriate standards or procedures. The aim is to ensure and define an acceptable measurement uncertainty. The implemented methods of calibration at Dräger are using following sources of gas: 1. low H 2 O 2 concentrations generated from urea hydrogen peroxide salt; 2. higher H 2 O 2 concentration generated by vapour pressure taken from the headspace; 3. actively vaporising H 2 O 2 solution ; 4. utilising sulphur dioxide surrogate gas. 1. H 2 O 2 concentrations from urea hydrogen peroxide An oven containing a permeation tube with urea hydrogen peroxide salt ( carbamide peroxide; CAS No: ; CO(NH 2 ) 2 x HOOH ) is kept at a constant temperature. A gas flow (compressed air) passing the urea carries the released H 2 O 2 to a calibration box which the sensors to be calibrated are connected to. The flow rate is adjusted to generate a H 2 O 2 concentration in a desired range of 5 to 20 ppm. The maximum achievable concentration by this method is approx. 30 ppm. The concentration is adjusted and monitored by means of a reference sensor. This reference sensor is calibrated regularly by applying a generally Accepted Procedure (AP) for traceability. The AP to determine the concentration for the calibration is the titration of H 2 O 2 with subsequent photometric validation. A defined gas sample is taken from the calibrating gas stream and passes through a solution generating a coloured titanium complex that is analysed with a photometer. The validated concentration is used to calibrate the sensitivity of the reference sensor. The accuracy of the photometer is regularly verified with a KMnO 4 complex. 2. Gas concentration generated by vapour pressure H 2 O 2 is soluble in water. H 2 O 2 solutions of certain concentrations are commercially available. The H 2 O 2 solution has a dedicated vapour pressure which is a function of the liquid H 2 O 2 concentration and the temperature. After time of stabilisation a defined concentration of H 2 O 2 vapour will adjust in a closed gas volume above the liquid. The concentration is defined solely by physical laws. The traceability is the so called Pure Substance Method PSM. This method will only work under static diffusion conditions but not with gas flow, meaning only single sensors can be calibrated at once. The concentration is not randomly adjustable. 3. Actively vaporising H2O2 solution Higher concentrations and larger gas volumes can be generated by actively evaporating hydrogen peroxide solution. Dropping the liquid solution on a heated plate will generate a predefined concentration in the gas-phase. This volume can be circulated along more than one sensor to be calibrated at once. The concentration will be controlled with a reference sensor, which is calibrated by the Accepted Procedure (titration and photometry). 3

4 4. Using sulphur dioxide as surrogate An additional alternative is utilising a surrogate gas. SO 2 (sulphur dioxide) causes a reliable crosssensitivity on H2O2 sensors. By applying a certified SO 2 concentration and knowing the crosssensitivity ratio, a cross-calibration can be performed. The sensitivity ratio between SO 2 and H 2 O 2 is a heuristic value and not a natural constant. The crosssensitivity value has a mean and a variance which contributes to the uncertainty of the H 2 O 2 measurement. It is expected, based on experience, that in standard applications the sensitivities track over lifetime. Nevertheless there is evidence that the cross-sensitivity ratio can have an unpredictable drift over usage which bears the risk of high measurement uncertainty. The transmitter s supported calibration procedure will only utilise the fixed mean sensitivity value which is stored in the sensor memory. Calibration gas uncertainty by technique H 2 O 2 from urea hydrogen peroxide salt 13 % H 2 O 2 vapour pressure headspace 7 % vaporising H 2 O 2 solution 13 % Sulphur dioxide surrogate gas (off certificate) 2% - 10 % Uncertainty of measurement The uncertainty of a measurement can be calculated by combining specified environmental effects with random uncertainties and systematic uncertainties. If the environmental conditions like temperature, humidity and flow are very similar to the conditions during calibration they can be neglected. Otherwise the impact can be calculated off the specifications from the datasheets of the different sensors. Figure 1 The repeatability (or also called precision) of the DrägerSensor H 2 O 2 is the standard deviation from analysing the results of multiple tests. Applying the same gas concentration under the same conditions multiple times to a sensor will result in a statistical spread of readings. The tolerance window around the mean value containing 68% of all readings is the specified repeatability. The value for the H 2 O 2 sensors is about ±5%. The accuracy (or bias) contributes the uncertainties from the calibration process and time related effects. Time related uncertainties are aging and drift which increase by time, starting with the last calibration. The combined standard uncertainty is been calculated by applying the law of uncertainty propagation. 4

