Standard Practices Overview for MAP Gas Analyzers
Laboratory and Portable Gas Analyzers have been used for several years to verify the mix constituents and accuracy of mix of headspace gas in MAP (Modified Atmosphere Packaging) applications. The purpose of this white paper is to clarify the issues concerning and the best practice of use for these gas analyzers. This White Paper is not directed to any specific manufacturer or model, but is intended to be used as a guideline of use to obtain the best accuracy and reliability of a gas analyzer in the industrial environment.
Use the analyzer as a reference gas transfer standard rather than absolute standard
Gas analyzers are intrinsically reference instruments. They are calibrated during manufacture on certified gas blends – and are designed to maintain accuracy over time and temperature variations. Their accuracy at the site of use can and should be verified and improved by simple testing using a certified reference gas blend. Doing so allows the analyzer to be used as a ‘transfer standard’ with the certified gas blend as the primary reference standard.
Always check analyzer accuracy on a certified gas blend locally before use. This is a relatively simple procedure that can be conducted before every line test series.
Certified gas blends are generally manufactured using gravimetric (weight) methods, whereby known masses of the constituent gasses are added to an evacuated cylinder – then the final mix is checked using very accurate analyzers. The result is a gas blend that is generally accurate within +/- 2% relative to the gas concentration values printed on the certification ‘Tag’. This is the standard to which the package headspace gas should be compared – not the gas analyzer itself – even if it has been calibrated at the factory. Generally, a certified blend of gases similar to that used in product packaging is available for purchase at reasonable cost from industrial gas blenders – and should be used to verify instrument accuracy as well as calibrate the instrument on site at will.
(Which is not to say that the instrument should not be calibrated by the means given in the product manual and using its instrument manufacturer-supplied gas – but at the end of the day, it should measure a representative gas blend accurately – and be able to be calibrated on-site to do so. The manufacturer of the instrument should buy into this approach unless he can say why he cannot – as measuring a certified gas blend by the same method used to measure package headspace is the best representation of the accuracy of the instrument use.)
The Criticality of Analyzer Storage and Operating Thermal Environments
Gas analyzers have classically been designed as laboratory instruments. As such, they generally are contained in a controlled environment, powered up each morning (or left powered on indefinitely), and then used periodically in a homogeneous and stable environment. This operating modality tends to reduce the effects of thermal transients on the equipment, thereby increasing their stability. Once they are allowed to ‘Warm Up’ (which can take between 15 minutes and an hour), they are quite stable. Since they remain powered on for long periods of time, they are at the ready at any time after the warm-up period to take relatively accurate and stable gas measurements.
This class of equipment is generally calibrated at regular intervals – during which the analyzer gas readings are compared to certified (generally /- 2% relative) calibration gas mixture, which is provided from the gas blender with a measured gas concentration level contained as a ‘Tag Value’ recorded on certification tag on the calibration gas cylinder. The analyzer is periodically adjusted so that it reads this gas mix accurately and reports the same values as are indicated on the certification tag. It is assumed that later readings of unknown gas mixes are maintained to the same accuracy as the calibration gas.
To achieve this accuracy, the analyzer must be both linear and stable. That is, it must maintain its accuracy over a range of gas values (linearity) and must be repeat ably accurate (stable). The stability of the analyzer can be relatively easily ascertained by periodic measurement of calibration gas. Linearity and cross-gas interferences can only be confirmed by delivering different gas mixes after calibration, a considerably more complex operation.
Portable and hand-held gas analyzers are subject to operation under increased thermal stress. This class of gas analyzers are generally left powered off to stabilize in their storage environment, and then transported to the point of application, powered on, and used to measure gases with relatively short warm-up intervals. As such, they are more subject to thermal transient conditions, which can mitigate their accuracy.
