Calibrating space

Companies who design and manufacture equipment destined for space must carefully calibrate their products to withstand the hostile conditions, and they must subject the products to rigorous test regimens before delivering them. One such company is Hamilton Sundstrand Space, Land, and Sea Systems-Windsor Locks (SLS-WL), which has been making space suits since the 1970s. The company also makes environmental controls for the Space Shuttle and the International Space Station as well as for aircraft and submarines.

Environmental controls in space must perform perfectly under extreme conditions; there’s no way to recall a product from orbit without great expense. Pumps, tanks, heat exchangers, motors, sublimators, evaporators, regulators, and electronics that control environmental systems need extensive testing before they launch. The systems that test these components must simulate conditions in space. The equipment that makes performance measurements and monitors the environment must be accurate—and that means regular calibration.

Simulating space and testing environmental systems requires SLS-WL to make many physical measurements, all of which require calibrated equipment. Instrumentation in test stations, called “test rigs,” measures temperature, pressure, liquid and gas flow, vacuum, humidity, dew point, and vibration. Some of the company’s test rigs date back to the Apollo program—SLS-WL provided the original lunar space suit—but they’ve been refurbished to meet current needs.

Electronic test equipment used throughout the SLS-WL facility measures voltage, current, resistance, time, frequency, power, and phase. The test rigs employ data-acquisition systems that record physical measurements from transducers such as temperature probes, pressure gauges, and flow meters.

As you might expect, the Hamilton Sundstrand facility is loaded with environmental chambers. Temperature chambers, for example, test products over the wide range of temperatures encountered in space, with temperature cycling from –100°F to 350°F. Temperature/humidity chambers test at –100°F to 250°F at 10% to 98% RH. Thermal vacuum chambers test products down to 10^–8 Torr and over temperatures from –300°F to 300°F.

The Hamilton Sundstrand space suits, or Extravehicular Mobility Units (EMUs), have environmental systems that include oxygen and water tanks, batteries, a sublimator, fans, and regulators. These systems reside in a backpack with astronaut controls accessible through a front display and control module (click to view Figure 1).

Pressure testers ensure that the tanks and tubes on the EMUs won’t fail. For example, an oxygen tank, which operates at approximately 7600 psig, is tested with supply pressures up to 10,000 psig. (The high tank pressure is necessary to supply enough oxygen for a spacewalk while keeping the tank sufficiently small.) Vacuum leakage testers verify that parts don’t leak water or oxygen into space.

Phil Noll, calibration lab manager at Hamilton Sundstrand, stands among the mannequins in the company’s space-suit museum.

The SLS-WL Thermal Vacuum Space Simulator includes a data-acquisition system that technicians use to make measurements from specifications written by test engineers. Several Hewlett-Packard 3852A data-acquisition systems have been in place since the late 1980s. As long as these instruments are calibrated, they continue to provide valid measurements.

“We’re not very quick to change instruments, because our test sheets are written to specific requirements and all equipment has been safety certified for each application,” said Noll. Because many of the products produced in Windsor Locks remain in production for years, so do their test rigs.

Not all data-acquisition systems date to the 1980s. For example, a thermal chamber that tests Flight Releasable Attachment Mechanism (FRAM) systems connects to relatively new measurement equipment: data-acquisition cards installed in an industrial PC that runs National Instruments’ LabView.

Used on the International Space Station (ISS), FRAM systems consist of a passive FRAM and an active FRAM, and they let astronauts transport cargo between the Shuttle Orbiter and the ISS. One passive FRAM is permanently mounted in the Orbiter’s cargo bay, and another is mounted on the ISS. An active FRAM containing cargo is mated to the passive FRAM in the Orbiter. Should equipment in the ISS need replacement, astronauts can transfer the active FRAM from the passive FRAM on the Orbiter to the passive FRAM on the ISS.

A FRAM thermal/vacuum test subjects the mechanism to temperatures from –140°F to 260°F with vacuum well below 10^–4 Torr. The FRAM system is mounted on a test stand where custom motors move FRAM mechanisms into place. Technicians use transducers and a data-acquisition system to measure torque (in. lbs) and force (lbs) while also measuring temperature and vacuum.

The FRAM test rig is one of several that requires calibration of its test instruments right at the rig. “We calibrate each parameter end to end in place,” said Noll. A technician rolls an equipment cart containing the required stimulus to the rig in order to perform the calibration.

Some tests require special test areas because of safety issues. For example, the company has an area outside the main building where technicians assemble the pumps and equipment that provide the high-pressure oxygen and nitrogen necessary for EMU testing. This test lab contains intrinsically safe and explosion-proof systems.

To test the EMU’s high-pressure O₂ tanks, technicians start with liquid oxygen or nitrogen, change it to a gas, and initially pump the tanks to approximately 2200 psig. While monitoring air quality and pressure with a data-acquisition system, they increase the pressure to 10,000 psig.

During testing of the O₂ tanks, an HP 3852A data-acquisition system collects temperature, pressure, and flow data and sends the data to a computer at the operator’s console. The console contains two monitors, one for the operator and another for a test engineer. While the technician runs the test and monitors the performance of the test rig, the engineer monitors the item under test, keeping track of parameters such as coolant flow, pressure, and temperature.

The measurements that these test rigs perform start with a specification. A design engineer who needs a measurement provides the requirement to metrology engineering. Metrology engineers write a measurement specification that contains information such as measurement type, range, scale, and accuracy. Often, an existing test rig has the measurement capability that the design engineer requires. A test technician and metrology engineer review the measurement specification and decide which existing test rig to use or, if necessary, what the requirements are for new equipment.

