PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques

Transcription

1 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques Introduction The amount of dissolved oxygen in process water is continually gaining importance in many industries as a critical parameter to be measured and controlled. The semiconductor and power industries are continuously requiring increased accuracy in dissolved oxygen detection as lower levels of oxygen are required in the process water. The purpose of the on-line, dissolved oxygen measurement is to continuously monitor the process water to ensure that the level of oxygen is within specifications. The dissolved oxygen concentration is typically measured at multiple points to determine the efficiency of the means used for oxygen removal - typically some form of mechanical degasification/deaeration and/or chemical injection - as well as to detect process upsets or leaks. However, plant personnel are often frustrated with the process of trying to obtain quantitative results for the measurement that are useful and meaningful. Many of the proper techniques used to actually obtain and correctly measure the dissolved oxygen concentration in a representative sample of the process water are not well understood, and the personnel responsible for the measurement are often very distrustful of the instrumentation as a result. When these techniques, consisting of proper calibration and sampling practices, are learned and understood, they can be used to effectively discern whether or not the dissolved oxygen levels in their process meets the specifications required. A semiconductor plant in Austin, Texas is one such example of a site where these techniques were effectively utilized to validate their sample lines and measurement instrumentation, and in doing so discovered a problem with a process component. The Problem The water is used as a rinse agent in the clean room process, which is why there is a focus on ever-tightening standards for controlling dissolved oxygen. Dissolved oxygen, when allowed to remain in solution, can cause oxidation in the mixed bed resins and can then be carried over to the clean room chip manufacturing level. Upon start-up of this facility in 1995, there was a dilemma in establishing a baseline for dissolved oxygen removal. Once a baseline was established, it would have to be accurately verified with a known standard, thus assuring optimum system performance. The vacuum degasification system removes the dissolved oxygen from the process water (Figure 1). Measurements are taken before and after the degasifiers, as well as on the supply loops feeding the factory. Although the vacuum degasification system under peak performance is capable of producing water in the 9 parts per billion (ppb) range, many variables must be met in order to maintain these operating specifications. The readings after Vacuum Degasifier #1 consistently read within the 5-20 ppb range, while the readings after Degasifier #2 read in the ppb range. Process streams which should have been identical consistently read a difference in dissolved oxygen concentration of more than 20 ppb. The sampling lines and instrumentation were always suspect since consistent and repeatable results were difficult. In order to ensure that the dissolved oxygen removal system was working correctly, the meters and sample lines would have to be proved first. To do so, a procedure was needed to test and prove that the readings were representative of the true oxygen concentrations. This was done by validating calibration methods and verifying that the sample lines were not contaminating the reading. The aforementioned semiconductor facility is one of the leaders in the semiconductor manufacturing industry, and as such focuses on producing the most advanced product with a major emphasis on quality. Ultrapure water is one asset that allows this facility to achieve this with minimal impact to its people and the environment.

