Chemical paper tape technology for gas detectors
Chemical paper tape technology uses chemically impregnated paper tape to detect toxic gases. The paper tape, much like litmus paper, changes colour when it encounters a corresponding gas; the paper tape machine analyses the change in colour of the paper tape and converts it into a gas concentration value, measured by means of a photocell.
The advantage of this system is that, as a result of the colour change reaction, the paper tape dispenser provides physical evidence of a gas leak (in contrast to electrochemical, catalytic combustion, solid metal oxide and infrared measuring heads which simply output a 4-20mA signal). In particular, they are also affected by interfering gases, but to a lesser extent than the electrochemical and solid metal oxide types, and are therefore more specific than they are. In addition, paper tape machines can detect more gases than electrochemical types.
The disadvantage of paper tape machines is that they can only be used for the detection of toxic gases and not flammable gases such as hydrogen. Because paper tape machines are expensive, they are usually placed in a central location and connected to individual measurement points via sampling tubes; the gas samples from each measurement point are pumped in turn. As a result there is a significant time lag between leakage and detection, and the sequential pumping can lead to some leaks being missed by the detection instrument. In addition, reactive gases (such as HF, Cl2, HCl, and NH3) can easily be adsorbed onto the sampling tube and not be "seen" by the instrument. Mechanical failures are always a problem with tape machines (rolled cartridge drives, dirty optics, bad pumps, clogged filters, and erratic flow rates), so regular preventative maintenance is required. Regular calibration of the optical system is also necessary. Manufacturers recommend replacing the paper cassette every six months, and while this is a simple process, the cassette can be very expensive to purchase and dispose of.