Sensors, meters, and other equipment used on test rigs as well as the test equipment used in manufacturing and engineering need calibration. That’s the job of engineers and technicians in the metrology lab. The lab consists of several calibration stations as well as a station for documenting and labeling equipment. (For a discussion of the metrology lab’s construction and grounding system, see "Into the earth." (Click to view a floor plan of the metrology lab.)

The electronic calibration station consists of two identical racks. Each contains a Fluke 5700 multifunction calibrator, an Agilent 3458A digital multimeter (DMM), and a Yokogawa power analyzer. In addition, the engineers can add instruments that provide any other required electrical stimulus. The stations calibrate DMMs, oscilloscopes, counters, power meters, data-acquisition systems, and other equipment that needs electrical input or measurement—about 1500 instruments. “We calibrate all the instruments used in SLS-WL,” said Noll. Table 1 shows the capabilities of the electronic calibration stations.

Technician Dave Bates certifies all calibration standards used in the Hamilton Sundstrand SLS-WL metrology lab.

At the time of my visit, technicians still performed manual calibrations on electronic instruments. To automate the calibration procedures, SLS-WL metrologists are implementing Fluke’s MET/CAL software. Although MET/CAL has many automated calibration procedures that cover much of the SLS-WL test equipment, most procedures need to be modified to meet specific lab needs.

Vibration is an important test for space-bound equipment, and the metrology lab has a dedicated accelerometer calibration station. At the time of my visit, the accelerometer calibration station—a rack of equipment—was about to be replaced by a benchtop instrument manufactured by PCB Piezotronics. The existing Bruel & Kjaer system had been in use since the 1980s and has a frequency range of 5 Hz to 10 kHz. The new system has a wider frequency range: 3 Hz to 50 kHz.

“Calibrating accelerometers requires a certain amount of finesse” said Noll. He pointed to the way an accelerometer under test was placed in the calibrator’s test fixture. The sensor’s wires must be held down so they don’t vibrate relative to the sensor.

Across the lab from the electronic and accelerometer calibration stations is the temperature-calibration area. Here, technician Dave Zisk calibrates temperature probes used throughout SLS-WL. He calibrates some probes in the metrology lab, but probes installed in test rigs must be recertified in place.

In the lab, Zisk has several options for calibration. An oven that contains a 25-Ω standard platinum resistance temperature detector (SPRTD) probe lets him compare a probe under test to a known calibrated probe. He also uses thermal wells and oil baths to calibrate “working” probes—those used in the facility’s test labs. Click to view a box highlighting the metrology lab’s temperature-calibration capabilities.

An instrumentation rack connected to the SPRTD and probe under test contains an HP 3456 DMM, which measures four-wire resistance from RTD and thermistor probes and measures voltage from thermocouple probes. A reference junction provides a 32°F reference for the instrumentation, and a switch system connects the probes and reference junction to the DMM. A PC running LabView stores resistance measurements from the SPRTD and probe under test. The software includes the calibration curve for the SPRTD and also converts the resistance measurements to temperature. Then, software generates a calibration curve for the probe under test from resistance or voltage measurements.

To calibrate probes installed in the test rigs, Zisk uses a “Black Stack” thermometer from Hart Scientific. The thermometer measures temperature from a standard probe under test, from which a laptop computer calculates the probe’s new resistance-temperature curve.

SLS-WL has six standard platinum probes. Technicians use five to calibrate working test devices. A sixth remains in the metrology lab as a check for the other five. These standard probes require calibration, too. Because they are the facility’s reference probes, they need to be calibrated at known, stable, and repeatable temperatures.

The metrology lab contains several Isotech fixed-point temperature cells that serve as primary temperature standards. The cells contain mercury, gallium, zinc, tin, aluminum, and water from which they achieve freezing points or melting points of these elements as well as the triple point of water (Ref. 1). Metrologists maintain a gallium cell—melting point 85.57628°F—at all times that they use as a quick check for damage to any of the six standard probes

“We use the primary standards at six-month intervals,” said Noll. When I asked how Hamilton Sundstrand can justify the cost of these primary standards, Noll replied that an SPRTD calibration can cost up to several thousand dollars, and the company comes out ahead in the long run by purchasing its own primary standards and by being able to obtain immediate verification of standards accuracy.

Engineer for methods and standards Scott Shepard (back) develops calibration procedures and provides technical assistance to the metrology lab. Technician Dave Zisk (foreground) calibrates and certifies instrumentation used for production testing of environmental controls.

For each item that requires calibration, the database stores information such as calibration results, calibration dates, the next recall date, and the location of each instrument and accessory used for electrical, mechanical, or physical measurements during the calibration. Every item that requires calibration has a serial number and bar code for easy tracking.

Because the database tracks hardware test information, engineers always know which products were tested with each gage. The system displays data on the lab’s intranet, where technicians and engineers can view the calibration information.

Figure 2 shows the flow of an item as it passes through calibration (click to view figure 2). The figure highlights two paths, depending on whether the item is to be calibrated at SLS-WL or at an outside calibration house. Following calibration, a technician places a green, white, or yellow label on the item before returning it to its location or placing it on the shelf. All items that fail calibration receive a red sticker. Table 2 shows the label colors and what they designate.

Hamilton Sundstrand’s Space, Land, and Sea metrology lab supports instrumentation used for numerous electronic, physical, and dimensional measurements. Calibration of measurement equipment used in test rigs ensures that the company’s environmental controls will keep astronauts safe and comfortable in space.