Fenceline Monitor Questions and Answers



Note: This is a FAQ made shortly after the installation of the Fenceline Monitor in 1995. The refinery has been sold twice since then and is now owned by ConocoPhillips. The fenceline monitors are still functioning and are soon to be improved. ConocoPhillips has been responsive to our suggestions and desires and we hope that our "good neighbor" relationship continues for years.

This FAQ has been updated in places noted..


Q: What is the Fenceline Monitoring system at the Tosco refinery?

A: The fenceline system is a network of ten optical-remote gas monitors placed along the North and South boundaries of Tosco's oil refinery in Rodeo.

Q: What are "optical remote sensors"?

A: These are a breed of monitor, also known as "open-path monitors" which use light beams to detect the presence of chemical gases in the open atmosphere. There were three major types of these devices available at the onset of this project, and while they all differed considerably in terms of their operation, all three types work by utilizing the same basic concept. A continuous light beam is projected between two points. As a chemical plume crosses this "beam-path", various molecules in that plume will absorb light energy in very predictable ways. It is by analyzing this absorption of light energy that open path monitors are able to detect and measure concentrations of chemical gases. The monitoring network at Tosco utilizes all three major open-path technologies: Infrared (FTIR), Ultraviolet, and Tunable Diode Laser (TDLAS).

Q: What is the system being used for?

A: The network is designed to serve two distinct functions. First, it is intended as an emergency warning system for refinery operators. The various open-path monitoring systems are all designed to provide warning very rapidly in the case of a chemical emergency; with response times ranging from a few minutes to a few seconds. The second function, perhaps equally important, is to track the day-to-day emissions levels of a wide variety of hazardous gases from the refinery

Q: What types of chemicals are being monitored?

A: The refinery is currently monitoring a list of 39 hazardous compounds. Included on this list are gases which pose both acute and chronic health risks. The chemicals from this list which are known to cause acute health problems (immediate symptoms experienced from short-term exposure) include hydrogen sulfide, chorine, ammonia and sulfur dioxide. Those chemicals known to cause chronic health problems (often delayed medical problems brought on by longer-term exposure) include suspected carcinogens such as benzene and MTBE, as well as potent neurotoxins such as toluene and carbon disulfide

Q: Does anything like this exist anywhere else?

A: Not really. Certainly not on this scale. There are a handful of facilities in the U.S. which have been required to install FTIR monitors on their perimeter, however, the system at Tosco does mark the first-ever commercial installation of either UV or TDLS "open-path" monitors in this country. We know of no other facility in the world which has more than two optical-remote monitors. Tosco now has ten on-line. Aside from the awesome technical achievement which this system represents, one of the most exciting aspects of the new system is the extent to which the community has gained ACCESS to the data generated by these monitors. This level of access to emissions data is truly unprecedented.

UPDATE: As of August 2003, as far as we know, this is still the only system of its type in operation. That will change shortly we expect.

Q: Who gets the data?

A: Tosco provides detailed monthly reports from the new fenceline system to the three signatory groups of the Good Neighbor Agreement: The Shoreline Environmental Alliance, (that's us) Communities for a Better Environment, and the Crockett / Rodeo Coalition. Besides the reports, we also receive from the refinery all of the databases and raw data output from the various open-path devices. In addition, one of our members in Crockett has been provided direct modem access to the fenceline system's main server, and can view (and record) emissions data in real-time.

Q: Do the various regulatory agencies also get this data?

A: No. They don't seem to be interested. The Memorandum of Understanding (MOU) we signed with the refinery requires them to grant complete access to the Bay Area Air Quality Management District and/or to the Contra Costa County Health Department, IF either of these agencies requests this access. Thus far, no such request has been made. Hopefully this will change.

Update: The County now gets the reports and posts them here.

Q: What does SEA plan to do with the raw data?

A: By examining the raw FTIR spectral data which we receive on CD-ROM disks, we are able to independently verify the accuracy of a large portion of the reports from the refinery's contractor, Terra Air Services. Also, in the case of these FTIR spectral files, we are able to search for and track the presence of other chemicals which are not being monitored by the refinery.

In the case of the UV and TDLS monitors, even though raw spectral files are not saved, and subsequent analysis of the type performed on the FTIR spectra is not possible, the database files we receive provide us with much more detail about detected emissions than is available in the monthly reports, and also provide us with information about instrument performance and other quality control parameters.

Q: What does SEA plan to do with the real-time data?

