Developing an Improved VXI Mainframe Cooling Measurement Technique

To achieve high levels of performance, the VXIbus standard addresses several pertinent technical issues. One of these is the evacuation of heat by a well-defined cooling system.

A VXI mainframe provides an operating environment for instruments that supply power, cooling, and communications interconnections. The capabilities of this operating environment create the boundaries that ultimately determine the maximum performance of the individual instruments.

Typically, high-performance VXI instruments consume 25 W to 100 W of power in each slot, and the majority of this power dissipates as heat. To remove this heat from the system, VXI mainframes provide a continuous flow of air.

As required by the VXI standard, module and mainframe specifications must state their respective cooling needs and capabilities. This allows the system designer to verify that the instrument modules are adequately cooled.

Each VXI instrument module has a characteristic power dissipation as it performs its function, and a VXI mainframe has a characteristic capacity to cool modules. Most VXI instrument modules specify an operating temperature rise of 10°C. If the module-cooling requirements are met, the temperature rise of the air flowing through that module will be less than 10°C.

To maintain this temperature rise, a mainframe must provide 1 liter/s of air for each 12.5 W of power dissipated by the module. If the module dissipates 50 W, then 4 liters/s of air must be provided to maintain a 10°C temperature rise.

As a VXI mainframe forces air through an instrument module, the restriction of the airflow produces a pressure difference between the top and bottom of the module. This pressure difference, which opposes the flow of air, is called back pressure. The back pressure reduces the capability of the mainframe to generate airflow through the module. If the back pressure is high enough, the mainframe may fail to generate a sufficient airflow to maintain the module-specified temperature rise.

Cooling Specifications

VXI instruments specify their cooling requirements as two values: the required airflow in liters per second and the back pressure produced by the module at that level of airflow in millimeters of water. These two values determine the minimum thermal operating point for the module.

VXI mainframes specify their cooling capability as a graph of airflow vs back pressure. For each level of back pressure produced by a module, the mainframe generates a characteristic airflow. At some high back pressure, the mainframe airflow is reduced to zero. At zero back pressure, a maximum airflow is achieved.

Figure 1 provides an example of a mainframe cooling curve and a module- specified operating point. The module operating point is one of the points on a characteristic curve of airflow vs back pressure for that module. In Figure 1, the specified operating point is 4.0 liters/s of airflow and a corresponding back pressure of 0.3 mm of water.

The mainframe cooling curve is the characteristic curve of the lowest performance produced by the mainframe at each level of back pressure. When the module is installed in the mainframe, the actual airflow generated through the module will be at the intersection of the module and mainframe characteristic curves.

In Figure 1, the actual airflow delivered to the module is 5.3 liters/s. Because of the conservative nature of the VXI specification, the cooling provided to the module will exceed the module requirements.

The VXI cooling specifications provide a high level of confidence that modules will be adequately cooled. This allows module manufactures to increase performance without compromising reliability.

Airflow Measurement Module

To ensure accurate measurement of module and mainframe cooling characteristics, the VXI standard defines the equipment and procedures to be used. These procedures have proven adequate for module measurement but have been disappointing in measuring mainframe cooling capacity.

Mainframe measurements are particularly difficult because they must be performed within the confines of a VXI module enclosure. Added to that constraint is the fact that the airflow through a module typically is turbulent and nonuniform over the length of the module. Commercially available equipment is not designed to provide accurate measurements under these conditions.

To combat this problem, we sought a new approach to measuring mainframe cooling capacity. A year of R&D resulted in an airflow measurement module that is accurate and simple to operate (Figure 2). It measures the total airflow in liters per second flowing through a VXI slot using the heat capacity of air to meter the airflow.

This is not a unique approach to airflow measurement. What is unique is the way this principle is applied to the measurement of airflow in a VXI mainframe.

The temperature of a volume of air rises as heat is applied to it. The change in temperature is proportional to the heat supplied for a given volume of air. If you provide the same amount of heat to a volume of air twice as big, the temperature rise will be half as large.

