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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Interpretation Response #10-0018 ([Symmetricon] [Ms. Susan Durr])

Below is the interpretation response detail and a list of regulations sections applicable to this response.

Interpretation Response Details

Response Publish Date:

Company Name: Symmetricon

Individual Name: Ms. Susan Durr

Location State: CA Country: US

View the Interpretation Document

Response text:

March 10, 2010




Ms. Susan Durr

Sr. Logistics Manager


2300 Orchard Parkway

San Jose, CA 95131

Ref. No. 10-0018

Dear Ms. Durr:

This responds to your January 15, 2010 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 ask whether a device containing ten micrograms (µg) 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 µgs 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 (0.40 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 µgs 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.


Charles E. Betts

Chief, Standards Development

Office of Hazardous Materials Standards

173.13, 171.1

Regulation Sections

Section Subject
171.1 Applicability of Hazardous Materials Regulations (HMR) to persons and functions