Ever since Interscan developed a portable hydrazine analyzer for Brooks Air Force Base, way back in the late 1970’s, our instruments have been widely deployed in all facets of hypergol (pertaining to fluid propellant) fuel and oxidizer measurements.
A new wrinkle was brought in when NASA requested an instrument with 10 parts-per-billion (ppb) sensitivity. Here is their official evaluation (1996) of the available technology. Bear in mind that little has changed (at least outwardly) in hydrazine sensor technology since this report was issued.
Development of a 10-Parts-Per-Billion Hydrazine Portable Analyzer
Both NASA and the United States Air Force (USAF) use hydrazine (HZ) in their spacecraft as a hypergolic fuel, either in the form of HZ, monomethylhydrazine (MMH), or unsymmetrical dimethylhydrazine (UDMH). These substances are highly toxic and are known or suspected carcinogens. Although the threshold limit values (TLV’s) established by the American Conference of Governmental Industrial Hygienists for personal exposure had been set for many years at 100, 200, and 500 parts per billion (ppb) (for HZ, MMH, and UDMH, respectively), the values have recently been changed to 10 ppb for all three compounds.
NASA, in conjunction with the USAF initiated a project at the NASA/KSC Toxic Vapor Detection Laboratory (TVDL) to develop a portable fuel vapor detector (PFVD) able to detect 10 ppb (PFVD-10). The PFVD-10 project was conceived as a two-phase effort: Phase I consisted of laboratory tests of potential technologies to identify those that could provide reliable detection at the 10-ppb level, and Phase II was more extensive laboratory tests on a production prototype based on the most promising technologies selected from Phase I. Three different technologies from three separate instrument manufacturers were evaluated in Phase I:
- An electrochemical cell from Interscan (the manufacturer of the current portable hydrazine detectors used at KSC)
- A colorimetric paper-tape detector from GMD Corporation
- An ion mobility spectrometer (IMS) supplied by Graseby Ionics Corporation
The Interscan electrochemical analyzer was the only instrument able to meet the basic performance criteria shown in the table (see below).
The GMD detectors were eliminated because, even though they could detect HZ and MMH from the 10-ppb threshold to above 400 ppb, their response times were slow and, even more importantly, they showed a major change in response due to relative humidity (very low readings at low humidity and very high readings at high humidity).
The IMS was also eliminated because it could not detect MMH or HZ below 25 ppb and because it experienced so many component failures and other problems during testing that its evaluation was terminated early before a characterization was completed.
Based on the results of Phase I testing, the Interscan electrochemical cell analyzer was selected for continuation into Phase II, and four production prototypes were procured from Interscan for a detailed evaluation at the TVDL. During Phase II testing, the production prototypes were tested. Although the Interscan instruments did not meet every specification for all three hydrazines during the Phase II tests, they performed well enough to be identified as the instrument of choice to provide a monitoring capability for hydrazine at the reduced TLV level of 10 ppb.
Acceptance test criteria—
|Precision||±10% maximum deviation from the mean value|
|Linearity/accuracy||±1 ppb, or ±15% of actual, whichever is greater|
|Zero drift||±2 ppb for 4 hours|
|Span drift||±15% of span value for 4 hours|
|Response||50% of span in 2 minutes at 10 ppb|
We would add that attempting to measure any gas in the parts-per-billion range is not for the faint-hearted. In the first place, considerable effort and expense will ensue in generating an appropriate calibration standard. But, that’s not all.
Think about how good your ZERO gas would have to be to assure that it is free from any measurable substance (either a target or interference) such that it can truly be “zero” for a 10 ppb measurement!
That being said, we at Interscan invite such challenging applications. Let us hear from you.