5 Examples of combined standard uncertainties after calibration DrägerSensor H 2 O 2 LC and DrägerSensor H2O2 HC Hydrogenperoxide calibration: Environmental conditions: Flow of calibration gas: 1 l/min ± 0,1 l/min Gas and sensor temperature: 21 C ± 1 C Barometric pressure: environmental Uncertainties: Uncertainty of concentration of generated H 2 O 2 gas (technique 1 or 3) 13% Repeatability of the sensor 5% Calculation U = ( ) 1/2 = 194 1/2 ±14 Combined standard uncertainty of a H 2 O 2 calibrated sensor: ± 14% Sulfur dioxide calibration Environmental conditions: Flow of calibration gas: 0,5 l/min ± 0,1 l/min Gas and sensor temperature: 21 C ± 1 C Barometric pressure: environmental Uncertainties: Uncertainty of concentration of SO 2 cal-gas (certificate) 2% Repeatability of the sensor 5% Calculation U = ( ) 1/2 = 29 1/2 ±5 Combined standard uncertainty of a SO2 calibrated sensor: ± 5% Pre-calibration performance Acceptable deviations during calibration procedure After a period of usage the sensor is checked for performance and measurement accuracy. A known test gas concentration is applied. The actual reading is been compared with the expected reading. The found deviation raises an issue about an acceptable tolerance. The decision has to take into account all measurement uncertainties including assumed drift over time and environmental effects. In the following examples a supposed drift of 3% over 6 months is assumed. The environmental effects are negligible if roughly the same conditions exist as during last calibration. At the end of the calibration procedure the transmitter gets tuned to read the applied concentration by adjusting the span. This process is call calibration. If the span reserve is been used up, after multiple calibrations, the transmitter will indicate an end of life message. Then the sensor needs to be replaced by a new one. Example of measurement uncertainty at point of recalibration for applied SO 2 gas on a H 2 O 2 calibrated DrägerSensor LC: systematic uncertainty of previous calibration ± 14% tolerance of certified SO2 test gas ± 2% cross-sensitivity uncertainty ± 10% assumed drift over 6 months ± 3 % standard sensor repeatability ± 5 % Calculation* U = ( 14 ) + ( ) 1/2 26 Combined standard uncertainty: ± 26% 5

6 Example of measurement uncertainty at point of recalibration for applied SO 2 gas for a SO 2 calibrated DrägerSensor HC: systematic uncertainty of previous calibration ± 5% tolerance of certified SO2 test gas ± 2% assumed drift over 6 months ± 3% standard sensor repeatability ± 5% Calculation* U = ( 5 ) + ( ) 1/2 11 Combined standard uncertainty: ± 11% Measurement performance Example of measurement uncertainty for H 2 O 2 with a H 2 O 2 calibrated DrägerSensor LC: systematic uncertainty of previous calibration ± 14% assumed drift over 6 months ± 3% standard sensor repeatability ± 5% Calculation* U = ( 14 ) + ( ) 1/2 20 Combined standard uncertainty: ± 20% Example of measurement uncertainty for H 2 O 2 with a SO 2 calibrated DrägerSensor LC: systematic uncertainty of previous calibration ± 5% assumed drift over 6 months ± 3% cross-sensitivity uncertainty ± 10% standard sensor repeatability ± 5% Calculation* U = ( 5 ) + ( ) 1/2 16 Combined standard uncertainty: ± 16% At a first glance the SO2 calibrated sensor looks better than the H 2 O 2 calibrated one. The reason is the uncertainty of the concentration of the H 2 O 2 test-gas compared to SO2. But the performance of SO2 calibrated sensor relies completely on the stability and repeatability of the sensitivity ratio between SO2 and H 2 O 2. There is evidence that in certain cases the ratio drifted for unknown reasons in both direction causing wrong readings. If the surrogate calibration was used the measured H 2 O 2 concentration is to be watched with high attention and care and is to be checked for plausibility. A sensor should not be used for more than two surrogate calibration periods. * (The uncertainty of the last calibration is a systematic uncertainty. Therefore in error propagation calculation it has to be considered as a linear sum with other random errors). Target gas calibration service Draeger France offers a calibration service for the calibration of DrägerSensor H2O2 LC and HC with the target gas hydrogen peroxide. This service is described on our Draeger internet homepage. The internet page describes how to receive a quotation and the handling and shipping process. To bridge the time when the sensor under calibration you might need to run a revolving process with extra sensors. Thus you can continue using continuous monitoring. 6

