Toxic exposure to hydrogen sulfide (H2S) is a well-recognized hazard in agriculture, particularly in livestock operations that manage large amounts of manure. Numerous fatalities have been observed, often multiple fatalities in a single incident, due to toxic exposure to H2S from manure pits at concentrations higher than 500 ppm. Direct-reading instruments that alarm workers in the areas when H2S concentrations are high may prevent these fatalities. However, monitors that are commonly found in industries with robust safety programs are impractical for agricultural use as they are often prohibitively expensive and require regular maintenance and calibration that may be above the expertise level of agricultural workers.
In more recent years, manufacturers marketed simpler models of direct-reading H2S monitors as “low-maintenance” or “maintenance-free” at a much lower cost than traditional monitors, which may cost $500 for basic models or more than $1000 for more complex models. The objective of this study was to test several models of low-cost, low-maintenance monitors in order to examine the features of each for comparison, as well as to test the performance of these monitors with no maintenance over time while under constant exposure to low levels of H2S.
Two types of monitors were examined: qualitative monitors that were lowest-cost (around $100) and provided only alarm settings with no concentration displayed (Honeywell BW Clip and MSA Altair), and quantitative monitors that cost slightly more (around $200) but displayed concentration readings (Dräger Pac 3500 and Industrial Scientific T40 Rattler). All models were exposed to H2S for a test period of 4 months, at concentrations slightly higher than typical background concentrations to simulate expected monitor exposure for a year in a barn.
The performance of qualitative (‘alarm-only’) monitors declined faster than over the course of the simulated barn year than the quantitative monitors, with both models of qualitative meters failing to alarm at the high setting before the test period was complete. The quantitative (‘concentration-display’) models showed fewer effects from long-term exposure over the duration of testing, but both models exhibited inaccuracies in the concentration readings when compared to calibration gas concentrations. The T40 Rattler provided consistently higher readings (+2.3 ppm) than the calibration gas concentration, while the Pac 3500 showed consistently lower readings (-3.4 ppm) than the calibration gas concentration. Serious acute health effects for H2S are not typically observed until exposure to concentrations above 500 ppm, so inaccuracies of this small magnitude are relatively insignificant.
Though each of the test monitors is advertised to be maintenance-free for two years, this study found that failures occurred within one simulated year in a barn. Bump checks should be performed regularly to ensure the monitor reacts to the presence of H2S appropriately, even when the manufacturer’s literature may say otherwise. Most importantly, agricultural workers should always inspect and bump check these monitors prior to any potentially high-risk activity such as manure agitation or pumping to ensure that the monitor is still providing the protection needed from a potentially toxic release of H2S.
This study tested each of these models within a clean chamber at room temperature to isolate the effects of long-term exposure to H2S. In an actual barn, these monitors may be exposed to variations in temperature and humidity, as well as other barn contaminants such as ammonia, dust, and chemicals. Each of these other exposures could also affect the performance of these monitors over time, and should be considered when storing and using these monitors. Furthermore, the potential interactions from other exposures is an opportunity for future study to better understand how these interactions may affect sensor performance in an agricultural environment.