Preventing FrostDoug George
by Doug George.
Some people ask, “Why do we use desiccant plugs?”
Simply put, they minimize life cycle cost and downtime.
The imaging sensor needs to be below freezing in order to control dark current and noise. If the sensor is exposed to the atmosphere it will quickly be encased in ice. This not only ruins the images, but can contaminate the camera and even cause damaging corrosion.
In order to prevent frosting, all cooled cameras include a sealed chamber. The sealed chamber will require an optical window, and it will require electrical connections. There will also be apertures to open the chamber for assembly and maintenance. So there will always be “holes” that need to be sealed. Common sealing methods include welding, high-quality epoxy, rubber O-rings of various sorts, indium metal seals, etc. All of these techniques have advantages and disadvantages in terms of reliability, quality of seal, maintainability, etc.
All seals slowly leak; it is just a question of how fast. No matter whether the camera contains dry air, nitrogen, argon, or vacuum, eventually outside air works its way into the chamber.
Most vacuum systems are periodically pumped down to maintain sufficient vacuum. Some manufacturers make “permanently sealed” vacuum units; however, inevitably the seal eventually fails, and this means expensive and time-consuming repairs to restore the camera.
Some manufacturers of inexpensive, small sensor cameras simply restrict the volume of air in the chamber. When you power up the camera, the residual humidity inside freezes out onto the coldest component, i.e. the back of the sensor. This works well as long as the camera cooler does not remain powered on for long periods of time; otherwise water vapor progressively diffuses into the chamber until the sensor freezes over. By this point the sensor pins are soaking wet and corrosion is a major risk. This type of camera should not be operated with its cooler running continuously.
Everyone else uses a high-performance desiccant, such as molecular sieve. With the right choice of desiccant, humidity levels in the chamber are maintained at a very low level, achieving a dew point below -40C.
These desiccants can maintain extremely low humidity until enough vapor eventually diffuses into the camera to saturate them out, at which point they become completely ineffective. Many manufacturers simply hide a large volume of desiccant inside the chamber. Eventually it saturates, and the camera must be returned for service, which includes opening the chamber, replacing the desiccant, and thoroughly cleaning the sensor and other hardware. Although the failure seems to be sudden, in fact it was gradually failing since the day it was built.
The problem with all of these approaches is that the user continues using the camera until they notice that it has failed. Unfortunately it is all too common for the user to take some time to notice the problem, resulting in a large quantity of frost. When the camera is powered down this frost turns to water, and the soaking wet sensor and associated circuitry starts corroding. All too often this results in damage and expensive repairs are the result.
Our approach is different. Our cameras have desiccant modules that can be easily removed and re-baked by the end user. Performing this simple procedure once in a while – we recommend once every six months to a year – avoids ever having to return the camera for recharging and cleaning. The module includes a special filter, which keeps any dust associated with the desiccant out of the chamber, while allowing vapor to flow through.
Simple periodic maintenance beats expensive service, and eliminates unnecessary downtime.