Scenario: Your Ludlum Model 9 Ion Chamber has been operating correctly since it was bought. After not being used for a couple of months, you need to use it. Batteries are placed in the instrument and it is turned ON. After the normal warmup period, the lowest range selection reads approximately 2 mR/hr. You depress the RESET button and the meter deflects to zero. When the button is released, the meter still remains at 2 mR/hr. The background radiation is known to be much lower than 2 mR/hr.
Question: What has happened? Why is the instrument not functioning properly?
The above scenario and questions are common to our Repair Department. Most of the tie the problem with this instrument is the desiccant located within the housing is saturated. These desiccants, which are blue or pink cylindrical shaped objects located inside the housing of the instrument, are positioned to keep moisture from affecting the ion chamber and related electrometer components. The ion chamber produces extremely low currents in radiation fields, therefore everything has to be kept at an extremely high resistance (1012 to 1013 ohm). If any moisture is collected inside of the chamber, the overall resistance is reduced enabling a "false" current to be produced indicating a "false" radiation field.
The equivalent problem may also occur in the air proportional detector. A loss of efficiency or premature detector breakdown can occur if moisture is allowed to enter the detector. Desiccant replacement will increase as the humidity increases.
Ludlum instruments are equipped with "indicating" desiccants. These desiccants have a dark bluish color when new, and as they absorb moisture will change from blue to a pink color. Ion chamber desiccants should be changed if there is a color change or if there is an offset on the instrument's meter dial. The air proportional detector desiccants should be changed in the case of color change or if there is a drop in efficiency.
Indicating desiccants can be purchased from LMI, or the old desiccants can be dried by placing them in a warm oven on 150 °F until they return to the bluish color. Improvements have been made in current Ludlum ion chambers for high humidity conditions. A new desiccant box which holds four desiccants instead of two and a new improved seal between the box and chamber has been utilized in the design of recent LMI ion chambers. Also, the mylar window frame has been strengthened to improve contact between the window and the chamber. These improvements can be incorporated into old-style LMI ion chambers if possible humidity problems continue to be encountered.
From September 1990 Newsletter