Whitepaper Measuring Pressure in Medical Sterilizers Selecting the Right Pressure Sensor OVERVIEW The sterilization of medical equipment is one of the most important processes in medicine. It kills all living microorganisms on medical and dental equipment, including bacteria, spores, viruses and fungi, allowing them to be used safely time and again. While there are several sterilization methods available, one method gaining popularity in medical service centers and hospitals, as well as dental offices, is hydrogen peroxide. Hydrogen peroxide is used because it is effective, safe and affordable. This paper discusses two sterilization techniques that utilize hydrogen peroxide and how to select the right pressure transducer based on the specifications and operational features of the transducer that are applicable for sterilization equipment. Sections in this whitepaper include: 1. Hydrogen Peroxide Sterilization Processes Process Pressure (Vacuum Manometer) Tank Level Pressure (Gauge Pressure Transducer) 2. Selecting the Right Pressure Transducer Pressure Range Accuracy Long-Term Stability Pressure Fittings Electrical Outputs Media Compatibility Overpressure Installation Orientation Agency Approval CONTENT Sterilization Processes Vacuum Manometers 2 Gauge Pressure Transducers 3 Transducer Selection Pressure Range 4 Accuracy 5 Long-Term Stability 7 Pressure Fittings 8 Electrical Outputs 9 Media Compatibility 10 Overpressure 10 Installation Orientation 10 Agency Approval 11 Setra Systems, Inc. All Rights Reserved.
1. Sterilization Processes Vacuum Manometer to Measure Sterilizer Process Pressure There are two different processes in which hydrogen peroxide is used for sterilization. Each requires a different measuring instrument to ensure proper operation. The first process is plasma sterilization, which uses a capacitance vacuum manometer to measure process pressure during the sterilization cycle. Medical devices are inserted into the sterilizer chamber. This includes scalpels and cardiac catheters, as well as dental equipment such as periodontal scalars and extraction forceps, and other equipment with recesses and grooves where microorganisms can grow. It is then sealed and evacuated which creates a vacuum. Hydrogen peroxide sterilant is then injected from a prepackaged cassette and vaporized into the chamber which interacts with the surfaces of the medical/ dental equipment. This causes a rise in pressure. Pressure is reduced again and a plasma is struck. Free radicals of the hydrogen peroxide interact with microorganisms, killing them. The hydrogen peroxide then breaks down, leaving only water vapor and oxygen. The medical equipment can be used immediately after sterilization or stored. Figure 1: Simplified Cycle Diagram of the Plasma Sterilization Process In order for the sterilization process to work effectively, a specific base pressure is required in the chamber when energy strikes the plasma. For this to happen the capacitance vacuum manometer is mounted to the sterilizer to monitor and/or control chamber pressure and confirm proper pressures are maintained for each step of the sterilization process. When selecting a vacuum manometer for your applications, work with a supplier that offers a family of products, each with a variety of full-scale pressure ranges from 10 to 1000 Torr with +/- 0.5 percent of reading accuracy (if possible, find one with a +/- 0.25 percent of reading accuracy option) and negligible temperature coefficients across its 0ºC to 50ºC compensated range. Setra Systems, Inc. All Rights Reserved. 2
Gauge Pressure Transducer to Measure Tank Level The second sterilization process uses vaporized hydrogen peroxide, which is stored in liquid form inside of a tank before its turned into vapor in the sterilization chamber. Liquid sterilant is ejected from the tank into the system to kill microorganisms on the equipment. In this application, a gauge pressure transducer is used. It senses pressure placed on it by the liquid in the sterilant tank, which can be translated to the liquid level of the hydrogen peroxide. Here is how it works: the pressure transducer is mounted to the bottom of the sterilant tank. The weight of the sterilant puts pressure on the transducer. A vent tube inside a cable in the back of the transducer allows the unit to measure pressure relative to local atmospheric pressure (gauge pressure measurement). As the sterilant depletes, the weight (pressure) lessens and the output signal from the transducer to a redundant monitoring system reduces accordingly. It can then warn sterilizing technicians of low tank level. When the transducer registers 0 PSIG, the tank is dry. Because gauge pressure transducers are referenced to atmosphere, they breathe through a vent tube which in some cases, is through a vent in the cable. To work properly and safeguard no liquid enters the transducer, the tube must be properly vented to a dry location, such as a control panel or a junction box that exhausts to atmosphere. A desiccant cartridge can be placed at the surface connector of the vented cable for additional protection. Setra Systems, Inc. All Rights Reserved. 3
