Month: November 2021

Temperature and Humidity Control Considerations with Environmental Test Chambers

November 22, 2021

Precise temperature and humidity control within an environmental chamber is a complex issue. To understand the nuances of humidity control, one must understand the importance of precise temperature control and the meaning of “relative” in relative humidity readings.

A good example of the importance of temperature control in relative humidity calculations is found in ASHRAE 1993: At a constant moisture content [a true measure of humidity] of a 12°C dewpoint at 23°C and 50%RH, a short-term temperature fluctuation of +1°C results in a fluctuation of relative humidity readings between 47% and 53%, even though the true moisture content in the humidity test chamber hasn’t changed.

This is the result of relative humidity being calculated by the measured moisture content of the air relative to the moisture-holding capacity of the air at any given temperature, referred to as the dry-bulb temperature. 

The warmer the air, the more moisture it can hold within the same volume of space, therefore it takes significantly more water vapor, in general, to elevate the relative humidity of air as it increases in temperature. The opposite is true as the air cools. Due to this fact, 50% relative humidity at a dry bulb temperature of 85°C is an exponentially higher moisture level than  50%  at a  dry bulb temperature of 10°C. 

At lower temperatures, the relative humidity measurement reading is affected by a change in overall moisture content of just a few grains per pound of air, whereas it may take grams of moisture to change the relative humidity reading at significantly higher temperatures.

 Stability is the Key to Humidity Control

The key then to precise humidity control in an environmental test chamber is stability. Typical environmental chamber control tolerance is +1°C and +5% relative humidity after stabilization. Stabilization is defined within the industry as the point when the chamber interior surfaces and unit under test (UUT) change temperature less than 2°C per hour, and without variations in load either external or internal (i.e., door openings or varying heat loads generated by the UUT).

Challenges exist with environmental chamber control when instability created by constant or intermittent changes in temperature and/or humidity is required within environmental testing profiles. These are commonly referred to as cycling profiles, which consist of infinitely variable combinations of “ramp” and “soak” steps in a specific sequence.

 A ramp step can change temperature, humidity, or both simultaneously in either direction up or down at linear rate over a set time period. A soak step holds a constant temperature and humidity setpoint for a set time period. In complex cycling profiles, the chamber heating, cooling, humidification, and dehumidification systems work together, essentially chasing the elusive ever-moving setpoints.

It is during cyclical profiles that maintaining typical stability tolerances can become all but impossible because instability is intentionally introduced. Setpoint changes across a short-range and/or time period create less instability compared to wide-ranging changes over shorter periods of time, which further impact the chamber’s ability to precisely control the ever-changing variables. This often results in short-term oscillations of temperature and/or humidity beyond the limits of stabilized control tolerances.

 Other Considerations for Precise Humidity Control

Temperature and humidity control can be further upset when chamber mechanical systems change from one state to another. For instance, a ramp step might require a chamber that is operating at a high temperature/low humidity to simultaneously change to a low temperature/high humidity within a span of “x” minutes. 

Contrary to what one might think, “cooling” and “humidifying” systems may not be initially required to operate, but instead the chamber control system simply reduces heating and dehumidification outputs, allowing the chamber to do a controlled drift which follows the moving setpoints of temperature and humidity. 

However, at some point during that ramp step, reducing heating and/or dehumidification fails to provide the response necessary to chase the setpoint, and the controller energizes the chamber cooling and humidification systems by requiring small but steady increasing outputs of cooling and humidification. During these changeover periods, it is also common to see short-term oscillations in temperature or humidity control.

There can also be situations where a cyclical profile has ramping requirements that exceed the capability of the chamber being used, but these are typically related to chamber capacity and not control.

Chamber Capability is Dependent on the Desired Profile

In summary, short-term oscillations that extend beyond normal stabilized control tolerances are common and expected with cyclical profiles. Considerations must be given to the ranges, ramp rates, and complexity of cyclical profiles. 

Except in very rare cases, these short-term oscillations have no negative impact on the quality or legitimacy of the overall test sequence, providing that the final setpoint conditions for temperature and humidity are fully achieved and are not beyond the capability of the chamber being used.

Russells Technical Products is your subject matter expert for thermal cycling chambers and humidity chambers. Our team of application engineers will walk with you through the process of understanding your testing specifications and applying the right solution to meet your requirements. Contact us to learn more today.

What Is An Altitude Chamber?

November 12, 2021

Pressure or altitude testing is essential for aerospace, industrial, and consumer applications where products or components need to function or be transported in low-pressure environments or withstand rapid decompression in combination with climate control. If your products encounter these environments, or you need to conform to a military or aviation test standard including but not limited to MIL-STD-810 or RTCA DO-160, an altitude test chamber may be the solution you need.

What are the Primary Uses for Altitude Chambers?

Unlike hypobaric chambers used to study the effects of high terrestrial altitude and hypoxia on human physiology, industrial altitude chambers create far more extreme and combined environments including altitude, temperature, humidity, icing, and in some cases, vibration. By combining multiple environments, your products can undergo the qualification testing necessary to simulate real-world conditions. Here are some considerations as you consider investing in an altitude chamber.

