Passive O xygen M onitors

A simple and inexpensive way to determine whether an atmosphere's oxygen concentration has been reduced to below 1000 ppm is to use a passive oxygen

Chapter 4

Figure 4.6

Oxygen-related color change in the azine dye methylene blue.

monitor. This is a ¼ in. (6.35 mm) diameter tablet that is placed inside the treatment container; it is blue in the presence of oxygen and red or pink under anoxic conditions. This color change is based on the chemistry of methylene blue, an azine dye employed in the formulation of the monitor (Fig. 4.6). Azine dyes can alternate between an oxidized or a reduced state, and this is indicated by color. Air keeps methylene blue in an oxidized condition with a corresponding blue color. In the absence of oxygen and in the presence of mild reducing compounds such as ferrous salts or aldehydes, the dye changes to pink.

Scientists at the Mitsubishi Gas Chemical Company have studied the chemistry of this color change and developed the Ageless-Eye oxygen indicator. They have

obtained a variety of Japanese, European, and United States patents covering products marketed under the Ageless-Eye trade name. One of their early formu- lations was based on glucose as the reducing agent in a mixture with triethyl-

amine, oleic acid, methylene blue, titanium dioxide, and glycerin (U.S. Patents 4,169,811 and 4,349,509). This formulation turned from blue to red at oxygen concentrations as high as 3.5%, which is much too high for killing insects. There were other problems as well with this early formulation. A relatively high

moisture content was needed for the chemicals to interact and provide a color change. In addition, over time, the pink-red color would take on a bluish cast, even in a very low oxygen environment. A more recent patent (Inoue,

Hatakeyama, and Yoshino 1994) disclosed that these problems can be overcome by substituting 1-amino-2, 3-propanediol for the triethylamine. This formulation provides a color transition that occurs under conditions closer to those needed for anoxia treatment. It illustrates how dependent accurate monitoring is on the nature and quality of the chemicals comprising the indicator, which may explain why conservators have had variable results using Ageless-Eye as an oxygen sensor. The composition of Ageless-Eye can be formulated to have the color transition occur at different oxygen levels, and two variations are marketed: type "C" and type "K." Type K changes from blue to pink at an oxygen concentration below 2500-3000 ppm. This is well above the transition zone of type C, which is between 1000 ppm and 1500 ppm. The color change appears to occur more slowly with C than K. Type C operates best at 15-35 °C and 60-90% RH, while type K is best at 5-35 °C and 30-92% RH. Type C Ageless-Eye has a deeper blue color than type K.

When exposed to an atmosphere containing 5000 ppm or more oxygen, Ageless-Eye is purple or blue. It will turn pink when the oxygen concentration falls sufficiently below this level. Quickly reexposing the Ageless-Eye to oxygen, within five to fifteen minutes, brings on the blue color. Going in the other direc- tion, from pink to blue, is slower and more difficult and takes several hours after anoxic conditions have been reached. The return to pink is brought about by the weak reducing agent contained in the formulated tablet. The chemistry of the

reduction requires water, a fact that is reflected in the moisture requirements for both types of Ageless-Eye. Gilberg (1989b) reported on the poor indicator

properties at low RH. He found that at 0% RH, Ageless-Eye failed to turn pink even after prolonged exposure to nitrogen atmospheres of very low oxygen content.

When Ageless-Eye is used in a treatment, it should be positioned to avoid direct exposure to bright light. With time and exposure to light and heat, the capacity of the reducing agent diminishes. This deterioration is generally the cause of

O perational Problems and Practices O perational Problems and Practices

Mitsubishi offers Ageless-Eye packaged three ways. Type K comes in single blue packets that have tiny openings in their windows to allow the treatment atmosphere access to the indicator. Type C is similarly packaged in single, colorless envelopes, but it also comes in a green-colored strip of ten envelopes. Here, too, the singles have openings to the atmosphere, but the strip sets must be punc- tured. The need for great care in storing Ageless-Eye made conservation supply houses reluctant to repackage the manufacturer's standard five-thousand-tablet container into smaller, more affordable units. However, the growing demand for this indicator appears to have overcome this problem, and inexpensive packages holding as few as twenty-five type C tablets can now be purchased.

Carbon Dioxide M onitors

A large number of carbon dioxide monitors have become commercially available to cope with recent stringent requirements for indoor air quality. While there are systems produced to cover any range of concentrations, environmental monitors normally provide measurements over two ranges: 0-2000 ppm and 0-5000 ppm. (The normal atmospheric concentration of carbon dioxide is approximately 350 ppm.) These ranges cover the safety limits set by the U.S. Office of Safety and Health Administration (OSHA). The monitors offer an accuracy that is 5% or better of full scale, but readings normally drift as temperature changes. Calibration must be done periodically and is based on pure nitrogen and a reference mixture of carbon dioxide and nitrogen. The determination of concentration is usually based on the absorption of infrared radiation at bands characteristic for carbon dioxide. The gas to be analyzed is passed through an infrared absorption cell operating over a narrow bandwidth where carbon dioxide absorbs strongly. Changes in radiation energy are detected, amplified, and sent to the signal-processing portion of the system for display or storage. The monitors contain either a diffusion cell, which provides low-power- consuming stable sensing at a low cost, or a sample aspirator, which has higher accuracy and long-term stability but at a higher cost.

An alternative to carbon dioxide monitors are oxygen monitors that can be used to determine concentrations of carbon dioxide in enclosures and chambers. When an enclosure filled with air is flushed with pure carbon dioxide, the oxygen concentration decreases. A reduction of oxygen from 20.9% to 10.45% cor- responds to an increase in carbon dioxide concentration from about 0.04% to 50%; an oxygen reading of 8% indicates that the carbon dioxide level is at 60%, the level that is favored for fumigation. The output from the oxygen monitor can be configured to read as carbon dioxide concentration. Bruel and Kjaer

Instruments Incorporated, Gas Tech, Inc., and Valtronics are established manu- facturers of these sensors.

Relative Humidity M onitors

Many types of RH monitors are available, including indicator strips, hair

hygrometers, and electronic instruments based on bulk polymer, capacitance, and chilled-mirror sensors. Most respond well in the range of 20-80% RH. More sophisticated and expensive instrumentation is needed outside this range. Selec-

Chapter 4 36 Chapter 4 36

The atmosphere in small sealed bags and pouches, where the change in RH is minimal and occurs slowly, can be monitored using inexpensive indicator strips or dial-type hygrometers. These can be calibrated at only one RH and tend to be less accurate than electronic sensors, which can be calibrated at both a low and a high RH. A more accurate and faster-responding sensor is needed in large chambers and bubbles where high nitrogen input, for a variety of reasons, obscures

a precise determination of RH. Among electronic monitors, resistivity bulk- polymer sensors are inexpensive but take up to five minutes to produce 90% of their response; monitors based on capacitance generally provide data in less than thirty seconds.

RH indicator strips are the cheapest sensors, while hair hygrometers can be found at a variety of prices. Electronic sensors range from a simple, low-cost,

bulk-polymer-based indicator, to a highly sophisticated and expensive chilled- mirror dew point sensor. Low-cost electronic RH sensors are available from the HVAC monitoring industry. ACR Systems, Inc., Edge Tech Moisture and Humid- ity Systems, Hy-Cal Engineering, General Eastern Instruments, and Vaisala, Inc., supply most ranges of RH sensors.

Passive O xygen M onitors