2. Selecting the Right Pressure Transducer Pressure Range Regardless of which of these two transducer types is right for your application, there are certain specifications and operational features to consider before making a selection. One of them is pressure range. Select a sensor with a full-scale pressure that is close to anticipated needs. If the application calls for 10 Torr, for instance, a 20 Torr product will provide best accuracy. In contrast, purchasing a 1000 Torr sensor would be wasting most of the measuring range. Do not over specify the operating range of the sensor just to be safe. Manufacturers state the sensor s safe over-range limits and this information should be sufficient. By over specifying your sensor range, you will reduce its signal magnitude and zero-based errors will increase as a percentage of the measurement range. 1 If a low pressure is to be measured, a transducer with the lowest full-scale pressure should be selected. This provides the most usable range as well as the most usable output. However, be advised that the lower a transducer s pressure range, the less robust it is. In other words, the more susceptible it is to over pressure failure. Therefore, engineers selecting a transducer to be used in an environment subject to very high overpressure would be wise to select a unit that is rated to withstand the expected overpressure. Vacuum manometers are available in standard ranges of 10, 20, 100, 200, and 1000 Torr. Some medical applications occasionally have a one-of-a-kind, nonstandard pressure range requirement, such as 147 Torr. In such a case, the design engineer should contact the transducer supplier s application engineering staff. The supplier may be able to customize a product to more precisely meet the desired pressure range. The same applies to system engineers and sterilizing technicians designing a new installation. The supplier s engineering staff can discuss the upcoming project, provide phone support, answer questions, and can recommend the best sensor for the application, saving a lot of time and angst. This image represents a pressure transducer that is expected to measure vacuum pressure but is exposed to some positive pressures (>14.7 psia) Setra Systems, Inc. All Rights Reserved. 4
A pressure transducer that is expected to measure vacuum & positive pressure and is also exposed to some additional positive pressures (>14.7 psia) Accuracy Accuracy, of course, is one of the most important considerations when selecting a vacuum manometer or gauge pressure transducer. In either case, the higher the accuracy, the better the process control. This is especially important during hydrogen peroxide plasma sterilization in which pressure must be precisely controlled at various steps for the process to work effectively. Determining accuracy for either a vacuum manometer or gauge pressure transducer is actually a measure of their inaccuracies. Complicating matters further, each instrument calculates accuracy differently. Take gauge pressure transducers. They typically express the deviation from the expected output value as a percentage of the full-scale error. Consider this example. A sensor has 10 a volt full scale output. The transducer s output ranges from 0 to 10 volts dc (VDC). Ten percent of that output should read 1.000 volt exactly, but because of error, the unit is off by say, 1 millivolt. To calculate accuracy as a percent of full scale (as the name implies), the 1-millivolt of error is divided by full scale output and is multiplied by 100, resulting in +/- 0.01 percent of full-scale error. Vacuum manometers, on the other hand, are typically rated as a percentage of reading error. In this same example, the 1-millivolt of error is divided by 1 volt because that s the reading at that point and then multiplied by 100, resulting in +/- 0.1 percent of reading. The same 1mv of error reported as a percentage of reading error is 10 times larger than the same 1mv error calculated as a percentage of full scale! Setra Systems, Inc. All Rights Reserved. 5
Percent of Full Scale (RSS) Error FS Pressure %FS Error Band Pressure Actual Response Allowable % Full Scale Error Band More accurate at the higher-end of the pressure range Percent of Reading Error FS Pressure Pressure Actual Response Allowable % Reading Error Band More accurate at the lower-end of the pressure range Setra Systems, Inc. All Rights Reserved. 6