Altitude (Vacuum/Pressure)

By utilizing vacuum pumps of various types and sizes, most altitude chambers are capable of standard vacuum conditions replicating 100,000 ft. with extended ranges available to 200,000 ft. To simulate sudden loss of cabin pressure, some altitude testing chambers can simulate rapid decompression by quickly and dramatically reducing pressure in 15 seconds or less. Alternately, using compressed air or inert gas, altitude chambers can be pressurized to simulate sub-sea or sub-site level altitude.

Temperature

Using either single-stage or cascade refrigeration, most altitude chambers are designed to provide combined altitude and temperature simulation up to 65,000 ft., allowing for a wide range of combined temperature and altitude profiles. Additionally, altitude chambers can be dual-purposed as temperature-only chambers with temperature ranges of -73°C (-100°F) to 180°C (356°F).

Humidity/Icing

The ability to simulate humidity at ground level in conjunction with temperature/altitude in a cyclical profile is essential in testing the durability and longevity of many critical flight components. In special cases like icing tests, combined environment testing must include control of temperature, humidity, and altitude simultaneously. Most combined environment altitude chambers can be dual-purposed as temperature and humidity chambers, providing ranges of 5% to 95% relative humidity (RH) within the bounds of an 85°C (185°F) maximum dry bulb and a 4°C (40°F) minimum dewpoint.

Vibration

Under unique circumstances, vibration compatibility with an electrodynamic or mechanical vibration system can integrate with altitude, temperature, and humidity chambers. Real-world applications require unique qualification testing for aerospace and avionics equipment.

Industry Requirements for Altitude Chambers

There are limitless altitude tests that can be tailored for consumer products and packaging, but for military and aerospace applications there are two primary sources of specifications and guidelines, which are often updated to reflect agreed-upon protocol changes and enhancements to ensure products meet the rigorous demands of the industry. Key industry requirements utilizing altitude testing include:

  • MIL-STD-810 – The United States DoD released the updated revision H, a successor to MIL-STD-810G, in January 2019. A MIL-STD is simply a list of testing guidelines and standards that ensure commonality in military applications. While MIL-STD-810H covers a wide range of testing methodologies, and several “Methods” within MIL-STD-810H specifically address altitude and/or combined environment testing.
  • RTCA DO-160 – This international standard, established in 1958 and most recently updated in 2011(DO-160G), is defined as the Environmental Conditions and Test Procedures for Airborne Equipment. The objectives of the standard are to demonstrate the performance characteristics of devices and equipment subjected to environmental conditions present in airborne equipment. Much like MIL-STD-810H, RTCA DO-160G covers a wide range of testing methodologies, including several “Sections” which specifically address altitude and/or combined environment testing.

Both standards make allowances for tailoring tests based on unique or specific products and/or flight or mission profiles, but in most cases test profiles are developed around four main procedural guidelines:

  • Storage – Tests materials and/or packaging transported or stored at high or low altitudes, with or without combined environment testing.
  • Operation – Evaluates performance of products or equipment at high or low altitudes, with or without combined environment testing.
  • Rapid Decompression – Evaluates the potential impact to personnel, products, or equipment resulting from rapid depressurization.
  • Explosive Decompression – Similar to Rapid Decompression, but simulating instantaneous decompression, such as when a pressurized aircraft cockpit becomes compromised.

Other Considerations for Altitude Chambers

With any purchase of environmental test equipment, there are several additional considerations for altitude chambers.

  • Chamber Dimensions – How large is your unit under test (UUT)? What quantity will you need to test at one time? Do you have move-in restrictions to your facility, or building infrastructure constraints for getting the chamber to its final place of performance?
  • Cooling and Heating Capacity – What is the temperature range requirement for your product? Does your product generate any heat under test? How quickly do you need to heat or cool the chamber? Will your testing capabilities expand in the future?
  • Altitude – What is the altitude range requirement for your product? How quickly do you need to change the altitude for your testing? Does your product discharge air or gas into the chamber during altitude tests?
  • Humidity – What is the humidity vs temperature testing range requirement for your product? Is humidity required in conjunction with altitude testing? Does your product discharge or remove moisture inside the chamber during tests?
  • Combined Environment – Is humidity, icing, and/or vibration required?   

The best altitude test chambers are designed and manufactured to meet your exacting specifications. The more information you can provide our Application Engineers, the better the outcome in meeting your requirements, both for now and the future. No two altitude chambers are exactly alike, which makes a collaborative effort between the customer and the manufacturer early in the development process critical.

Russells Technical Products understands altitude test chambers and has the solution to match your requirements. We provide one-on-one attention to our customers to make sure that from start to finish, you stay involved in the process and communicate with a single point of contact at the factory. That’s why we’ve been trusted to design and build some of the most complex altitude systems ever created. Contact us today to start a conversation and learn for yourself why we’ve been supplying superior altitude test chambers for over 50 years.