2 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques 2 Calibration Dissolved oxygen probes designed for measurements in the parts per billion range generate a current proportional to the oxygen partial pressure. For the typical levels of oxygen partial pressure present in the process and calibration mediums used in the semiconductor industry, the signal produced by the probe is essentially a linear function of the partial pressure.1 In order to obtain an accurate representation of the dissolved oxygen, current values must be calibrated to the partial pressures which they represent. Few dissolved oxygen instruments display the partial pressures of oxygen, however. Rather, the dissolved oxygen concentration, which is directly proportional to partial pressure of oxygen for the range of interest 2, is the commonly displayed unit of measure, and will be used in the following discussion along with partial pressure. The common calibration method for most dissolved oxygen systems involve exposing the probe in air, since air will always have a known partial pressure of oxygen dependent upon atmospheric pressure as well as temperature-dependent humidity, if present. Air-saturated water can also be used for calibration, but guaranteeing a precise partial pressure of oxygen in the water is difficult and problematic. Air calibration remains the tried and true procedure for calibrating dissolved oxygen probes. However, air calibration can easily can be done incorrectly, yielding significant error if the characteristics of the air in the immediate vicinity of the probe are not taken into consideration. It is vitally important that the user understand the effects caused by variations in any of these variables in order to ensure an accurate and consistent air calibration. Pressure The constituents of air have been well defined, and it is known that air contains % oxygen. 3 Since the total pressure in the air is the sum of all of the partial pressures (Dalton s Law), an atmospheric pressure of 760 millimeters Mercury (mmhg) in dry air will contain a partial pressure of oxygen (po 2) of approximately 159 mmhg (760 mmhg * ). Changes in atmospheric pressure will cause a directly proportional change in the partial pressure of oxygen in the air. Atmospheric pressures will vary depending upon altitude and local weather conditions. Some average pressures for varying altitudes are listed in Table 1. The relationship between oxygen partial pressure and total atmospheric pressure should be understood and incorporated into the air calibration in order to minimize calibration error, which could be as high as 5-10% dependent upon altitude and local weather conditions. Most dissolved oxygen meters that have any sort of advanced air calibration (such as temperature compensation, which will be discussed in a later section) will be based upon an atmospheric pressure of 760 mmhg. Most tables of oxygen solubility are referenced to this value.3,4 Because of the change in oxygen partial pressure with changes in atmospheric pressure, a correction must be made when the pressure varies from this value. A simple means of incorporating pressure changes is listed in the correction factor shown in Table 1. The value listed is a rough multiplier which can be used once the initial oxygen concentration is determined based upon temperature and relative humidity. A more accurate calculation for incorporating pressure will be discussed after relative humidity and temperature effects are investigated. A few newer dissolved oxygen meters contain a pressure sensing device which provides compensation for pressure effects when an air calibration is performed. Since most meters do not have this, it is usually necessary to note the average pressure in the local vicinity of the probe, which will be mostly altitude-based, and adjust the calibration using the simple correction factor or the more complex calculation performed later. A mercury barometer located in the immediate vicinity of the meter will give a relatively accurate measurement of the local atmospheric pressure if an older meter with no pressure sensor is used. Relative Humidity and Temperature The discussion of pressure effects were based upon atmospheric pressure with dry air (no moisture content). Whenever air contains a certain amount of moisture, the atmospheric pressure contains another source of partial pressure -- water vapor. If a comparison of the oxygen partial pressure in air with 100% relative humidity and air with 0% relative humidity is done while both are at the same atmospheric pressure, the air with 100% relative humidity will have a lower oxygen partial pressure due to the presence of the water vapor pressure (ph2o). Water vapor pressure in air varies with temperature, and is well defined.3 The effect of temperature on oxygen partial pressure in moist air is such that higher temperatures yield lower oxygen partial pressure, while lower temperatures yield higher pressures. This relationship is shown in Figure 2.

3 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques 3 Note that the effects of relative humidity and temperature can cause errors when air calibration is performed in dry air, since most of the current tables and meter temperature compensations are based on air containing 100% relative humidity. Table 2 shows both the oxygen concentration, which is linear with the partial pressure of oxygen, that would be present at 100% relative humidity and 0% relative humidity. The values only differ by a few percent in ambient air conditions, and thus is generally ignored. Most dissolved oxygen meters have temperature compensation for air at 100% relative humidity, and no manual correction is necessary. However, many older meters do not have temperature compensation included, and therefore this calculation must be done manually. If temperature is not compensated for in the calibration, the error can be as much as 20 to 30 % for every 10 degrees difference from 25º C, and therefore temperature compensation is standard on most dissolved oxygen meters today. Since the effects of relative humidity is minimal at all but the highest temperatures, no current dissolved oxygen meters incorporate any kind of relative humidity sensing device. In order to ensure an accurate temperature and current reading, the probe must be exposed to the air for enough time to allow thermal equilibrium to occur. There are often significant temperature differences between the process water and the ambient air. Larger temperature gradients between the two necessitate additional time for thermal equilibrium to take place. For instance, a 20º C difference between ambient air and process water can cause a calibration delay of about 30 minutes in many probes for the probe to fully equilibrate to ambient temperature. Since most temperature gradients will not be this large, allowing approximately 15 minutes is usually a safe assumption. It is common for users to calibrate the unit before the dissolved oxygen meter is reading the stabilized temperature and current value, which can cause significant error since a difference of even 5º C from actual can cause the reading be off by 5 to 10%. It is often useful to have a calibrated temperature sensor, accurate to 1º C or better, at the calibration location to know when the probe temperature is reading the correct ambient air temperature. Equation 1 should be used with air with 100% relative humidity, and Equation 2 should be used for air with 0% relative humidity. Equation 1 (100% Relative Humidity): S = (S ) * (P - p) / (760 - p) where: S = Oxygen solubility at barometric pressure of interest S = Oxygen in saturation at one atmosphere (760 mmhg) at a given temperature P = Barometric pressure of interest p = Vapor pressure of water at the temperature of interest Example 1: The user wishes to calibrate a dissolved oxygen probe in air at an altitude of 3500 feet. The temperature is 30º C, and the relative humidity is 100%. At an altitude of 3500 feet, the atmosphere pressure will usually be about 668 mmhg (Table 1). The sample temperature is 30º C, and the relative humidity is 100%. From water vapor pressure tables, the water vapor pressure at 30º C is 31.8 mmhg. 3 The oxygen saturation level at 760 mmhg and 30º C is 7.54 ppm (Table 2). Substituting these values in the above equation gives the following: S = (7.54) * ( ) / ( ) = 6.59 ppm Example 2: Assume the same conditions as in example 1, but with a relative humidity of 0%. In this case, the value used for the oxygen saturation level would be 7.87 (Table 2), not The calculation will change since there will be no water vapor pressure. Equation 2 (0% Relative Humidity): S = (S ) * (P) / (760 mmhg) Substituting the above values into the equation yields the following: It is useful to have an equation which can be used to determine oxygen concentrations in air based upon temperature, relative humidity, and pressure. 2 Since the full equation is quite lengthy and complex, two easier versions are presented to the user, along with Table 2, to determine the correct oxygen concentration in air. S = 7.87 * (668) / (760) = 6.92 ppm Note that the multiplier of (668) / (760) is actually the simplified correction factor listed in Table 1 for an altitude of 3500 feet (0.88). Table 3 lists calibration values for varying temperatures pressures at relative humidity levels of 100%.

