One of the mining industry's goals is to establish a standard monitoring device that will primarily monitor gas levels and airflow side by side in real-time to assist as mine safety triggers and in Ventilation Air Methane (VAM) monitoring purposes. Unlike in most Australian mines, continuous real-time air velocity and gas monitoring has been practiced in South African coal mines for over three decades. Envisaged benefits from real-time velocity monitoring over current monthly manual ventilation monitoring are, viz., consistent and continued diagnosis of underground environment and managing catastrophic risks such as fires, explosions, and spontaneous combustion through gas make values; ability to determine real-time carbon monoxide, methane and other noxious gas make, estimation and reconciliation of specific gas emissions during panel development and longwall retreat, determining goaf capture efficiency, accurate determination of heat loads and air cooling capacity, and improving the confidence in ventilation air methane (VAM) emission data. Currently, industry is faced with the persistent and complex challenge of obtaining a 'reference true gas monitor' for ‘accuracy’ determination of quintessential VAM parameters, viz., CH4, CO2, air velocity, and temperature. Despite, supplier or external reviewer’s claims, that one monitoring system is superior than the other in terms of its measurement ‘accuracy’, i.e., when compared with the “true measurement device”, in almost all cases, validating these claims was not possible due to lack of data evidence. Therefore, use of measurement system/s that are deemed to provide a practically acceptable, reliable and safe system to provide transparent measurement data is important. Underground operators are often faced with the famous and simple audit question on an important area of ‘accuracy’, i.e., the difference between ‘true’ value and measured value. There are suggestions of “slight inaccuracies” are being acceptable but currently, no such guidance or value exists. None of the studies or available guidance documents provides guidance on choice of an ‘accurate’ instrument for VAM monitoring. For example, it is acceptable to have an air velocity measurement error of 5 to ± 20 % that are based on research and operational practices. AS2290.3 (1990) outlines an acceptable tolerance measurement limit for instruments. For example, working limit for 1.0 % true concentration of CH4 is 0.91% for real-time (electrochemical /pellistor sensor) with 5% range and 0.90% for tube bundle system with 100% range excluding span gas ranges of ± 0.2%. Considering the above inherent instrument inaccuracies expected, a true measure of instrument performance is to obtain side-by-side results that can demonstrate the difference between the monitoring systems exposed to the same atmosphere. This paper demonstrates that over and beyond the inherent minimal instrument measurement differences, it is those operational factors that are critical to the recording of concentration of gas levels which the instruments are exposed to, viz., airflow that would affect the concentration of CH4 and CO2, barometric pressure, shaft cage effect, longwall coal production levels, magnitude of gas levels, longwall production, which is the main source of the U/G VAM.