Frequently Asked Questions

The following questions and answers are of a general nature and should provide you with basic information on some of the aspects and disciplines encountered in the field of environmental testing.

What is environmental testing?
Environmental testing is a ‘catch-all’ expression, which covers a number of disciplines. Generally we can take any number of environmental conditions that can be found on or around our planet such as temperature, humidity, pressure and vibration and apply them to a product to see what happens to it. Sometimes we are testing a product to see if it can survive under certain conditions. At other times we may wish to apply conditions in a controlled way as part of a process in order to change the structure or appearance of a material. Back to Top

Why should I carry out environmental testing?
For any number of reasons. For example; you may wish to ensure that your product will perform as specified under the conditions that it will be used; you may wish to build robustness into the design well before the manufacturing process begins; you may wish to determine how long a product or device will last in the field; or you may wish to have a good idea of its condition after a number of years in operation. An environmental test programme can achieve all this and more. Back to Top

What conditions can be applied to a product?
Pretty much any condition or combination of conditions that can be found on or around the planet; temperature, humidity, pressure, vibration, pollution, rain, ultra-violet light, sand and dust, noise, corrosive atmospheres and radiation are some of them. Back to Top

What is combined testing?
Applying more than one condition simultaneously to a product. Usually this means vibration and temperature cycling. Back to Top

What sort of products can be tested?
Anything from cardboard packaging and food containers to mobile phones and aircraft components can be tested. Most industrial sectors carry out environmental testing on their products. Electronics, Telecommunications, Aerospace, Automotive, Food and Drink, Oil and Gas exploration, Pharmaceuticals, Packaging and Defence are a few of the industries that have established environmental test programmes. Back to Top

What is accelerated ageing?
This is a process whereby a product is stimulated by cycles of temperature and humidity in order to determine its future condition. An effective accelerated ageing process can show in just a few hours or days what a product will be like after several years. Do not confuse accelerated ageing with accelerated life testing which will take a product to failure in order to determine how long it may last in the field. Back to Top

How do I go about environmentally testing my products?
The item to be tested has to be exposed to the test condition or environment in a controlled way. In the case of mechanical stimulation such as mechanical shock or vibration the product is fixed to a specialist piece of equipment such as a shaker, vibrator, slip-table or rate-table. For most other types of environmental testing the usual method is to place the product inside an insulated enclosure, such as a climatic chamber or environmental chamber. The test space can then be heated, cooled, humidified, dehumidified, evacuated and pressurised as required, in a controlled and repeatable way. Measurements can be taken and data logged for further analysis. Back to Top

Will I have to buy a range of expensive equipment?
Not necessarily and in some cases not at all. You will have to assess the type of testing you need to carry out particularly with regard to how much and how often. If your requirement is only occasional then one of a number of excellent Test Houses will more than likely offer a cost-effective solution. Another alternative may be to hire the equipment for the time you need it. As with any other business decision you will need to look at cost-effectiveness in both the short term and the long term. Some companies make the strategic decision to sub-contract their entire test programme while others prefer to keep the work in-house. There are valid arguments for either. In fact, you could always do the routine work in-house using standard equipment and then sub-contract occasional work where perhaps a specialist piece of kit is required. Back to Top

What effect does temperature have on my products?
Depending on the material, high temperature will accelerate ageing. Heating and cooling of certain devices can lead to fatigue and eventual failure; rapid temperature cycling will accelerate failure in materials or components containing latent weaknesses. Back to Top

What range of temperatures can I expect to achieve inside a chamber?
This will depend on how the chamber is constructed; particularly the materials used and the type and thickness of the insulation. The method of heating and cooling is also a consideration.
Most test chambers will be heated by electrical elements and the upper range limited to about 180°C. Cooling can be by means of mechanical refrigeration or the expansion of a cryogenic liquid gas injected directly into the chamber. A single-stage refrigeration system would normally be able to achieve -40°C and a cascade system -70°C. Chambers using liquid nitrogen are able to achieve temperatures below -100°C. Back to Top

What is stress screening?
Stress screening is a process whereby latent faults in a product are converted to hard failures so that conventional testing or inspection can identify them. Failure mechanisms seen by a product in real life, such as vibration, mechanical shock and thermal cycling can all be reproduced in the laboratory. Stress screening can be divided into two disciplines, vibration stress screening and thermal stress screening. Depending on the material or device to be screened either one may be more effective and in some cases both vibration and thermal stress screens should be applied. Back to Top

What is thermal stress screening?
Very rapid cycles of temperature are applied to products causing them to expand and contract. This movement causes mechanical stresses, which will concentrate at any weak point to bring about failure. The process is often applied at a design stage in order to build robustness into a product. It is also used at various stages in a manufacturing process to ensure that reliable product is shipped to the customer and so reducing the potential for field failures.
An effective thermal stress screening process will call for approximately 6 or more very rapid cycles of high and low temperature. The upper and lower temperature extremes and the optimum number of cycles can be established by experimentation. They will vary from one product type to another and will depend on such factors as the product mass and material type. Back to Top

How does thermal stress screening work?
Subjecting a product to rapid cycles of heating and cooling will cause it to expand and contract. If a device is made up of a number of different materials then their different thermal expansion coefficients will cause them to expand and contract at different rates and by different amounts. This leads to high levels of mechanical stress concentrating at any point of weakness. Robustly designed and well-manufactured products will be able to withstand these stress reversals, however weak product will fail be identified by conventional testing or inspection. This process is particularly effective at finding latent faults caused by poor design, component failure or manufacturing problems in electronic assemblies or devices. Back to Top