For vacuum manometers, look for an accuracy rating of +/- 0.5 percent of reading. For transducers, find an accuracy rating of +/- 0.20 percent FS. Keep in mind that accuracies are specified for room temperature. So if a sensor is installed in an environment other than typical room temperature, the change in environment temperature will cause the transducer s output to change. Sensor manufacturers disclose temperature error as a function of temperature, which allows sterilizer technicians the ability to calculate that error and determine the true accuracy of the pressure reading. Long-Term Stability Long-term stability is another very important consideration. Long-term stability is a measure of how much the output signal will drift over time under constant operating conditions. Drift is caused by the cycling of pressures, temperature extremes, environmental changes, as well as vibration, shock and aging. Long-term stability is typically expressed as a percentage of full scale over a 12-month period. For a capacitance vacuum manometer, look for +/- 0.5 percent FS/YR (+/- 1.0 percent FS/YR for ranges less than 100 Torr full scale when operated at 80 C). In the case of the gauge pressure transducers, expect stability of 0.5 percent FS/ YR. Long-term stability and accuracy are assured for sealed sanitary gauge-type pressure transducers. That s because of a capacitance-sensing element, coupled with a signal conditioned IC-base circuit. All transducers experience some drift over time, regardless of how well they are built. The critical question a sterilization technician needs to ask is: How accurate does the transducer need to be for my application? A sterilization technician measuring a sterilizer s tank level may not be concerned that a transducer drifts a little over time, especially if a redundant system is used. As a result, the technician adjusts it, say, every two years instead of every year. In contrast, a vacuum manometer that controls pressure should be verified more often. Fortunately, drift can be corrected in the field by adjusting the zero potentiometer, or by calibration, depending on what the needs are for the application. In the case of the gauge transducer, the load on the diaphragm must first be eliminated. A potentiometer screw on the back of the unit is turned to return settings to 0 volts at 0 PSI. To set span as a function of full scale, it is recommended that the transducer be returned to the factory or a certified calibration house. Recertification is dependent upon the criticality of the situation. Therefore, it is highly advisable that system engineers and sterilization technicians factor the possibility of recertification into their overall design to ensure the sensor is accessible, and can be easily and safely removed. Setra Systems, Inc. All Rights Reserved. 7
Resetting a vacuum manometer, however, is much more difficult. First, it must be removed from the process chamber. It is hooked up to a pump capable of evacuating it to a pressure below the sensor s minimum resolution. Then the manual zero potentiometer is reset to achieve the required setting of 0 volts at 0 PSI. Further, if it s a critical application, it is recommended that the unit be returned to the manufacturer or a certified calibration house. Pressure Fittings Normally, medical processes that require gauge pressure transducers and absolute vacuum manometers are sterile applications. Accordingly, there can be no threaded connections on the sterilizer. Any fitting or connection with trapped volumes, where microorganisms can grow, including NPT threads and SAE fittings, must be avoided. In the case of gauge pressure transducers, a Tri-Clamp sanitary fitting made of stainless steel or Inconel is used. This is the industry standard. It meets 3-A Sanitary Standards. Tri-Clamp sanitary fittings have no threads; they are clamped on. These fittings are applied after the gauge pressure transducer diaphragm is mounted flush to the process chamber, or whatever material it is connected to. A Tri-Clamp connection consists of two ferrules (i.e., a transducer and process chamber) joined by a 1.5 or 2-inch Tri-Clamp with a gasket like a gasket sandwich. Each ferrule has a flat circular face with an outer groove. The grooves anchor the gasket, ensuring a sanitary and secure seal between the fittings. Neuter-style ferrules simplify installation, disconnection, and removal. The Tri-Clamp fitting allows direct mounting for installations that must withstand harsh cleaning conditions, as well as external high-pressure wash downs. Look for a small, lightweight unit with a welded stainless steel housing, and Tri- Clamp sanitary pressure fittings that allow the sensor to remain installed on the tank during Clean