Flow System Calibration in Medical and Bio-medical Production Processes.

Transcription

1 Flow System Calibration in Medical and Bio-medical Production Processes. Speaker / Author: Mark Finch Fluke Calibration Norfolk, Norwich, NR6 6JB, UK mark.finch@fluke.com Phone: + 44 (0) Abstract The manufacture of drugs or medical equipment requires many complex production operations, none more than the ability to ensure the correct amount of drug is contained in a tablet or deposited on a product. Many items for use within the body, i.e. a stent to open a blocked artery or a pacemaker to control the hearts rhythm require that they are coated in drugs that will stop the human body from rejecting the device. The manufacturing processes of this type of equipment uses Mass Flow Controllers or Mass Flow Meters that can control or measure the amount of drug being deposited within the production process. In the production of incubators or other hardware used for controlling breathing we need to verify the volume flow rate of the air being delivered. Flow meters are used in the production process to verify these parameters. In all cases the production test equipment needs to be periodically calibrated in the most efficient and effective way to ensure minimal production down time. This paper discusses a technique to calibrate gas mass flow and volume flow devices using a commercial mass flow measuring system that has no moving parts, enabling simplification and automation of these tasks within the calibration laboratory. 1. Introduction There are numerous items used within the realm of medical science that can only be produced or tested if mass or volume flow of gases within a system is known. Artificial hardware that is placed in the body like a heart pacemaker, a stent in an artery or even a replacement hip or leg joint need to be coated in the correct amount of drugs to ensure they are not rejected by the human body. In the production of tablets of all kinds we need to ensure the correct amount of different drug is being deposited. In the manufacture of incubators and ventilators a manufacturer needs to ensure the correct volume of air is delivered and in anaesthetic machines the correct flow of gases. There are various devices that manufactures use to control or measure this on the production line. Mass Flow Controllers (MFC), Mass Flow Meters (MFM), variable area flow meters (Rotameters) and turbine flow meters to name just a few. Of course these items require periodic calibration.

2 2. The Basics 2.1 Mass Flow Mass flow is the movement of a quantity of material through a system over a period of time. The SI unit for mass flow is kg/s but is typical converted to other mass flow units like g/s or mole/s. More commonly used units are those that are volumetrically based such as sccm, slm etc. The s that prefixes these units indicates standard and defines standard conditions of kpa absolute pressure, standard atmosphere and a temperature of 0 C. When a gas flow system is in steady state mass flow is the same throughout a system and is independent of the gas pressure and temperature. Examples of mass flow devices are MFCs and MFMs. 2.2 Volume Flow Volume flow is the measurement of a volume of gas through a system over a period of time. The SI units for volume flow are m³/s. Other units are l/m and cc/m. In a gas flow system at steady state, volume flow will be different throughout a system and will be dependent on the pressure and temperature at the point of measurement. This means that the volume flow will change due to natural changes in pressure and temperature that naturally happen as a gas moves through restrictions caused by valves, regulators, filters and plumbing. Volume flow devices are common in industry because they are simple in design. They do not contain temperature or pressure sensors that would be required to measure mass flow and are therefore relatively low cost. Rotameters, turbine meters and orifice plates are examples of volumetric flow devices. These units are calibrated to indicate volumetrically based units like sccm and slm. The output of these devices is based on a specific gas at specific operating conditions. When calibrating these devices the laboratory conditions will typically not be the same and therefore will not indicate the correct flow rate unless these differences are accounted for. 3. Traditional Methods Traditional calibration of MFCs, MFMs, Rotameters, turbine meters etc, would have been carried out using piston provers, bell provers and other variable volume devices. However these devices have many practical limitations: The operator making the measurement has limited time before the device needs to be reset causing temperature and pressure variations that affect the calibration. They need to operate at atmospheric pressure limiting the device under test (UUT) operating conditions. They have many moving parts making them complicated to use and the cause of instability in the device being tested. Many older devices use mercury as a piston seal. 4. Modern Approach The modern approach to the calibration of these devices is to use a system based on laminar element or sonic nozzle technology. This is not new technology. Laminar element and sonic nozzle technology has been around some time and the system I describe here has been

3 available for fifteen or more years but for most applications it has probably been too accurate for the workload. Now as technology improves and more industries strive for higher accuracy measurements this system has been given more attention as its superior accuracies are now required. 4.1 Laminar elements In accordance with well known laws of gas behaviour that we will not discuss in this paper, the flow of a known gas in the laminar flow regime can be calculated from the flow path geometry and the gas pressure and temperature [1]. We call our elements molblocs, mole being the SI unit for the amount of substance. The laminar molbloc contains a laminar element suitable for the mass flow range being calibrated, two pressure ports either side of the element to measure the differential pressure and two temperature sensors. See figure 1. Figure 1, Section through Laminar molbloc The molbloc also has a digital connection to an EPROM. This is where calibration constants for the bloc are stored. When used the molbloc is connected to a molbox terminal that contains a pressure sensor to read the pressure differential, circuitry to read the two temperature sensors and the ability to read the calibration constants and range of the molbloc. The molbox also contains look-up tables of different gas densities making it easy to calibrate different gas species at the push of a few buttons. The units of measurement can also be easily changed to match those of the UUT. Laminar molblocs cover the range 10 sccm to 100 slm. 4.2 Sonic elements The sonic element measurement principle follows established critical flow venturi (CFV) nozzle flow theory [2], that states With the proper mechanical configuration to define an isothermal, unidirectional flow stream; and the inlet operating conditions that cause the flow to reside in the critical flow regime, the mass flow rate can be calculated from the flow path geometry and the gas pressure and temperature. In practice the molbloc-s technology requires that the ratio of the outlet to inlet absolute pressure known as the back pressure ratio, (BPR) be less than a pre-defined critical value. Once this occurs, the velocity of the flowing gas reaches its attainable maximum that of the speed of sound. At this point the volumetric flow rate is at its maximum, and cannot be affected by increasing the inlet pressure or changing downstream conditions. Sonic molblocs cover a range of 2 slm to slm. The sonic molblocs, although larger than the laminar molblocs, look very similar externally however they contain a sonic nozzle with pressure ports to measure the BPR plus two temperature sensors.