What sort of chamber would I need in order to carry out thermal stress screening?
An effective thermal stress screen requires a specialist piece of equipment capable of very fast temperature transitions of up to 100°C per minute. This is quite beyond the scope of standard off-the-shelf equipment. Liquid nitrogen is used to provide the rapid cooling capability and open element heaters provide the fast response when heating. Powerful fans would be employed to provide high airflow velocity around the products with careful consideration given to even distribution of the conditioned air throughout the working volume of the chamber. This is important in order to achieve the same level of screening to all the products inside the chamber. There is also an optimal chamber volume of around 700 – 800 litres in order to keep the internal chamber mass to a minimum. It is unlikely that the desired temperature ramp rates will be achieved in a chamber of more than 1000 litres. Back to Top

Do I have to use liquid nitrogen for thermal stress screening?
Yes. There is no viable alternative, as even very powerful mechanical refrigeration plant will provide air-cooling rates of not much more than 20°C per minute, slowing down considerably at low temperatures. Back to Top

Do liquid nitrogen cooled chambers cost more than conventional ones?
Generally a liquid nitrogen (LN2) cooled chamber will cost considerably less than a mechanical cooled one of equivalent size. Maintenance costs are also lower and their footprint is smaller too so they take up less space and they are relatively quiet in operation. However LN2 is more expensive than electricity, it requires specialist installation and as a refrigerated liquefied gas you will need to be aware of some potential hazards. Back to Top

What are the hazards associated with using liquid nitrogen?
The major hazard is a risk of asphyxiation due to oxygen depletion in the event of a leak. Oxygen depletion alarms should be installed in the vicinity of the installation. Contact with the liquid will cause cold burns and the possibility of frostbite. A proper risk assessment should be carried out. Back to Top

What is humidity?
Put simply, it is a measure of how much water vapour is being held in the atmosphere. There is a relationship between temperature, atmospheric pressure and moisture. The warmer the air, the more water vapour it can hold. In the environmental test industry when we talk about humidity we are usually referring to relative humidity. This is the amount of humidity of the air expressed as a percentage of the total amount of water vapour that the air could hold at a given temperature. 100%RH (or saturation) means that the air cannot hold any more moisture at that temperature. Should the air temperature increase its capacity to hold more water vapour will increase and the relative humidity will fall. Back to Top

What is dew point?
Dew point is the temperature at which moisture will condense out of the air. People who wear glasses can see the effect of this; their lenses steam up when entering a warm room after being outside on a cold day. The temperature of the glass is below the dew point of the air in the room and moisture condenses on the cold surface. Back to Top

What effect does humidity have on my products?
Some materials will absorb moisture in humid conditions. High humidity accelerates corrosion and can be the cause swelling and dendritic growth. It can lead to delamination in composite components and degradation of fibrous material. Levels of high humidity will also support fungal growth. Low humidity or very dry conditions can cause cracking and shrinkage. Back to Top

What range of humidity conditions can be generated inside a chamber?
A general-purpose humidity chamber will usually be able to generate humidity conditions from 10%RH to 98%RH at temperatures ranging from +20°C to +90°C (sometimes called the climatic range). However there are usually some restrictions at the limits of the ranges, high humidity at high temperatures and low humidity at low temperatures. More specialist equipment would be needed should testing be required at these extremes. Back to Top

How would changes in pressure affect my products?
Mechanical performance can be altered as well as the thermal characteristics of some materials and components that are exposed to changes in atmospheric pressure. Back to Top

What could be the effect of sand and dust on my product?
In some materials they can be the cause of mechanical abrasion. They can degrade the efficiency of heat sinks and heat exchangers. In electronic circuitry sand or dust can cause tracking particularly in damp or humid conditions. Back to Top

What is vibration stress screening?
The mechanical failure mechanisms a product is exposed to in its natural environment can be reproduced on an electrodynamic shaker system. High levels of mechanical stress will concentrate at any point of weakness. Robustly designed and well-manufactured products will be able to withstand these stresses, however weak product will fail and can then be identified by conventional testing or inspection. Back to Top

Can the performance of my test chamber be improved?
Possibly. Upgrading an older chamber can often be a cost-effective solution when new requirements and different test regimes are called for. However, the capacity to improve the performance of a chamber by upgrading refrigeration, heating, humidity and control systems may be limited by physical constraints such as the volume of the chamber, size of evaporator, type and thickness of insulating material etc. Back to Top

How often should I have my chamber calibrated?
The majority of quality regimes require at least an annual calibration. Back to Top

How often should maintenance be carried out?
This depends largely on the usage of the chamber. In order to keep equipment running smoothly an annual inspection should be carried out. However if the chamber is being used continuously, then more frequent inspections are advisable. Preventative maintenance contracts can usually be tailored to suit chamber availability and calibration can also be scheduled at the same time. Incidentally, if a chamber has not been used for some considerable time it might also require checking before putting it back into operation. Back to Top

The answers provided here might well raise further questions or comment. If this is the case please feel free to e-mail us with your enquiries and we will try and get back to you with the information you need.

Environmental Test Equipment Specialists