FTA vs FID: 7 reasons a FID can have a large reading error

Our next topic in the comparison of the Flammability (FTA) Analyzer and Flame Ionization (FID) Detector for use in process applications, is reading error.

The best way to calibrate any analyzer is with a known concentration of the solvent vapor mixture being used in the process. Furthermore, this mixture should be sampled at the process temperature, because LFL values are lower at elevated temperatures. The task of using heated solvent mixtures for calibration is difficult, and so a more easily handled reference gas is typically used. When a reference gas is used for calibration, the analyzer meter reading must be adjusted to account for differences in the analyzer’s response to the reference gas versus the actual solvent vapor mixture at process temperature. 

Fire and Safety codes require that the resulting readings must be accurate to +/-10%. Calibrating a Flammability Analyzer using a reference gas results in meter readings that are accurate within +/-3% of full scale or +/-10% of applied gas, thus meeting this requirement. However, an FID (depending on its design) can have a reading error as large as -50% to +150%.

Here are 7 reasons why:

1. Many processes employ more than one solvent. When a sample contains more than one solvent, safety codes require that the analyzer be calibrated to the vapor to which it is least sensitive. This causes the analyzer to overreact to the other solvents in the sample, generating a high reading known as a “safe-side” error. Safe side errors are problematic because they produce nuisance alarms, making it impossible to operate a dryer at its intended speed. Users learn to ignore nuisance alarms, which can result in a catastrophic event. 
2. Because the FID is measuring the current flow created by ionized carbon produced when the sample is incinerated, converting this signal to a % LFL meter reading is not a direct measure of the flammable hazard present in the process.
3. Different solvent molecules have differing numbers of carbons. If more than one solvent is used in the process, then the exact recipe of that solvent mixture must be known in order to calculate a correction factor to convert the signal into a meter reading. 
4. If the process oven or dryer produces many products using different solvent recipes, then the FID must be recalibrated whenever the product being manufactured is changed. Managing these corrections factors is difficult.
5. FID analyzers consist of capillaries as part of their flow system. Capillaries are small and are prone to clog by sample particulate or moisture, reducing sample flow which could result in lower than actual readings. There is no indication of a clogged capillary to alert the user that the analyzer has failed. 
6. Not all solvent vapors respond to FID incineration in the same manner. For example, not all of the carbon in a halogenated hydrocarbon is ionized when incinerated; therefore the meter reading will be inaccurate (too low). On the other hand, oxygenated hydrocarbons such as isopropyl alcohol tend to produce higher readings in FIDs.
7. A non hydrocarbon flammable sample (ie. hydrogen fuel or CO) will go undetected, causing a FID to grossly under report flammability.