Reduce Thermal Transients
To limit these effects, the thermal transients to which the analyzer is exposed should be reduced. One way to do this is to keep the analyzer at as constant a temperature as possible. Most analyzers have warm-up compensation built into them that assumes a stable external thermal environment. That is, the analyzer start-up thermal transient begins with the inside of the analyzer at the same temperature as the outside, and gradually warms up until it reaches its final interior temperature. If the analyzer is stored at one temperature and used at another, the warmup conditions are further complicated by this change in external temperature, causing a degradation in accuracy. Accuracy and stability can be degraded even by moving a warmed-up analyzer from one ambient temperature condition to another, although this is certainly less severe. The point is that the analyzer should be operated in as stable external temperature environment as is practical to ensure the greatest accuracy.
Good Practice Procedures Before Use
Use filters at the needle, not at the analyzer. Keep the sample line clean and dry
Make sure the analyzer protective filter is installed at the needle before the sample line, not at the analyzer. Doing so will keep the sample line clean and dry.
Use the manufacturer-supplied needle
Gas analyzer manufacturers design the gas analyzer as a complete system – including the sample needle and filter. Using a low-cost replacement (usually bevel-cut hypodermic) needle is a bad idea for two reasons: First, it is designed for ease of piercing and liquid transport, just the two things you do not want to occur. If a bevel-cut needle is pushed through a septum and plastic film, it will cut through them rather than spread them. Doing so can cause a piece of the septum and/or film to be drawn into the needle – blocking the gas path. Secondly, the needle can easily pierce product with the same effect, or draw a bead of water into the system. While you should have a protective filter installed on the needle, you will experience considerable needle and filter contamination problems using a hypodermic needle. Thirdly, the use of a hypodermic needle is dangerous to the operating personnel due to the likelihood of needle sticks. The bottom line is that hypodermic needles are dangerous to the operator and ineffective in preventing contamination and plugging.
Use the manufacturer-supplied filter
The use of generic filters can also cause problems. There are many filter designs which will fit on the industry-standard needle and sample line gas interfaces and look similar to the manufacturer-supplied filter, but are substantially different in detail filter material design, and can seriously degrade the analyzer accuracy and functional reliability. The filter provided by the manufacturer is designed to provide contaminate protection with a minimum of flow restriction. Generic filters are often designed to pass liquids under high pressures, and so are not appropriate for this application. We have seen customer-installed filters that run the gamut of reducing the gas flow to the analyzer so much that it can no longer make accurate measurements to providing no protection to ingested water or contamination.
Make sure the sample line fittings are tight
Verify before gas measurement that all of the needle/filter/sample line/analyzer fittings are secure and gas-tight. Small gas leaks at any of these locations can cause significant measurement errors.
Use a septum to seal the sample site – even if you are taking just one sample. It prevents errors due to uncontrolled ambient air dilution.
The conscientious use of a septum at the package sample site provides a gas seal against the ambient air and greatly reduces the chance of ambient air leaks reducing gas accuracy during testing, especially when testing small headspace package volumes.
The usual sample septum can be used 4 or 5 times – it depends on the manufacturer recommendations. However, it is good practice to use a septum regardless, as this is a relatively minor cost item, especially when compared to the cost of an inaccurate test.
Good Practice Procedures During Use
Power-up the analyzer immediately
The longer the warm-up time before use, the better. It is better to power on the analyzer immediately upon removal from its charging station and carry it to the location of use than it is to wait until the analyzer is at its use location to power on the analyzer. Sometimes only a few minutes of warm-up time can make a big difference in analyzer accuracy.
Check the Analyzer Accuracy using a local Certified Gas Blend
Before testing product, make sure the analyzer reads a certified gas blend accurately. This is a short test and the best way to confirm that the analyzer is going to give accurate test results during use – when it reads gas values of unknown gas mixes. Verifying that the analyzer reads a known gas blend accurately is a critical step in maintaining accurate line test results.
Ambient Air Zero frequently or before a critical test run
If there are air-reference technologies (infrared and most oxygen measurement technologies commonly use ambient air as a reference gas), the analyzer should be given an ambient air reference calibration if possible before critical measurements are made. This will serve not only to freshen the oxygen calibration, but stabilize infrared measurement processes – both of which are subject to long-term drift.
Watch the gas values being developed during testing
The gas analyzer should be able to display gas values it is obtaining during testing. This allows the operator to confirm that the test itself is valid, and correct error causes if it is not. The following items should be noted.