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8 Linearity Diffusion limited electrochemical sensors have been proven to be linear over a wide dynamic range of concentration. Tests have shown that the electrochemical H 2 O 2 sensor generates a current signal proportional to the applied concentration. As long as the electronic circuitry is able to drive the necessary current all H 2 O 2 gas up to 3500 ppm will react at the working electrode. The threeelectrode sensor guaranties that the electrochemical potential, favourable for the chemical reaction, stays stable during the reaction. The speed of reaction ensures that no accumulation of H 2 O 2 at the working electrode happens. Due to the fact, that no consumables are participating in the electrochemical reaction, no deficiency of any substance involved is known. Reaction: 2 H 2 O 2 => 2 H 2 O + O 2 The highest tested concentration for linearity in the lab on the LC sensor was 1500 ppm and 3500 ppm on the HC. 2-Point calibration Derived from the measured linearity it is sufficient to calibrate a sensor at zero (zero calibration) and at one other concentration (span calibration) within the measuring range. The Polytron transmitter, the sensor is operated in, is designed to support a two point calibration. The measured zero-signal and the signal at the acknowledged gas concentration are used for calculating the H2O2 sensitivity which is revised in the memory of the sensor afterwards. This sensitivity is been used to calculate the respective concentration within the entire measuring range. If the sensitivity of the sensor after a calibration is less than a certain threshold the transmitter submits an end of life indication. Then the sensor has to be replaced by a new one. Residual effects If H 2 O 2 gas is generated by evaporating a hydrogenperoxid/water-solution condensing easily can happen on surfaces like tubing, pumps and adapters. These surfaces get saturated and covered during exposure with a H 2 O 2 /water film. After the exposure, H 2 O 2 is slowly released by evaporation leading to a prolonged decline time. Active heating or purging with dry air can accelerate the recovery from residual effects. Sampling systems should have short tubing and be on the same temperature level as the system gas. Calibrated shelf life An unused sensor on the shelf in its original package will experience the same ageing as a sensor in use under the same environmental conditions. Due to the fact that H 2 O 2 does not wear the sensor and does not consume any internal chemicals the H 2 O 2 exposure has almost no aggravating impact on the expected performance. Physical stress like extreme or swinging environmental conditions (temperature, pressure, humidity and flow) can have an impact on the decay of sensitivity due to wear and tear on components like membranes, catalysts and electrolyte. The expected accuracy can be calculated as descried in chapter above. Calibration period DrägerSensors are calibrated in the factory and come along with a factory predefined calibration interval of six months. In a suitable Polytron transmitter 6 months after the last calibration there will a warning massage in the display that this sensor needs recalibration. The factory pre-set calibration interval is a default only. Ideally the user has to define, derived from his requested accuracy and the specified drift, for the application his individual time interval. This can be entered into the Polytron transmitter as a maintenance reminder. Expected Life Time, End of Life A sensor will lose sensitivity with time regardless whether it s been in use or on the shelf. This loss is individually different and can only be described statistically. The specified drift is the observed change of the mean value of sensitivity over time of a sensor population under normal conditions. Whether a sensor is still operable can only be checked during a calibration. When performing a calibration the gain of the transmitter gets adjusted to compensate for the sensitivity loss of the sensor. This will reinstall the maximum accuracy of the system. But at a certain 8

9 point the sensor s signal to noise ratio becomes prone to false alarms. Now the sensor has to be replaced. It s the end of the usable life. Some Polytron transmitters support the feature to inform the user during calibration that the sensitivity is within a 10% range approaching the end of life threshold. Although the sensor is still operable it should be replaced soon. Sensitivity Ratio H 2 O 2 / SO 2 Electrochemical sensors are sensitive to more than one gas. On a H 2 O 2 sensor sulphur dioxide SO 2 will also generate a signal of certain sensitivity. Additionally it is assumed that an aging H 2 O 2 sensor will lose sensitivity towards H 2 O 2 the same rate as on SO2. Even though of the difference in chemistry the correlation of sensor properties between both gases is assumed to be sufficient. If Hydrogen peroxide is not available for calibration a surrogate gas like SO 2 could be used. But this adds an additional measurement uncertainty. The statistically derived mean value of the relative cross-sensitivity ratio between SO 2 and H 2 O 2 is about 0,7 with a statistical tolerance of <± 10% for a confidence interval of 68%. This is based on lab experience with new sensors. Caution, a sensor in use could become contaminated at the sensor front with impurities ( e.g. grease, dirt, chemicals or metal chippings). Such contaminations could selectively impede the sensitivity on either gas. This will never become noticeable nor compensated until the sensor is calibrated with target gas! A surrogate calibrated sensor should only be used in indicatory applications never for reliability and quality tasks. Flow Sensitivity Due to different design the H 2 O 2 sensors LC and HC show different flow sensitivities. The DrägerSensor LC for low concentrations has an "open design" to achieve best sensitivity. The drawback is a flow sensitivity. If the flow differs from the flow been used during calibration the sensor will show a deviating concentration (see figure 2). The factory calibration is done with 0,3 m/sec which reflects standard convection conditions. Flow sensitivity LC rel. sensitivity 4 3,5 3 2,5 2 1,5 1 0, m/sec Figure 2 The DrägerSensor HC because of a different design has no flow dependency. Sampling Applications For sampling application special attention is to be paid to H 2 O 2 adsorption and desorption effects on exposed material surfaces. For sampling small concentrations, in opposite to the recommendation in the Polytron 7000 manual, the LC sensor shall be positioned before any sampling pump, as close as possible to the inlet. Thus is to reduce H 2 O 2 adsorption effects The DrägerSensor HC can be affected by strong pulsing pressure from a pump in sampling installations. This will affect the gas diffusion in the capillary in front of the sensor and can lead to 9

