# Symmetricon — Hazardous Materials Safety Interpretation

**Citation:** 10-0018  
**Type / status:** guidance / guidance  
**Agency:** Pipeline and Hazardous Materials Safety Administration  
**Effective:** Not stated  
**Published:** 2010-03-10

10-0018 response to Symmetricon concerning 171.1.

## Document text

<<<PAGE 1>>>

U.S. Department of Transportation Pipeline and Hazardous Materials
Safety Administration
1200 New Jersey Ave, SE
Washington. D.C. 20590
MAR 1a 2010
Ms. Susan Durr
Sr. Logistics Manager
S ymmetri con
2300 Orchard Parkway
San Jose, CA 95131
Ref. No. 10-0018
Dear Ms. Durr:
This responds to your January 15, 20 10 letter concerning the applicability of the Hazardous
Materials Regulations (HMR; 49 CFR Parts 171-180) to a device containing non-radioactive
cesium metal (Cs-133). Specifically, you askwhether a device containing ten micrograms (Ilg)
or less of cesium, a Division 4.3 dangerous when wet material, is subject to the HMR.
According to information provided with your letter, the device is a chip scale atomic clock
(CSAC) that contains ~ ten Ilgs of cesium metal enclosed in a hermetic cell filled with inert gas
at < four psig (0.26 atm». The cesium in the CSAC is contained within four layers of
mechanical protection. A worst case scenario analysis of the temperature and pressure if the
cesium is exposed to oxygen or water indicates about a 1°C temperature increase in the cell and
about a six psig (0040 atm) pressure rise. You also provide video evidence of destructive testing
indicating no perceptible hazardous effects.
Based on your analysis and based on a prior interpretation provided to your company by
PHMSA that less than one gram of rubidium (also a Division 4.3 dangerous when wet material)
contained in an atomic clock does not pose a hazard during transportation, it is the opinion of
this Office that a CSAC containing ~ ten Ilgs of non-radioactive cesium metal does not pose a
hazard during transportation. Therefore, provided the material does not meet the definition of
any other hazard class, the CSAC is not subject to the HMR.
I hope this information is helpful. If you have further questions, please contact this office.
SinCerelY'~
t~.Be~
~dards Development
Office of Hazardous Materials Standards

<<<PAGE 2>>>

..----:9
Symmetri~orT"
l)er lLlndt~ ~
January 15, 2010 U.S Department ofTransportation PHH-l0
1200 New Jersey Avenue, SE East Building, 2nd Floor
Washington, DC 20590 Attantion: Mr. Edward T. Mazzullo
~Kceff-'()~s
10 - GO 1'3
Dear Mr. Mazzullo,
Symmetricom, Inc. has developed a new chip scale atomic clock (CSAC) which is more than 100
times smaller than existing atomic clocks. The CSAC contains a very small amount (10
microgram) of cesium metal. Because of the very small quantity of cesium and the inherent
hermetic construction of the CSAC product within the scope of 49 CFR § 173.13.
Symmetrlcom seeks a DOT Special Exemption Letter to define the CSAC as exempt from
HAZMAT transport regulations within the Continental USA and Its territories/lslands.
Symmetricom has conducted analysis and testing on the CSAC which confirm its non-hazardous
properties, as documented in the attached analysis. We provide the analysis with an
accompanying movie ( CSAC_cel'-fracture_20091222.avi ) of testing; this movie may be viewed
using most media viewers (eg Windows Media Player).
Should you have any questions please do not hesitate to contact the following individuals for
further clarity and/or information:
Administrative: Susan Durr
sdurr@symmetrlcom.com
voice: 408-964-7624
Technical: Dr. Michael Garvey
rmgarvey@symmetricom,com
voice: 9782321417
We look forward to a favorable decision from the Department ofTransportation.
~/73·J3
Susan Durr
Sr. logistics Manager
Symmetricom
2300 Orchard Parkway
San Jose, California 95131

## Provenance

- Official: Yes
- Source: <https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/legacy/interpretations/Interpretations/2010/100018.pdf>
- Source ID: `phmsa`
- SHA-256: `8a97d479abab4122c73fbf852a5dd3fce2a28cc11c96d890ee1277223c153046`
- Retrieved: 2026-08-20T00:59:31.977Z
- Exported: 2026-08-23T08:06:28.985Z
- Document slug: `phmsa-interpretation-10-0018`

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