Written on: November 5, 2020 by Bob O'Brien
The cadmium sulfide cell, or cad cell as it’s more commonly called, has provided more than a half century of yeoman service, but that time may be ending soon for a couple of reasons.
To review, how does the cad cell do its job? The cad cell eye is made from a cadmium-coated disc with a conductive grid encased in a protective enclosure. The cad cell eye has a very high resistance in darkness and a very low resistance in the presence of visible light. It is basically a variable resistor that alters its resistance in relation to the light it sees.
The cad cell’s primary control uses this resistance to determine whether a flame is present or not. The ohm level that is determined to be an unsuiable flame and to cause the primary control to lockout is called the threshold. Resistance is measured in ohms and must only be checked on de-energized circuits. Failure to do so will usually result in the need for a new multimeter!
In darkness, the ohm reading will be well over 50,000 ohms and is typically in the hundreds of thousands ohms. When subjected to a typical oil burner flame, it will drop to under 1,000 ohms and most often in the 200–300 ohm range. Cad cell resistance also varies in relation to CO2 readings; lower CO2 will raise cad cell ohm readings and vice versa. As an example, at 12% CO2, the ohm reading may be 300; at 10% CO2 it could be double that.
The primary control or cad cell relay must have a very high resistance across the F-F terminals to allow the burner to start—this is to prevent burner operation when a flame is already present. After the trial for ignition (TFI) period ends, the cad cell must provide a resistance under the lockout threshold to prove the presence of flame and allow continued burner operation. The lockout threshold varies by manufacturer and is field selectable on some newer primary controls. All microprocessor-based controls have a higher threshold than older electromechanical types, but 1,600 ohms will be sufficient for even older legacy primary controls.
Room for improvement
The electro-mechanical controls are operated on a very reliable, robust system that has proven itself over the years. If it isn’t broken, then why fix it? There are two main reasons—first is the cadmium itself. Cadmium and its compounds are highly toxic and exposure to this metal is known to cause cancer, targeting the body’s cardiovascular, renal, gastrointestinal, neurological, reproductive and respiratory systems. The danger is primarily to process and production workers rather than service technicians, as they do not handle cadmium directly. Riello has already switched to a new flame detector without cadmium in response to a European Union (EU) directive preventing the use of hazardous materials in production processes.