1.0    Background

1.1     Historical

In the mid-1980’s, Interscan developed an instrument to be used in the clearance of structures treated with Vikane® gas fumigant. At the time, the reentry level was set at 10 ppm (parts-per-million). The measuring range of the instrument was 0-50 ppm, with an integral analog meter. The units were usually calibrated with span gas at a target level of 40 ppm.

Some years later, the reentry level was lowered to 5 ppm. To accommodate this change, operation of the integral pyrolyzer (or furnace) was modified to produce better conversion of the SO2F2 to SO2, and thus, improved sensitivity of the instrument. A procedure was subsequently introduced calling for more frequent calibration, within 30 days of use.

Recent federal re-registration of this pesticide by the US EPA has once again lowered the reentry level—this time to 1 ppm. About one year prior to this label change, the span gas was lowered to a target level of 5 ppm.



1.2     Purpose of this document and technical scope

Although the Interscan was originally designed to detect levels of 10 ppm and subsequently modified to detect at 5 ppm, increased care in operation and maintenance of this instrument will effectively achieve satisfactory performance to the first division on the meter, 1 ppm.

This document will focus on methods that will optimize instrument performance under these new operating conditions. Certain of these methods are subject to change as modifications (including digital meter kits) are made to your Interscan unit, and revisions to this document will be issued at that time. Detailed discussion of accuracy, resolution, and calibration is beyond the scope of this document.



2.0    New Instrument Operating Procedures

2.1     Zeroing the instrument

2.1.1  Current procedures, as outlined in the GF-1900 instruction manual, call for zeroing the unit with ambient air, which is being drawn in by the integral sample pump. Whether one was operating in the 10 ppm or 5 ppm era, prudence would always dictate that this zeroing be done only in an environment that would be free from contamination with sulfuryl fluoride gas.

2.1.2  Now, however, the slightest contamination might be enough to compromise the zero, and therefore the fumigant clearance reading.

2.1.3  Best practice shall now be to use the following procedures to assure that only clean air, free from any sulfuryl fluoride gas, be utilized to zero the instrument. It is recommended to zero the instrument a minimum of 10 feet upwind from the fumigated structure and outside of carports, porches, or other areas with overhanging roofs attached to the fumigated structure.

2.1.4  Current procedures, as outlined in the GF-1900 instruction manual, call for waiting until the READY light illuminates before zeroing the instrument. In addition, the user is cautioned to wait until the meter stops moving, before attempting to zero the instrument. Given the critical nature of the zero setting, initial meter stability must be especially emphasized, even if stating this instruction is technically nothing new. Thus, best practice shall now be as follows:

As before, wait until the READY light illuminates, but after this occurs, check that the meter is stable (needle steady and not moving) before attempting to zero the instrument. Depending on when the instrument was last used, this could take an extra few minutes.

2.1.5  The new zeroing method, described in 2.1.3 and 2.1.4 shall be performed just before each clearance.


2.2    Taking readings in the structure to be cleared

2.2.1  Random motion imparted to the instrument, from walking around while holding it, could cause the meter to bounce around mechanically, rendering a reading of one division difficult. In addition, holding the instrument at odd angles may affect meter movement.

2.2.2  Best practice shall now be to stop walking, hold the monitor parallel to the ground, and stand still while taking a reading.


2.3     Interpreting a one ppm reading

2.3.1  Users of the GF-1900 are well familiar with the difference between artificially induced spikes (often caused by excessive radio frequency interference) and true instrument response. A true instrument response is characterized by a smooth, steady rise of the meter needle.

2.3.2  As such, best practice shall be to define a one ppm reading as a smooth, steady rise of the meter needle to a point coincidental with, or beyond the first meter division. To distinguish this from possible noise, this reading must persist for at least 5 seconds.


3.0     Conclusions

The move to a 1 ppm clearance level has put increased burdens on all of us: The fumigant manufacturer, the fumigator, the instrument company, and the regulators. Even the most sophisticated analyzers are pressed to the maximum to read 1 ppm, and these are priced at a point nearly ten times higher than the GF-1900.

This Best Practices document, as well as subsequent mod kits and newer models of the Interscan sulfuryl fluoride monitor, will help our valued customers in their continuing efforts to provide safe and effective pest control by effectively achieving the 1 ppm clearance standard.