4 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques 4 Calibration Procedure The following steps can be followed to ensure an accurate calibration. 1. Expose the probe to air. Since meters that incorporate temperature for calibration purposes base the compensation on 100% R.H. air, the calibration will be most accurate whenever the air around the probe is high in relative humidity. 2. Wait until the dissolved oxygen and temperature readings stabilize. This usually takes about 5 to 45 minutes, depending upon the temperature differences between process water and air, as well as the oxygen difference between the process and the air. If the meter does not have a temperature reading, it may not have a temperature sensor either, in which case the probe will be ready for calibration as soon as the dissolved oxygen reading stabilizes. When no temperature reading is available from the meter, the user should have an accurate temperature sensor to determine the ambient air temperature for calculation purposes. 3. Compare the temperature and pressure reading on the meter to a calibrated temperature and pressure reading in the same vicinity of the probe. If the meter doesn t have a pressure sensor available, use a local mercury barometer to determine the atmospheric pressure. If a barometer is not available, just use the value from Table 2 associated with the altitude at the location of the meter. 4. If the pressure and temperature agree with the local readings, then perform the air calibration. If the temperature and pressure reading on the meter differ significantly (pressure by more than about 5 mmhg or temperature by more than about 2º C), recalibrate them, if possible, or adjust the readings manually. If the temperature and pressure readings are not included with the meter, the manual calculations described earlier should be used to determine the correct calibration value. 5. Place the probe back in the process. It is useful to adjust the flow up to 300 ml/min or more for the first few hours in order to flush out any oxygen that may have become entrained as a result of the air calibration. Sampling Correct sampling is perhaps the most misunderstood requirement in obtaining valid dissolved oxygen readings at low ppb levels. Effects such as long recovery times from air calibrations, errors in comparisons between on-line probes and wet chemistry tests, and incorrect readings of the actual process dissolved oxygen levels are typical examples which affect more sampling systems than users know. Sampling lines often have dead legs (spaces where stagnant water or air is present), trapped air bubbles, and/or leaks in them. Deadlegs are found in areas of piping that do not always have flow through them, causing an area of stagnant water to be present. This can result in an area of trapped air, which will cause a seemingly high dissolved oxygen reading. Air bubbles can be found in piping that is flowing in a downward direction. The natural buoyancy of air will cause the bubbles to resist the flow of water in downward flowing piping at lower flow rates. Deadlegs and air bubbles will eventually dissolve (they will dissolve faster at higher flow rates), but may take hours or days to dissolve if the flow rate is low (100 ml/min or less). While air bubbles will eventually dissolve, leaks will continuously cause erroneously high readings. While leaks are usually minimized by higher line pressures due to successive stages of smaller sampling lines, their presence can still cause large errors in readings, especially at the low levels of oxygen present in most power plant and semiconductor process lines. A way of testing for leaks is to increase the flow by 50 to 100% and observe the dissolved oxygen reading for the total length of time that it would take water to travel from the process to the probe. If a drop in reading is noticed, it may be due to a leak. The flow should be returned to normal for 10 to 20 minutes, and the test repeated. If the same drop in reading is noticed again, there is most likely a leak somewhere upstream of the probe. Higher flow rates can help to minimize leaks, since higher flow rates will dilute the oxygen ingress. Leaks can be caused by nonmetallic piping, fittings, rotameters, and even probe mountings. Flowmeters, a common cause of leaks, can often be placed after the probe and perform the same function without introducing a leak until after the probe reading. Non-metallic sample lines have been observed to allow oxygen ingress even when new, and often get worse over time, so they should be replaced with stainless steel piping. Loose fittings are also a common source for leaks and may need to be redone with additional teflon tape or sealant of some sort. Incorrect mounting of the probes are common and can cause leak problems as well. The manufacturer of the probe should be contacted to ensure that the correct procedure is being followed for mounting the probe.

