Tektronix Encore


When Temperature and Humidity are Under Control

April 02, 2019

Environmental conditions can vary widely from place to place and season-to-season, often making it necessary to evaluate electronic products while temperature and humidity are well controlled. Testing a device under test (DUT) or equipment under test (EUT) under adverse environmental conditions can often reveal problems in a design and help discover ways to improve a design meant for use over temperature and humidity extremes. An essential tool for performing such testing is an environmental chamber capable of accurately controlling temperature and humidity.

Environmental test chambers are available in many sizes and power levels, depending on measurement requirements. An environmental test chamber well suited to an application should provide the amount of control needed in terms of adjusting temperature and/or humidity, enough volume to contain a desired number of DUTs or EUT, and adequate isolation from the environment outside of the test chamber. The test chamber should also provide interconnections for the probes and sensors needed for tracking the changes of temperature and humidity within the chamber as well as interconnections for remote computer control and for any desired measurement equipment, such as voltmeters and oscilloscopes.


Taking the Temperature

Temperature-controlled test chambers, which are also known as temperature-forcing units, range in size from compact table-top units for testing a few DUTs at a time to much larger, floor-standing systems. They can be sorted by temperature ranges, displayed in thermal units such as degrees Fahrenheit (°F) or degrees Celsius (°C), as well as power consumption and size. Thermal test chambers may also use British thermal units (BTUs) as a measure of energy, signifying the amount of work needed to raise the temperature of one pound of water by 1°F. Temperature-controlled test chambers are often also evaluated by how well they control the rate of change in temperature, by temperature display resolution, and by the accuracy of the reported temperature. Most temperature-controlled test chambers are capable of local control, such as by a chamber-mounted keypad, or by remote control, by means of a computer running test software.

Temperature-forcing units achieve their control of colder temperatures by chemical or mechanical means. Units equipped with liquid-nitrogen (LN2) fittings for connection to external LN2 supplies, for example, can achieve subzero temperatures quickly and quietly, while mechanical refrigeration systems typically require multiple compressors to reach subzero temperatures. For test applications that may require temperature cycling, temperature-controlled test chambers capable of making fast transitions from cold to hot temperatures and back again can be advantageous when setting up time-efficient measurement stations.


How About Humidity?

Test chambers with the capability to control humidity can be quite useful in evaluating how well devices or EUT responds to real-world humidity conditions. Test chambers equipped with humidity control should include a drain for condensation and a supply of soft water as the source of humidity. This contrasts with hard water, which can lead to scale conditions within humidity chambers. Because test applications can differ widely, such as requirements for EUT designed for marine versus non-marine environments, humidity-controlled test chambers may also provide the flexibility of providing condensing as well as noncondensing humidity conditions, perhaps in combination with temperature control within the same chamber.


Exploring Examples

How do these guidelines fit with currently available temperature- and/or humidity-controlled environmental test chambers? An example of a temperature-forcing unit for rent from Axiom Test Equipment, the THERMOSTREAM ATS-810-M from inTEST TEMPTRONIC Corp., achieves a wide temperature range from -80 to +225°C. It does this without LN2 or liquid carbon dioxide, using nontoxic, nonflammable refrigerants. It provides a display of temperature with resolution within ±0.1°C and with temperature accuracy within ±1°C of the temperature shown on the display. For applications requiring fast changes in temperature, this temperature-forcing test chamber features transition time of about 10 s to rise from -55°C to +125°C or to drop from +125°C to -55°C.

The thermal test chamber can be operated under local control or by means of a personal computer (PC) with MS Windows operating system (OS). The test chamber includes Ethernet, GPIB, and RS-232C ports for connecting test instruments and PCs. This is a good-sized chamber, weighing 805 lbs. (365 kg) and mounted on four wheels for mobility. It is designed for 60-Hz power systems and built for safety, allowing an operator to set high- and low-temperature limits (a high-temperature limit of +230°C is set at the factory).

Similarly, the ThermalAir TA-5000A temperature-forcing unit from MPI Thermal, also built for speed and efficiency, operating from 200 to 250 VAC, 50/60 Hz supplies. Available for rent from Axiom Test Equipment, this temperature-controlled test chamber can achieve subzero temperatures without LN2 or liquid carbon dioxide, and can make thermal transitions from -55°C to +125°C or from +125°C to -55°C in less than 10 s. It has a total temperature range of -80°C to +225°C with ±0.1°C display resolution and ±1°C calibrated system accuracy. At 584 lbs. (265 kg), this is not a small system, but it is well equipped with multiple USB ports, computer interfaces, and remote-control buses for computer hookups. In addition, the temperature-controlled chamber includes multiple (K-type, T-type, and RTD) temperature sensors and two front-panel touch-screen displays for local control.

For those seeking simultaneous control of temperature and humidity, a combination temperature and humidity chamber such as the Test Equity 1207C is required. Although this test chamber provides slightly less of the total temperature ranges covered by the two temperature-only test chambers, from -73°C to +175°C, it does feature a test volume of 7 ft.3 in which humidity conditions from 10% to 95% relative humidity (RH) can be accurately simulated even as temperature is being controlled with 0.1°C resolution and high accuracy. Users can program changes in temperature and humidity to create test profiles for different DUTs and EUTs. As with the temperature-only test chambers, the Test Equity 1207C has multiple instrument interfaces for remote control, including GPIB and RS-232C connections.

This is a small sample of the environmental test chambers for rent or purchase from Axiom Test Equipment, with many variations in size, temperature range, and test capabilities available depending upon the requirements of an application. In general, test chambers can be operated locally, from the front panel, or automatically, under computer control, to follow specific test profiles for a DUT or EUT. More information or datasheets on any of these temperature/humidity-controlled test chambers is available on our site under Temperature and Humidity. For help selecting a temperature chamber contact Axiom at either 760-806-6600 or sales@axiomtest.com

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