Chapter 5 - Materials and Methods

Abstract

The most accurate commercially available humidity measuring devices have a precision of ±1.7% RH in the 95–99% RH range. This is not sufficient for monitoring and controlling the humidity during low-pressure (LP) storage because the error spans the entire range between a small water loss and extreme desiccation. During LP, a wet- and dry-bulb temperature measurement with ±0.05°C thermistors shielded from radiation with Mylar® is used to compute the chamber RH with an accuracy of ±0.1% at 99.9% RH. The wet-bulb measurement corresponds to the chamber dew-point temperature since heat transfer to the wet-bulb by convection is limited by the low chamber pressure, and Mylar® limits heat transfer by radiation. The RH is computed from the dew-point and dry-bulb temperature measurements. Pressure is measured with a temperature compensated absolute capacitance transducer. A vacuum breaker controls the pressure by adjusting the rate at which atmospheric air enters while the vacuum pump withdraws low-pressure air at a constant rate. A vacuum regulator controls the pressure by adjusting the rate at which the vacuum pump evacuates the chamber while atmospheric air enters at a constant rate. Hypobaric intermodal containers operate intermittently over-the-road at altitudes that cause a vacuum breaker to close or a vacuum regulator to open if they have a barometric reference pressure. To prevent this from occurring the reference pressure is controlled ± 0.2mmHg by a low capacity self-relieving absolute pressure vacuum regulator having a sealed absolute pressure reference. A single-stage, oil-sealed, rotary veined, gas-ballasted, air-cooled vacuum pump is used in almost all hypobaric systems. Before the low-pressure mixture enters a vacuum regulator or the vacuum pump, the 0–15°C process gas’s temperature is increased to 25°C, decreasing the low-pressure air’s RH by 53–81%. Sufficient “gas ballast” air is admitted to cause the vacuum pump’s exhaust valve to open before the water condensation pressure is reached. The air-change rate during LP storage is determined by the pumping speed of the vacuum pump and can be measured at atmospheric pressure with a flow meter located upstream of a vacuum breaker. Airflow through a mechanically humidified hypobaric system controlled by an absolute vacuum regulator cannot be calculated based on the rate at which atmospheric air enters the system because the regulator increases (or decreases) the flow rate from the chamber to the vacuum pump in response to transpired water and the altered air and water vapor pressures which develop at the elevated regulator temperature. Total CO2 production and O2 consumption due to respiration and fermentation is measured by comparing the [CO2] and [O2] concentrations present in air entering the hypobaric chamber vs. their concentrations in air exhausted from the vacuum pump. Methods are described to measure and control the air-change rate and measure O2, CO2, ethanol, and acetaldehyde in the air-change and within the stored commodity.