5 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques 5 Sampling for comparison readings with a portable meter should always be done in parallel lines and never in series, as one probe will contaminate the downstream reading. Wet chemistry readings should always be done in parallel lines as well. Most users have a separate line for these readings which is turned on for a few minutes at 200 ml/min or less before the reading is made. Doing such will often result in a higher-than-actual reading, since even a few hours at this flow rate may not be enough time to totally remove all the entrapped air or air bubbles in the sample line. In addition, a period of time (approximately 5 minutes) is often needed in many sample lines just to ensure that the sample contains current process water, and not process water from the last test. At a minimum, it is recommended that the wet chemistry sample line be running for at least four hours at a high flow rate (300 ml/min or more) before sampling in order to ensure a good reading. More extensive work on proper wet chemistry sampling has been reported elsewhere. 4,5 Results Based on the information provided regarding proper measurement system calibration, a procedure was developed to ensure consistent, accurate air calibrations. The meter used employs temperature compensation for air with 100% relative humidity. No pressure compensation was available on the model used, so the air calibration was adjusted manually if pressure deviated from the norm. Complete electronic calibrations were also done on the dissolved oxygen analyzers, with several discrepancies found and repaired at that time. After full electronic and air calibrations, the meters and probes sampling the outlets of Degasifier #1 and #2 were switched. The readings at the sample points remained the same, verifying that the probes and meters were working correctly. This pointed to the sampling lines potentially being the source of the problem. The sampling lines were entirely stainless steel, but leaks were suspected from fittings and sample line flowmeters. Leak testing was done with little change in reading, thus indicating that the discrepancies between Degasifier #1 and #2 were probably not due to sample line leaks. As a precaution, the sampling system was further pressurized in order to eliminate any potential leaks as a possible point oxygen filtration into the lines. At this point, readings were trended and the data logged to pinpoint possible discrepancies and changes occurring within the system. From this information and a new confidence in the sampling system and analyzers, discrepancies between supposedly identical streams pointed to the vacuum pump system supplying the degasifiers. This system is vital in order to maintain levels of vacuum useful to the process, and is assisted by seal water and eduction venturis. By checking vacuum levels in the degasifiers and proving seal water flow rates to the pumps, all indications point to the eductors at the pumps. It was determined that the cause was pressure drops in the eductors, causing ice buildup which plugged the orifice holes, drastically reducing pump performance. Elimination of this problem caused readings at the outlet of Degasifier #2 to change from the 25 to 50 ppb range to peak performance of about 10 to 15 ppb. This proved beneficial through money savings, system balancing, and improved process quality, which in turn has gained the trust for the instrumentation measuring such a critical parameter in the system. As technology continues to advance, even lower levels of dissolved oxygen will become more critical to detect for applications in today s competitive semiconductor market. This means that baselines must first be established for allowable dissolved oxygen concentrations in the process. However, specifications concerning process equipment and all controlling variables must be addressed prior to questioning the integrity of the dissolved oxygen analyzers and sample lines at this semiconductor manufacturer s facility.

6 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques 6 Water from UV Sterilizers Ambient Air Eductor 2 Dissolved Oxygen #1 Vacuum Degasifier Water Out 1 Vacuum Pumps 2 Dissolved Oxygen #2 Process Water 1 Figure 1: Dissolved Oxygen Sample Points for Vacuum Degasifier Altitude (ft) Pressure (mm Hg) Calibration Correction Factor Sea Level Table 1: Oxygen Value Corrected for Pressure (25º C) 4

7 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques Partial Pressure, mmhg ph2o po2, 100% R.H. po2, 0% R.H Temperature, Degrees Celsius Figure 2: Partial Pressures of Water Vapor and Oxygen vs. Temperature

8 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques 8 Temperature DO (100% R.H.) DO (0% R.H.) Temperature DO (100% R.H.) DO (0% R.H.) (Degrees Celsius) (ppm, mg/l) (ppm, mg/l) (Degrees Celsius) (ppm, mg/l) (ppm, mg/l) Table 2: Dissolved Oxygen Solubility vs. Temperature2,3

9 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques

10 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques Table 3: Oxygen Concentration (ppm) for varying pressures (mmhg) andtemperatures (Degrees Celsius) at 100% Relative Humidity

11 PPB Dissolved Oxygen Measurement - Calibration and Sampling Techniques 11 More Information For more information on using Dissolved Oxygen Measurement, visit or contact your Honeywell account manager. Honeywell Process Solutions Honeywell 1250 West Sam Houston Parkway South Houston, TX Honeywell House, Arlington Business Park Bracknell, Berkshire, England RG12 1EB UK Shanghai City Centre, 100 Junyi Road Shanghai, China SO ENG January Honeywell International Inc.