Evaluation of Ammonia Sensors for Controlled-Atmosphere (CA) Applications
Under sponsorship from the Washington Tree Fruit Research Commission, staff at the US Department of Energy's Pacific Northwest National Laboratory (operated by Battelle) are initiating a project to evaluate the performance of sensors for ammonia leak detection in controlled atmosphere (CA) fruit storage facilities.
The project is comprised of two tasks:
- Perform long-term testing of ammonia sensors
- Conduct sample gas transmittance measurements using a variety of tubing materials and flow conditions.
During this project, a number of commercially-available ammonia sensors will be subjected to periodic controlled ammonia exposures over a period of 6 to 9 months to assess measurement accuracy, calibration drift, reliability, and sensitivity to interferences (e.g., ethylene which evolves from ripening fruit). A relatively new vapor characterization technology, Fourier Transform InfraRed (FTIR) spectroscopy, will also be evaluated in this experimental study. Carrier gas for all sensor exposures will be representative vapor (oxygen, water vapor, carbon dioxide, etc.) drawn from active CA rooms at Stemilt Growers in Wenatchee, Washington. Gas sampling tests and the development of optimal vapor sampling protocols will be performed at Battelle facilities in Richland, Washington.
This investigation is motivated by several factors:
The existing generation of ammonia sensors employed in CA facilities is viewed as unreliable (e.g., they malfunction with no indication and/or they false alarm).
Recent insurance claims for fruit damage from ammonia exposure have been high with the result that insurance underwriters will soon require certification that installed ammonia sensors are periodically calibrated and verified to be operational.
Insurance companies attempt to assign blame (and therefore financial liability) for fruit damage to any and all parties involved in the design, construction, and maintenance of the malfunctioning CA facility. Consequently, there is considerable interest amongst CA facility designers and suppliers to reduce their personal and corporate liability for ammonia-damaged fruit.
The goals of the project are as follows:
Identify commercially available ammonia sensors suitable for long-term monitoring of ammonia in CA rooms.
A variety of optical and electrochemical vapor sensors have traditionally been used for CA ammonia detection. Performance data on these sensors is largely anecdotal and there is little or no quantitative CA data for these devices.
Identify sensor interferences and failure modes.
A number of the ammonia sensors in CA use exhibit responses to vapors other than ammonia. These responses need to be quantified and documented. In addition, sensor failure modes are undocumented and poorly understood.
Determine the influence of the gas sampling system, especially the sampling lines, on ammonia alarm performance.
It is likely that many of the CA room vapor sampling and transport systems currently in use result in degradation of the sample (e.g., loss of ammonia due to the presence of liquid water in the sampling line). These sample degradation effects need to be verified and quantified and recommendations for high quality sampling methodologies made to the CA industry.
Overview of the Ammonia Sensor Testing System
An overview of the ammonia sensor testing system appears in Figure 1. The subsystems are summarized below:
The vapor sampling systems to be used for these studies are presently in operation at the Stemilt Growers facilities in Wenatchee, WA and are used to monitor oxygen and carbon dioxide levels in each of the commercial or research CA rooms. The vapor sample for the ammonia sensor evaluation system will be drawn from the output of the existing sampling manifold.
The injection/mixing system combines the CA room vapor sample with controlled, periodic injections of ammonia vapor and interference gas (ethylene). These gases are provided from high pressure bottles equipped with pressure regulators and computer-controlled solenoid valves. Vapor mixing is performed in an inert mixing chamber prior to transfer to the ammonia sensor array.
Ammonia Sensor Array
Ammonia sensors under test are connected in parallel to insure that they "see" identical vapor samples. Flow rate to each sensor is individually adjustable to manufacturers' specifications. If feasible, two sensors of each type will be evaluated simultaneously in order to obtain data on sensor-to-sensor variability.
Complementary Gas Sensors
Effluent from the ammonia sensor array will be passed through the infrared absorption cell of an FTIR vapor analysis system. In addition to monitoring ammonia concentration, this sensor will provide concentration data for water vapor, carbon dioxide, ethylene, and other vapors (esters, alcohols, aldehydes, etc.) eluting from stored fruit.
Control and Data Acquisition System
The control and data acquisition system performs four functions:
- Tt controls ammonia and ethylene injections
- It records ammonia sensor output signals
- It records the outputs of any ancillary sensors (flow, temperature, pressure, etc.)
- It performs daily transmissions of all data to Battelle facilities in Richland, WA.
The ammonia sensor testing system is presently being assembled at Battelle facilities in Richland, WA. Installation of the system at the Stemilt Growers facilities in Wenatchee, WA is anticipated in late Spring of 1998. Continuous, long-term evaluations of the ammonia sensors will performed over the duration of the 1998-99 storage season which begins in October, 1998 and extends through June 1999.
Dr. Jeffrey Griffin1, Seth Poulson and Lawrence Carter2, and Dr. Nate Reed3
1Battelle Northwest Laboratories, Richland, WA
2Dept. of Electrical Engineering and Computer Sciences
Washington State University, Tri-Cities Campus
3Stemilt Growers, Wenatchee, WA
14th Annual Postharvest Conference, Yakima, Washington
March 10-11, 1998