Sometimes a client will ask for the impossible and expect it at low cost. Or worse yet, the sales engineer in your company will win a project on a low bid because he/she thought it could be accomplished in a “creative” (but impossible) way.

What if you were tasked with a wirelessly communicating embedded project whose electronics, circuit, components, PCB, and all, were required to fit inside a furnace? Typically, a thermocouple or RTD would be placed inside a furnace and wiring would take the sensor signal into a box that holds the electronics. But what if you were asked to create a project where the sensing devices, a 7-segment LED display, and wireless communication were all placed inside the furnace?

Yeah, sometimes engineers get asked to do the atypical. In some cases, the challenge is met with creativity and innovation. And sometimes you must admit that the request can’t be done with today’s technology. With decades of experience, this would be my train of thought if asked to complete such a project.

Think at a higher level: what’s the real goal?

What is the objective here? Is to measure furnace temperature or build an embedded design that can withstand 125°C continuously? So many thoughts come to mind when I read this headline beyond just finding a technical solution for the chosen path. The first engineer I ever worked for gave me several pieces of advice I still practice today. Much of the advice can apply to the headline above.

Like me, you might have heard, “There must be a dozen ways to accomplish the same goal and more than a thousand ways to get it wrong.” Measuring and data logging temperatures well above the operating temperature of the electronic control devices is not a unique problem. Metal smelting furnaces and glass kilns operate between 1,000°C to 1,500°C. The material and walls of these ovens appear bright red at these temperatures! Why limit the suggested design to just 125°C? Why not make it 1,500°C? Obviously, because the design would melt in second; like Arnold Schwarzenegger in the finality of “Terminator 2”.

MIL-SPEC components are available at 125°C. These are considered high-reliability parts since they are designed, tested, and certified to operate continuously at 125°C. Component qualifications are submitted to life tests that subject sample lots to full power at a 125°C ambient temperature for 10,000 hours, perhaps cycling 1 ½ hours on and ½ hour off. The power cycling is supposed to further stress the expansion and contraction of the internal component materials. However, MIL-SPEC components are not actually used at 125°C continuously in practical designs. Rather, the fact that the components will operate at an acceptable failure rate at the extended temperatures, they are even more reliable at lower ambient temperatures.

Just because some electrical components are available at a 125°C rating doesn’t mean that placing them inside the oven they intended to measure is the only solution. Obviously, problems quickly arise trying to source all the components that can withstand the continuous temperature.

If someone asked me to do this project, I would take a step back and look at how others have accomplished this goal. What’s wrong with that? A more traditional approach would be to mount the thermocouples inside the furnace and wire them to the outside of the furnace. Depending on the design requirements, the thermocouple might be just insulted with high-temperature insulation or enclosed in a metal sheath. The thermocouple wiring would be long enough to reach the embedded control or monitoring electronics that are mounted in a more reasonable temperate environment.

The engineer must also consider that there is not always a direct commercial-to-military equivalent for all components. In fact, due to the time it takes to qualify high-reliability components for life testing, MIL-SPEC components often lag commercially available components by years. Many commercial components just don’t have an equivalent due to the demand and cost that it takes to qualify these components. In military, aerospace, and avionics applications, the reliability factor is more important than the latest technology.

What about the mechanics that are related to an embedded design?

Modifying the mechanics of the furnace for wiring access or mounting sheathed temperature probes might quickly get away from the skills and abilities of the electrical engineer who is designing the embedded system. People don’t want to be wrong, especially engineers, and tend to stick with the familiar. An electrical engineer will tend to provide an electrical solution. A mechanical engineer will tend to offer a mechanical solution. A software engineer will give you a software solution but blame the electrical engineer or mechanical engineer if their solution doesn’t work as expected (insert lopsided grin; no one is perfect).

However, the tongue-in-cheek statements above don’t mean that the electrical engineer shouldn’t seek the advice of someone qualified to change or modify the mechanics of the system. Another piece of advice from my first boss was, “You don’t have to know everything, you just need to know where to find the answer.” You got me…. these were in the days before the modern internet, so finding the answer meant more than just googling for answers. There is no substitute for talking to people and asking questions. (For which forums can be a good enough substitute, but not as good as whiteboarding with some experienced engineers.)

Although I’ve seen engineers in some companies hold tightly to their disciplines, the best designs are a balance of mechanics, electronics, and software. There are many advantages to seeking out a multidisciplined balance if you are ever presented with a tuck-it-inside-the-furnace problem. (This is not to say that I haven’t run across mechanical engineers who provide wiring access holes large enough for the wire bundle, but not the terminated connectors, too!) The approach needs to be a cooperative give-and-take of knowledge and ideas.

Who is asking for a narrow solution?

However, exchanging ideas with other engineers isn’t the only problem with the design objective. Is the client asking for the control and datalogger to be installed internal to the furnace without understanding the technical challenges and limitations? Maybe the client isn’t knowledgeable enough about what they are asking for, and the engineer isn’t confident enough to educate them and diplomatically suggest alternatives. There is a difference between doing what the client says and providing what the client needs. Again, if the realistic objective is to control and monitor furnace temperature, why is the engineer trying to mount the embedded controller inside the furnace?

If you look at the requested design, it’s asking for the entire embedded project, including the readout, to be placed inside the furnace. The question should be why you would try to source a seven segment LED display rated for 125°C? You will likely not be able to find one, because a human cannot survive reading an LED display while trapped in a 125°C oven. There is no such Commercial Off the Shelf (COTS) device; it would need to be custom made at a very high price. One application in need of such a rugged LED display might be on the planet Venus.

Better designs exist for the more fixable problems

If mounting several thermocouples externally on a furnace makes the wiring runs too long to multiplex (MUX) them at a single controller, then don’t take that approach. One possible solution could be that separate controllers are needed to read each thermocouple and then upload the data to a central controller. (Definitely cheaper than a custom 125°C tolerant display.) How you solve the problem all depends on the size and layout of the furnace that the engineer is trying to measure.

You don’t have to take the initial approach that’s requested and plow through with a suggested solution when so many others seem possible, which leads me to my final piece of advice from my old boss, “Why is there never enough time to do things right, but always enough to do things over?”Yes, the engineer who tries to put the whole project, electronics and all inside the furnace can finish a functional design, but it will never be reliable.

Taking a step back to seek more traditional solutions would not be a waste of time. Sometimes you have to take the right road and reject the unreasonable request….unless you have massive funds to commission custom devices or decades to do the R&D necessary to create a currently unavailable device.

This FAQ was inspired by a conversation on EDAboard.com. Check it out here.