Selecting Thermal Sensors: How big? How Fast?

How to choose an appropriate temperature sensor for thermal test:

Selection criteria of an appropriate temperature sensor for a given application can be detailed but it often does not need to be that complicated.  Two main points we are going to address today are sensor size and speed of response.  Prior discussions covered thermocouple v. RTD and in the future, we will likely cover more of the environmental issues of temperature sensors such as temperature range and resistance to moisture or other abuse.

Small low mass sensor to screw-mount to the device surface

The sensor is typically used to sense Device Under Test. This sensor might be considered Medium-Massive and sensitive on both sides, results will show some smoothing of instantaneous temperature readings and although attached to the device there will be some weighted averaging with air temperature.

Thermocouples are by nature generally smaller, often cheaper, and lower mass but can be ultimately be packaged very much similarly to RTD sensors. Smaller, lower mass sensors will pretty much respond faster. When using a temperature sensor to control, the faster the response, generally the better. Likewise, when just logging a temperature fast response helps.

A slower response can solve or hide some other problems but often is it better to look more at the real problem. An example might be a sensor on a device being temperature conditioned on an uncovered thermal platform. The sensor may be subject to fluctuations in air temperature instead of the intended device.  In this case, a cover is recommended as well as a sensor that is designed to be sensitive on one surface to reduce the possible negative effects of a sensor with a fast response.

Smaller, faster sensors although more fragile are often desired when a device under test is small relative to the sensor and when the temperature of a very specific location is desired instead of a larger package. You don’t want the sensor to be so big that it occurs as a heat sink to the device being monitored.

There are times when some temperature smoothing effects (smoothing over a surface area/volume and overtime) sensed by a more massive sensor is helpful but on the other hand, often modern monitoring/controlling systems can add response smoothing over time through software if averaging is required.  There is generally limited ability to compensate for a slow response.  Smoothing over a volume or surface is likely best monitored with larger or multiple sensors. Often once a device can be thermally modeled/characterized with multiple sensors, a single sensor can provide good test data for ongoing tests.

Inside a temperature chamber or on a thermal platform, the temperature reading of a device under test is often desired. Many systems now have advanced temperature control algorithms that rely on both the chamber (or platform) temperature plus the temperature of the device.  If possible a small pocket or hole on the device to locate the temperature sensor, make good physical contact and isolate it from the air can be helpful as well as a sensor designed to be sensitive on just one surface.  In many cases, a small rugged sensor with insulating adhesive tape on one side is a good choice for making good physical contact to take device temperatures.  Some sensors such as the top two pictures have a small mounting hole used to secure the sensor to the device under test.

Adhesive mount RTD surface sensor

Many modern temperature instruments offer the ability to read several different types of sensors including RTDs and thermocouples however I have found that often the instruments are more optimized for one range or one type of sensor than another.

We previously indicated our preference for RTDs over thermocouples, we stand by that opinion but there are also some very high-quality thermocouples that remain accurate and repeatable over a range of abusive conditions such as extreme temperatures or rapid ramping rates and vibration.  We additionally generally find that the RTD sensors made of wound platinum wire are more durable than those which are made with platinum wire printed on a substrate.

100 ohm DIN RTD element made of platinum plating on the substrate.

Temperature sensors designed to measure air temperature can be elaborate with fins or special materials to transfer heat from the air but often work best in a simple thin-walled aluminum sheath to protect the sensor from contamination and abuse. A small amount of conductive grease inside helps improve heat transfer from the sheath to the sensor element inside. Likewise, a small amount of thermal grease between the sensor housing and the device under test is helpful.

Some POINTS TO REMEMBER specifying a temperature sensor.

• Small size and speed optimized for the size of the device under test and controller
• Review best compatibility with instrumentation and device
• Ability to attach to the device to be sensed
• Low mass v. ruggedness of sensor for repeated usage

A final note:

There are a wide variety of temperature sensors available, generally smaller is better for faster response, large enough to make contact with the surface you really want to measure but not so big that there are resulting lags in the readings. Let TotalTemp’s many years of experience help you select the right sensor for measuring the temperature of your device to be tested.

Using Purge Gas When Thermal Testing

Applying Purge Gas Provides Two Main Benefits When Thermal Testing

INTRODUCTION:

A choice to be considered when selecting the best thermal test system is the use of purge gas. Purging with gas serves two purposes during environmental thermal testing. Most often it is used to keep condensation and frost from accumulating on the surfaces of the DUT (Device Under Test) at cold temperatures. Additionally, it is also used to prevent the oxidation of metals surfaces at high temperatures.

Heavy frost forming at -40C ultimately becomes wet.

High-temperature corrosion 

Generally, the condensation of any moisture is to be avoided during the environmental testing of electronics.

