1. Source Decay. On day one the source will be sufficient, but will decay steadily until too weak to operate the detector. Cobalt sources decay fast, Caesium decays to half value in 33 years. We calculate the source size to be still sufficient at 20 years life time. The cost to replace a source is considerably more than the acquisition thereof.
2. We size the source to allow for a certain amount of build-up on the walls of the vessel. Unless the customer specifies this as a special problem, we allow for one half of a half value to ensure reliable operation.
3. The detector deteriorates constantly with time and has a total life span of 10¹² pulses. To avoid the early replacement of this expensive item, we will allow another one half of a half value to ensure a good life expectancy from the detector. All this is over and above the allowances made under items (1) and (2) and results in a source size above the absolute minimum for good operation on day one.
4. Maintenance adjustments are done less often when a system has considerable tolerance built into its design.
5. Our level gauges are super reliable, being dual output type, and should it fail, it immediately (via the second relay) advises the operator that the system is non-operative. This feature is especially important in Low-Low level applications where the Low-Low level is not intended to occur ever. The concern will be whether the gauge will work on the rare occasions when this level is reached. Our gauge will immediately announce the failure of the system to prevent damage or other problems.
6. Background radiation (noise) forbids the use of very sensitive detectors. For long term reliability, one has to be well above noise level. As the source decays, one moves closer to noise level. Also, the noise level tends to be seasonal and varies from area to area. To be safe, we work with a large signal and ignore the noise pertaining to a specific area, process material etc.

The practical value of the above is that none of the gamma level gauges we have supplied ever required replacement, while, on the other hand, we often replace others.

Process Automation gamma gauges last the lifetime of the plant it is installed in.