A: We are simply recording this data as another method of insuring that we receive accurate reports from the refinery. The MOU which we signed with the refinery prevents us from sharing data with anyone for at least three days after any given release. SEA does not have the capability or desire to act in the role of emergency responder for the county. We insisted on access to the real-time data only as a further method to verify the truthfulness of the refinery's reports.

We believe that because we are recording the real-time screen output, it would be much more difficult for anyone to effectively tamper with the data files which we are sent, or to fake "down-time" after the fact.

UPDATE: Through cooperation with Contra Costa County and ConocoPhillips, we are now able to make public on the web the real time screen output of the fenceline monitor. This is another "first"!

Q: Why the three different monitor types?

A: The short answer to this is that none of these systems, on its own, was capable of doing everything we wanted it to do. Some chemicals absorb light very strongly in the infrared range, others chemicals absorb in the ultraviolet. Some absorb light well in both ranges, some in neither.

Here's the long answer:


The Infrared, or FTIR monitors were the first monitors we settled on, and they were demonstrated successfully in the 6-month "test period" that preceded the final installation. FTIR monitors offer several distinct advantages: These are the most flexible of any of the various monitor types; capable of detecting more than 350 gases (although not all at once). The FTIR units at Tosco are now monitoring for 30 gases simultaneously in real-time. The gases being monitored can be changed quickly and easily in the analytic software, and, importantly to us, the raw spectral data from the FTIR monitors is saved electronically-and can be checked later for the presence of other "unknown" or "interfering" chemical compounds.

The FTIR units were also the only type of unit to have been successfully deployed in a commercial installation in the U.S. prior to the Tosco installation.

The problem with the FTIR units is that they do not do a particularly good job of detecting some of the chemicals of greatest concern at the perimeter of an oil refinery. Specifically, the detection limits for the BTEX group of chemicals (Benzene, Toluene, Ethyl Benzene, Xylenes) generally cannot reach into the low parts-per-billion range. Fortunately, these compounds all absorb light very well in the ultraviolet range, and can be detected reliably in the low ppb range with UV open-path equipment.


Generally speaking , UV monitors can detect chemicals at lower levels, and more quickly than can FTIR units (response times of a few seconds compared to a few minutes), although they cannot detect as wide a variety of chemicals. Unlike FTIR systems, changing target compounds on a UV system requires costly hardware modifications. There is no saving of spectra for subsequent analysis on most UV systems.

The market for open-path UV monitors is one where there promises to be a great deal of competition between manufacturers in the future. Currently there are at least four or five firms marketing UV devices, which are all fairly similar in terms of capabilities and specifications. None seems to have emerged as a clear favorite to dominate this market.

UPDATE: The technology has changed significantly since the fenceline monitor was installed, and we are suggesting that new UV sensors be installed that will drastically improve the sensitivity, accuracy, and reliability of the system, while reducing maintenance costs to the refinery.



Neither the FTIR or UV systems can reliably detect hydrogen sulfide gas at low levels. It is because of this that we opted for a third type of monitor as well, the Tunable Diode Laser System (TDLAS). TDLAS monitors can be "tuned" to detect only one gas, and cannot be switched to another gas without costly hardware modifications. At the Tosco facility, there are currently two separate TDLS monitors operating on each fenceline - one for hydrogen sulfide and one for ammonia - for a total of four monitors.

UPDATE: New UV technology would supercede and replace the TDLAS monitors.




Many of the commercially available open-path monitors are available in both bi-static or uni-static configurations.

"Bi-static" open-path systems employ a transmitter on one end, and a receiver on the other. This configuration is often employed by manufacturers of UV monitors, as longer beam paths can be achieved using this design. Of course, electrical power is required at both ends of such a system. Precise optical alignment is required at both ends.



"Uni-static" systems employ a single optical device (telescope) to act a both transmitter and receiver. Light is bounced off a passive "corner-cube reflector" or "retroreflector". Optical alignment is much simpler in a uni-static system. At the Tosco site, the Infrared and Tunable Diode Laser systems share the same retroreflectors.




This is a photo of the West end of the fenceline monitor installation on the North side of the refinery. The small unit mounted up high is the UV transmitter. The larger cylindrical object is the housing for the 120-cube retroreflector, which is shared by the FTIR and both TDLAS monitors.

This is the East end of the installation on the North. This exact setup is duplicated on the South fenceline. On the North, the total beam path is 920 meters. The beam path on the South is 855 meters.  







Q: What if I have more questions?

A: e-mail them to the webmaster, by all means!



rodeo fenceline site