in Place and Sanitize in Place procedures without damage. Care should be taken not to spray directly on the diaphragm because it could result in exceeding the unit s proof pressure. Similarly, standard sanitary fittings are available for absolute capacitance vacuum manometers. It should be noted that transducers vary in design and consequently will have different outputs when flush mounted in otherwise identical situations. For example, consider a bottom-mounted gauge transducer measuring liquid level in a tank in which the diaphragm is brought up flush against the bottom of the tank. Flush mounting eliminates any trapped volume/dead volume between the connection and the diaphragm. As a result, when the tank is empty, it reads 0 PSI. Setra Systems, Inc. All Rights Reserved. 8
However, another transducer might be designed to have the diaphragm set back slightly in the pressure fitting, which creates a small space between the diaphragm and the connection. However, because of the trapped liquid, it will not read 0 PSI, even though the tank is empty. Again, system engineers and sterilization technicians should be aware of this condition, and if this type of transducer is installed, they must be prepared to compensate for it. The zero potentiometer could be used to adjust for this trapped liquid volume ( head height ). In this situation, the system designer needs to be sure that the transducer zero adjustment has enough range to compensate for the head height. Electrical Outputs Vacuum manometers and transducers are available in three standard electrical outputs: current, voltage, and digital. In the case of current, a standard 4 to 20 ma output is available from most suppliers. Voltage outputs, however, are available in a number of ranges. They include 0 to 5 VDC, 0 to 10 VDC, 1 to 5 VDC, 0.5 to 5.5 VDC, and so on. Some suppliers also offer digital communication such as RS-232 or RS-405. While this may seem obvious, output selection can be a critical decision because choosing the wrong output for a particular application could result in system failure. Selecting a voltage output for an extraordinarily long cable run will results in a significant voltage drop due to resistance causing an inaccurate signal. For installations requiring long runs, select a unit with a true 2-wire, 4 to 20 ma output signal. Another serious concern in the selection of vacuum manometers is the zero condition, also known as dead zero. It is possible to have negative pressures in some applications such as in hydrogen peroxide plasma sterilization. If you have selected a transducer with a 0 to 10 VDC output, it is assumed that 0 PSI indicates zero pressure. But if the pressure goes below 0 PSI, the vacuum manometer might not send a voltage signal below 0 VDC. Thus, the vacuum manometer output signals do not accurately reflect falling chamber pressures. Again, this could result in serious problems, including system failure. To correct such situations, suppliers have provided a variety of output ranges above the zero value. They include 4-20 ma, 0.1 to 5.1 VDC, 0.5 to 4.5 VDC, 0.2 to 5.2 VDC, etc. If a 4-20 ma signal is used, 4 ma represents 0 percent of the measurement range and 20 ma represents 100 percent. In this example, 4 ma represents the live zero the lowest possible pressure that can occur in the application. Any reading below 4 ma indicates a malfunctioning transducer or power loss. System engineers need to take this into consideration from the very beginning and choose a transducer appropriate for the pressure ranges of their application. Setra Systems, Inc. All Rights Reserved. 9
Media Compatibility There are various media used in sterilizers. Hydrogen peroxide, in both liquid and gaseous state, is gaining popularity because it is a stable high-level disinfectant for a wide range of microorganisms. It is highly effective due to its oxidizing ability. However, that same feature causes it to react poorly with the oxidizable materials that could be in the sensor, such as copper, brass, bronze, and iron. To circumvent this problem, most suppliers offer a 316 stainless steel (SS) housing and a stainless steel diaphragm. Even with the 316 SS, the hydrogen peroxide can have an adverse affect on the unit it may very well bubble or discolor. While the transducer performance is not affected, it leaves a visual image of oxidation and corrosion on a product in an otherwise hygienic medical environment. Making matters worse, sterilizer manufacturers are increasing concentrations of hydrogen peroxide, which is accelerating the oxidization on the stainless steel transducer. In response to this, it is suggested a sensor be ordered with high-nickel content, such as Inconel. This will help ensure the unit will maintain its sanitary