10 increased sensor readings. Recovery time A sensor doesn t stock H 2 O 2 internally. The recovery time is mainly driven by the vicinity of the measuring electrode of the sensor. The load of absorbed and adsorbed H 2 O 2 on surfaces is feeding the sensor for a long time until everything is evaporated. The evaporation rate of H 2 O 2 is 13 times slower as of water. The duration of evaporation can be accelerated by effectively purging even the surfaces with fresh air or additional heating the air or surface. A suitable material selection can also contribute to the recovery time in particular when tubing, ducts or pipes are used to transporting samples of the air. Example: The alarm threshold is set to 1 ppm H 2 O 2. The maximum cycle concentration is assumed 1000ppm. The DrägerSensor LC is exposed to the concentration during the entire decontamination cycle. The recovery time from high to low, following an e-function, is the T 99,9 time which already is a huge step-change. Due to desorption and purging it will last more than an hour to clean the volume and surface, archiving a reading of 1 ppm constantly and below. Down to the real zero-clamping at 100 ppb it will take additional time. The total recovery time is more than the exchange of air it is the recovery of the whole exposed system. A prediction is impossible. When interrupting the purging process a rise of the measured H 2 O 2 concentration can be observed proving the release of stored H 2 O 2 from the system. Calibration gas concentration All our EC sensor are linear by nature. That has to do with Fick s law of diffusion and the range the sensors are operated in. Derived from that it is regardless what concentration is been used for calibrating the span. And a twopoint calibration is sufficient (span and zero). The other restriction is that it is no possible to choose and adjust to a specific concentration, because H2O2 gas is not available in cylinders you can use for blending. Thus we utilize the concentration be randomly archive with our proven method of gas generation (see Compendium) each time verified by analytical means. Typical values in our production for the LC sensor are in the range between 15 and 25 ppm and for the HC to 300 ppm. The applied concentration is written in the calibration certificate coming with each sensor. 10

11 Specification (Nov 2012): Specification DrägerSensor H 2 O 2 LC measuring range min 0 to 1 ppm default 0 to 5 max 0 to 300 lower detection limit LDL 0,1 ppm linearity tolerance ± 1% of reading repeatability zero ± 0,05 ppm span ± 5 % of reading temperature effect none flow effect 0 to 6 m/s zero none span see figure 2 long term drift span ± 6% per year T90 time 10 sec typical relative sensitivity on SO2 10ppm SO2 = 7 ppm H 2 O 2 ± 10% accuracy factory calibration (H 2 O 2 ) better than 15 % standard confidence level Cross sensitivities of H2O2 LC on other desinfectants: Gas Concentration Effect Alcohol R-OH 500 ppm < LDL Ammonia NH3 50 ppm < LDL Ethylene oxide C2H4O 30 ppm < LDL Acetic acid CH3COOH 5% aqueous solution 0,25 ppm strong drift Formaldehyde CH2O High doses damages the catalyst by polymerisation Chlorine Cl2 5 ppm -1 Ozone O3 1 ppm - 0,5 Chlorine dioxide ClO2 Negative Peracetic acid PAA CH3COOOH 0,04 % aqueous solution Negative reading strong drift Specification DrägerSensor H 2 O 2 HC measuring range min 0 to 1000 ppm default 0 to 4000 max 0 to 7000 lower detection limit LDL 100 ppm linearity tolerance ± 1% of reading repeatability zero ± 10 ppm span ± 5 % of reading temperature effect none flow effect 0 to 6 m/s zero none span ± 5% of reading long term drift span ± 3% per year T90 time 30 sec typical relative sensitivity on SO2 1000ppm SO2 = 650 ppm H 2 O 2 ± 10% accuracy factory calibration (H 2 O 2 ) better than 15 % standard confidence level Values are typical and subject to alteration. The data are for information only and don't justify any liability claim nor extend the use of the product as specified in the sensor datasheet. Dräger reserves the right to amend the design and specification of the sensor without prior notification. 11