4 Figure 2, Section through Sonic molbloc The Sonic molblocs have the same connections as the laminar blocs and are connected to the molbox in the same way. Using a molbox/molbloc system has some great advantages: Continuous flow output No moving parts Compact and portable No mercury Low uncertainty, as low as ± 0.125% of reading 5. Continuous flow Most process flow sensors like those already discussed have real time and continuous output indications. It is important to capture the output of the reference and the UUT at the same time. The molbox/molbloc system has real time and continuous output indicators making it a good solution to calibrate these flow sensors. It allows the operator to make direct comparisons between the UUT and the reference for as long as required to obtain the required results. Once the flow rate has been set conditions within the system like line pressure, differential pressure and temperature do not change. A manufacturer of piston-provers recommends setting the desired flow rate and measuring it with their reference, then changing the connection to divert the gas flow into the UUT, to determine the UUT output. If you use this method to calibrate a flow device, you cannot ensure that the flow rate and operating conditions remain constant, or that the UUT output did not change. Another concern with these types of devices is that the stroke time is short so the manufacturer recommends taking averages across multiple strokes. This is not ideal, because the piston oscillations cause fluctuations of the gas flow stream that can affect the UUT. Figure 3 [3] shows that averaging multiple proving strokes with this type of device can cause an error due to even small amounts of flow instability. This could be likened to the filling of a bucket with water from a hose pipe. You know the volume of the bucket and you can time how long it takes to fill that bucket and calculate the volume flowing through the hose, however once the water reaches the top of the bucket you have to stop filling the bucket, empty it, and then start to fill it again interrupting the measurement.

5 The molbloc system has no moving parts providing a continuous reference flow to the UUT. The molbloc system is compact and easy to transport making it an ideal system for calibrations outside of the laboratory environment. Bell provers and piston provers are bulky and heavy and do not lend themselves to portable calibrations. Figure 3: The error in averaging readings between a continuous and non-continuous reference. 6. Leakage Good measurements in mass or volume flow rely on leak free connections between the reference and the UUT. The molbox allows the user to check for leakage by looking at the pressure decay in a system allowing the user to correct for any leaks. 7. Volume Flow Devices As we have already discussed many popular flow devices are based on volume flow measurement, the output or flow rate of which is based upon a specific gas at specific operating conditions. This is known as normal operating conditions. This means that measurements performed in conditions that are not normal will need correcting so that correlation can be made between the reference flow and the UUT. 7.1 Square Root Density Correction This correction is used for devices that are based upon the Bernoulli phenomenon that relates volume flow to differential pressure. Examples of these devices are Rotameter, orifice plates and sub-sonic venturis. The calculation for the corrected reference flow is: Where: is the reference flow if the device was operating at its normal conditions is the uncorrected molbloc/molbox reference flow rate is the density of the process gas at the UUTs normal conditions is the density of the calibration gas at the UUTs actual conditions

6 7.2 Proportional Density Correction This correction is used for devices that are volume based, output in Mass flow units, but do have their own density compensation. Examples of these devices are turbine meters, soapfilm meters and critical flow venturis without electronic hardware. The calculation for the corrected reference flow is: Where: is the reference flow if the device was operating at its normal conditions is the uncorrected molbloc/molbox reference flow rate is the density of the process gas at the UUTs normal conditions is the density of the calibration gas at the UUTs actual conditions For Volume flow calibrations we also need to know the temperature and pressure at the input to the UUT. This can be performed using a hand-held thermometer and a pressure monitor. This data can be entered into the molbox that will correct for the differences. The Volume flow and the density correction calculations could be calculated manually but could also be handled by software. We have dedicated software for flow that will help perform the calibration, calculate corrections and flow rates, control the molbox/molbloc system, allow the UUT readings to be entered manually or automatically if the hardware allows and produce a calibration certificate. As with all calibration software, it helps to standardise a calibration of a UUT and helps produce consistent repeatable results. 8. Conclusion The use of laminar and sonic elements for a reference in a flow system allows the fast calibration of mass or volume flow measurement equipment used in the manufacturing of medical devices. It allows the operator to switch quickly between different gas species and the units of the UUT being tested. There are no moving parts ensuring a continuing measurement of a UUT over an infinite time period with very small uncertainties. These devices are compact and easy to transport making them ideal for on-site calibration. 9. References 1. Pierre Delajoud, Martin Girard, A High Accuracy, Portable Calibration Standard For Low Mass Flow, XIII IMEKO World Congress of Metrology, Torino, Italy, September Pierre Delajoud, Martin Girard, Michael Bair, The Implementation Of Toroidal Throat Venturi Nozzles To Maximize Precision In Gas Flow Transfer Standard Applications, FLOMEKO, Peebles, Scotland, June, Fluke Corporation, Four Steps to more effective gas flow calibration. Application Note, 2010.