appearance. When in doubt, consult with a vacuum manometer or transducer manufacturer that can help advise and select the proper transmitter for the application. Overpressure Another time in which it is appropriate to contact a transducer manufacturer for support is when your application is prone to have occasional overpressure spikes. For instance, let s say your full-scale pressure range is 30 to 50 Torr, but infrequently spikes to 1000 Torr. While the sensor will not report that overpressure reading, it is subjected to that pressure, and may fail because of it. Therefore, discuss this problematic installation with the transducer s engineering staff to help choose a sensor that strikes a balance between being able to accurately report process pressures, while also having overpressure protection capabilities. Installation Orientation Another instance in which your transducer engineering staff should be contacted is during the design phase when the application calls for the transducer to be installed in a different orientation than was intended. That s because the diaphragm could deflect as if process pressure was being applied. As a result, the transducer will generate an output signal equal to that sensed pressure. To prevent this problem from occurring, the transducer engineering staff can calibrate the sensor for any atypical installation or the installing contractor can do this if forewarned. In the field, the contractor installs the sensor and simply turns the zero potentiometer to be zeroed in the orientation in which it will be used. Setra Systems, Inc. All Rights Reserved. 10
Agency Approval If the sensor is going to be installed next to a pump or in an area that is electrically noisy, then there is risk that this RF interference will cause the unit to send erroneous signals. To avoid this problem, select a sensor with RFI immunity. Look on the unit s specification sheet for the CE mark. It guarantees a minimum level of product performance as it relates to low-voltage equipment. The CE mark designation certifies that the sensor conforms to all the essential requirements of the relevant European Directive. Furthermore, the CE mark can designate different levels of immunity from RFI interference. For example, the CE mark can describe how well the sensor performs under different field strengths, ranging from 1 volt per meter for field strength to 100 volts per meter. For extremely important applications, the medical facility may want to ask for a Declaration of Conformance from the manufacturer. This document details information about the essential characteristics of the transducer. Without the CE mark, a critical process can be seriously compromised even ruined by RFI interference from a simple cell phone. Another important designation to look for is RoHS compliance. This means that the unit is limited to the maximum allowable concentrations of six hazardous substances. Check the unit s specification sheet for this designation too. Conclusion Selecting a vacuum manometer or pressure gauge transducer to monitor and/ or control pressure in a hydrogen peroxide sterilizer is an extremely important decision. System designers and sterilization technicians must carefully examine a wide range of operational considerations the expected pressure ranges, the ranges the sensor will actually be exposed to, transducer accuracy, long-term stability, pressure fittings, electrical outputs, media compatibility, and sensor orientation. Then, and only then, can they zero in on and select the optimal sensor for their application. About the Authors: Kevin Bourbeau is an Engineering Manager at Setra Systems Inc. He has worked for over 12 years in research, design & product development. Prior to Setra, Kevin worked on new product development at other sensing companies. Kevin has a Bachelor of Science in Mechanical Engineering from UMass Lowell and holds 2 patents on Setra products, with others pending. Melanie Cavalieri is the Product Planner for Industrial Products at Setra Systems, Inc., She is responsible for new product development and marketing for Industrial, Refrigeration and Medical markets. Prior to entering the marketing role at Setra, Melanie worked in Quality and Manufacturing for Setra Systems and Kollmorgen Motors in Radford, VA. Melanie also has R&D experience at Kodak Versamark in Dayton, OH and Ethicon Endo-Surgery in Blue Ash, OH. Melanie has a Bachelor of Science in Mechanical Engineering from Purdue University. 1 Sensors Magazine, October 2011 Setra Systems is a leading global designer and manufacturer of pressure sensors, and transducers, humidity transmitters, current switches, current transducers, and calibration equipment for the HVAC and Industrial markets. Setra Systems, Inc. All Rights Reserved. 11