The airflow measurement module uses this characteristic of air, heating the air as it passes through the module and measuring its temperature as it exits the heat source. If the rate of power dissipation and the temperature rise are known, the airflow through the module can be calculated.

In a VXI module, the airflow is not uniform. Typically, there will be higher airflow in the front portion of a module than in the rear. The airflow also is turbulent. It does not follow a straight vertical line from inlet to exhaust. This poses a significant problem.

The airflow measurement module deals with these problems with a heat source and thermometers that measure the temperature rise. The heat source must be assembled so that a volume of air receives a constant quantity of heat each second it remains in the module, regardless of the path it takes. It must provide a constant, homogenous source of heat over a 3-D space inside the module. The temperature rise is then proportional to the time spent in the heat source that is, in turn, proportional to the airflow produced by that volume of air.

We accomplished this by using an array of 400 resistors positioned in a 3-D pattern. The module is thermally insulated to ensure all of the heat provided is taken up by the airflow.

Because the airflow exiting the module is not uniform, the temperature rise at each point of the exhaust will vary. Consequently, the temperature of the exhaust air must be sampled at many locations to determine the temperature gradient at the module exhaust.

The airflow measurement module samples the exhaust temperature at 64 locations with a resolution of 0.05°C. The ambient inlet air can be assumed to be well mixed and uniform in temperature. A single thermometer is sufficient to measure inlet temperature.

Given a known power dissipation and a sufficient number of temperature-rise measurements, the total airflow can be calculated. A simple algebraic formula is applied to each sample to produce an incremental airflow value:

where: D T = temperature rise

w = power dissipation

r = air density

c = heat capacity of air

G = total airflow

To simplify the measurement process, the airflow measurement module incorporates thermometer scanning circuits, an A/D converter, a microprocessor that performs measurement control and airflow calculations, and an alphanumeric display that indicates the measured values directly. This eliminates interpretation of the measured values, ensuring consistent, accurate measurements.

Cooling Capacity

The airflow measurement module also produces a new measure of mainframe cooling capacity: front-to-back airflow percent variation. Due to the small space available to the cooling structure in a VXI mainframe, it is difficult to ensure that the same level of airflow is provided over the length of the instrument module.

Typically, the airflow in the front of the module is higher than the airflow in the back. This can cause components toward the rear to get excessively hot.

The front-to-back percent variation specification uses the standard deviation of the airflow samples taken over the length of the module. The standard deviation is divided by the total airflow to provide a figure of merit relative to the cooling capacity provided. Lower percent variation values indicate better airflow balance over the length of the module.

To verify this new technology, extensive wind-tunnel testing was performed. The airflow measurement module was attached to a calibrated wind tunnel, and airflow between 3 liters and 35 liters/s was forced through the module. The airflow measured by the module was compared with the values produced by the wind tunnel.

The data showed a correlation with errors no greater than 5%. Additional tests were performed, blocking 30% and 70% of the inlet area to force a nonuniform turbulent flow. The correlation error increased to 7% but remained well below the goal of ± 10%.

Conclusions

The airflow measurement module provides accurate, repeatable measurements of VXI mainframe cooling capacity. Tektronix has proposed that the VXI Consortium adopt this new approach to measure VXI mainframes. Greater accuracy in these cooling measurements will allow module manufacturers to take full advantage of the higher power levels provided by the VXI standard. Customers will benefit because modules will run cooler, improving reliability and reducing downtime.

About the Author

Michael S. Hagen is an engineering manager at Tektronix. He has worked for the company since 1975, participating in the development of the original VXI specification and the VXIplug&play effort and chairing two key committees (the VXI-8 Cooling Specification Technical Working Group and the VXIplug&play VPP-2 and VPP7). Mr. Hagen studied electrical engineering at the University of Portland. Tektronix, P.O. Box 500, MS 39-122, Beaverton, OR 97077, (503) 627-1573.

Copyright 1997 Nelson Publishing Inc.

November 1997

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