Thermal Platform with Probing Cover & Nitrogen(g) Inlet

In simple terms, condensation occurs when the air temperature drops below the dew point.  Above the dew point, more moisture evaporates from surfaces than condenses, below the dew point more water condenses on surfaces than evaporates. The dew point is a temperature determined by the combined effects of the surface temperature and the relative moisture content of the air (assuming the same atmospheric pressure).

Generally, condensation is to be avoided during the environmental testing of electronics. In simple terms, condensation occurs when the air temperature drops below the dew point.  Above the dew point, more moisture evaporates from surfaces than condenses, below the dew point more water condenses on surfaces than evaporates. Thus, the farther below the dew point, the faster moisture will condense.  The Dew point is a temperature point that is determined by the combined effects of the surface temperature and the relative moisture content of the air (assuming the same atmospheric pressure).

The easiest way to avoid condensation on a DUT is to lower the moisture content of the air. The easiest way to do that is with dry Nitrogen purge gas into the space around the DUT. The Nitrogen displaces moisture-laden air so that dew point condensation of high-temperature oxidation cannot occur. Nitrogen from a Dewar or portable tank works very well because the moisture content of the Nitrogen is extremely low and it is often readily available at the test station. Nitrogen is also notably inert, that is to say, it does not easily react with other elements or materials.   If you are going to extreme temperatures and condensation can cause problems with your devices, you should take measures such as purging gas to reduce condensation.  Critical or sensitive metal surfaces exposed to hot temperatures above 170C will be preserved better with purge gas as well.

Sometimes the question comes up if CO2 (Carbon Dioxide) can be used as a purging gas. The quick answer is no. Although CO2 from a tank is dry and displaces moist air, CO2 can react with water to produce carbonic acid which of course is corrosive.  As long as there is no moisture, to begin with, CO2 would work but it is generally not recommended.  Other gasses such as Argon can also be used but are generally more expensive. If your facility provides clean dry compressed air, often this can be successfully used to purge, especially if your air is really dry (some facilities add a desiccator system to air supply) and temperatures are not so extreme (~above -20° C as a rule of thumb).

Your requirements may vary but as a starting point in a small benchtop enclosure or chamber 15 – 20 SCFM for an initial flow rate to displace moist air followed by a reduced rate of 3-5 SCFM two to three minutes after openings have been closed. Too much flow on an ongoing basis can create air temperature gradients inside the box.

Consult your application engineer about options to automate these processes. Application of purge gas to a thermal platform or chamber is generally very easy. A pressure regulator can attach right to your Nitrogen tank. The flowmeter is normally located at the tank as well and is used to set the flow rate. Typically a small 1/4″ OD uninsulated hose can connect the output of the regulator/flowmeter to the hot-cold plate cover or chamber with a simple push to connect fittings.

Temperature Testing Practices

Many items require thermal environmental testing. Items going through product development, requiring validation for high-reliability applications, and products exposed to extreme environments among others require specific thermal testing.

Diligent adherence to requirements results in performing a meaningful test and ultimately higher quality products as opposed to simply taking the easiest route to put a testing checkmark in the box.

A few important aspects of the testing can help ensure that best practices in thermal testing are followed to get real results from testing efforts.

1. 1. Testing can be time-consuming.  Properly applied automation will increase repeatability and can produce consistent results. Test results can be automatically stored on the test equipment, in the cloud, or automatically printed to a network printer.
      
   2. Automation also reduces some unexpected uncertainties in the testing such as reducing the temptation to open the door/lid of the chamber during the test.
      
      In addition to disturbing the temperature uniformity of the test, if cool air is introduced when the system is at extreme hot or cold temperatures, moist air can result in unwanted condensation or corrosion on the devices being tested.
   3. Best heat transfer will get the job done most effectively.  When possible conductive heat transfer will achieve the setpoint in less time with less energy.  When heat transfer by conduction is not possible or practical a convection chamber or a hybrid combining conduction and convection is a good idea.

   HBC144 Hybrid Benchtop Chamber

   

   4. When using a temperature chamber, for best heat transfer, be sure that airflow across the device is as great as possible to transfer heat effectively as possible.

   Using shelves improves DUT gradients

   

   C460-N Triple Stack

   
   

   5. Use of a secondary sensor in a Temperature Chamber or Thermal Platform can be applied to either verify that critical zones in the product to be tested actually achieve the specified or with a controller that employs smarter temperature control algorithms which can consider both the system temperature and individual device under test (DUT) temperature.
   6. Prior to purchase, always give plenty of consideration to the most preferred cooling method given the testing requirements and facilities available.

   TotalTemp Technologies can guide you through all the trade-offs, including cooling methods. We not only design, manufacture and sell some of the best equipment out there but also offer free dedicated consulting to get you to the best thermal testing equipment that meet your unique requirements.

   Call us (888) 712-2228 or contact us if you have any questions.