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As members of the Slovenian Metrology Association and leaders in the field of temperature measurement, we would like to impress upon you the importance of choosing the right temperature sensor for your process.

Sensors for energy efficiency

By controlling sensors, we can effectively influence, improve and control energy efficiency and, above all, make huge savings. Here we describe the issues and give a concrete example of why sensors are the key to process energy efficiency. The expert content was part of the event “Metrology to lower costs” and the lecture of the Vice President of the SIMER Section, Ms. Aleksandra Lepenik, on “Sensors for energy efficiency”.

What is a sensor?

A sensor is a sense of smell. Similarly to the human body, the senses detect a change (hot, cold…). The signal travels to the brain, the brain triggers the muscle and a reaction happens. If a sense of smell is impaired, it works less well or not at all.

A sensor is a technological sense. He feels a change in himself. And development engineers make sure that the sensors present the detected changes to users in a readable and visible language – that is, on a visible display, as data called “data”. Today, everyone is talking about BIG DATA, i.e. big data. This data is produced by sensors. The sensor is always first and nothing starts without it, nothing starts without sensing this change. These three things are useful to remember:

  1. The sensor always senses a change on itself.
  2. Produces a piece of data.
  3. The sensor is always first! It is the SOURCE, the INPUT, the INPUT, the MEASUREMENT.

If we don’t pay attention to the source, everything else is in vain!

What is the problem and why do we link sensors to energy efficiency?

A young person can see, hear, type and everything else perfectly. Like the new sensor, which does its job perfectly – it’s accurate. Then life happens, the sensor takes action. If we live within our means, calmly and steadily, without sudden ups and downs, we are likely to have a long life. The same applies to the sensor. But if we are more adrenaline-driven, constantly jumping out of bounds and testing the limits of our abilities, our lifespan is expected to be shorter. And not only will it be shorter, but for the duration of the operation, we will at times, or even permanently, be worse off for our surroundings. The environment will not have what it expects from us. The same applies to the sensor.

Where is the problem?

We will give you a concrete example of a problem our user is having.

The large industrial furnace had an operating temperature of 1050 °C, measured by K-type thermocouples. Occasional problems were encountered, due to the acidic environment eating away at the sensor’s protective tube, but no other problems were encountered in principle. Then, over two years of operation, the temperature was gradually raised to 1150 °C. This allowed them to speed up the process quickly, achieve better targets, produce more product and be more efficient. They have learned that if they raise the temperatures, they can immediately achieve a greater effect without any other more demanding intervention and extra work. At the beginning of last year, the technologists wanted to raise the temperature again, as they needed to quickly increase the capacity due to the material. Management agreed. The maintenance staff were cautious and asked us for advice.

We immediately explained to them that the operating temperature in this version should not exceed 1100 °C and that the thermocouple type should be changed to platinum and the protective housing to ceramic. Of course, this is where the problem comes in, ceramics are more delicate, especially when changing. There is also a price difference, as platinum was five times more expensive in this case. All the cabling would need to be replaced and the control settings would need to be adjusted. But at the moment he called us, the user did something else that probably everyone would have done. He has consulted the internet for the maximum temperature of a K-type thermocouple. He found information on the SIST EN IEC 60584 standard, which talks about tolerance values for thermocouples, below:

Figure: Table of tolerances for type K thermocouples from SIST EN IEC 60584

Thermocouple tolerance values

In doing so, the user forgot to look at another table called the recommended temperature. This is about the dependence of the thermocouple thickness up to a certain temperature. The user had a fi=3 mm thick conduit. This thickness means that it can measure up to 1100 °C, which was its specified operating temperature. In the short term, it can measure up to 1200 °C, but this short term has certain negative effects. It happened that a user decided to follow the information on the web because he thought that the standard must be correct, which it is. But it did not take into account the additional clarifications, the related standards and the advice of the manufacturer, who have years of practical experience from similar cases. He also did not know that the standard contains extreme limits with short lifetimes, which are intended for researchers and testing laboratories. Thus, he wrongly assumed that it was perfectly safe up to 1230 °C. He thought he could work in the area with this type of thermocouple, but he did not know the complementary effects and did not expect any problems.

The consequences of the wrong solution

He had this type of thermocouple in use:

Figure: Illustration of a K-type thermocouple and its operating ranges

A flat thermocouple with a metal protective outer tube, attached to the stove with a flange to allow quick change of the element. The thermocouple is labelled with the operating temperature (Tdel) and the maximum temperature (Tmax). Type K operating temperatures are up to 1200 °C in the most powerful versions. From this temperature onwards, we work exclusively with platinum thermocouples, type S or R, or B.

The user made his decision and set his system to 1230 °C. As the process is continuous and involves a chemical reaction, they cannot stop the process or lower the temperature for the purpose of changing the temperature sensors, but must change the sensor during operation. So you can imagine that a sensor designed for an operating temperature of up to 1100 °C and a short-term temperature of 1200 °C cannot operate permanently at 1230 °C. Of course, it has experienced frequent failures of the articles themselves. These post cancellations are linked to frequent changes. Changing a temperature sensor on a stove, where access is more difficult, temperature is high, taking it out, putting it in, is not easy. The biggest problem here is that because it is very hot at the opening, the slow or gradual insertion of the temperature sensor into the stove, which is prescribed at 100 mm/2 min, is not taken into account. The user therefore took out the old thermocouple and immediately installed the new one fully in the opening. At this point in this implementation, surely no one is thinking about micro-cracks in the material.

Why do thermocouples start to deviate?

It is important to remember that thermocouples are made of natural materials. In the case of type K, one core (one pole) is a nickel-chromium alloy, the other pole is pure nickel. When they are used, these natural materials slowly break down. And that is the key to the problem that is happening.

We use the more familiar term – that thermocouples creep or “drift”. Drift happens because the material is essentially falling apart and no longer has the same values as those tabulated in the standard. So when they are used, they break down, oxidise and bind with other substances. If the elements are temperature sensitive, they start to decompose more rapidly at certain temperatures. These temperatures, where they start to decompose more rapidly, are above the recommended operating temperature. This is why the recommended temperature (from-to) is written in the standard, so that users do not have to investigate how long a temperature sensor can really last.

Sensors for energy efficiency

As a result, when solving such a problem, it is important to realise that materials decay. As materials decay, and in particular decay more rapidly outside the recommended ranges, we measure a lower thermovoltage than we would before decay. The control system detects this as a lower temperature than the set temperature, so the automation imposes a task on itself, what we call an irreversible black energy cycle. It is important to remember here that sensors are subject to material decay because they are based on natural materials. When materials decay, something is added to them, which means they will no longer give the same values as they did in their new state. Automations are designed to go to a set value, so they constantly adjust that value. This triggers an irreversible black cycle that speaks of:

the temperature sensor is worn, no longer accurate, has severe creep, shows less;

the temperature rise further increases the degradation of the thermocouple’s natural materials, its casing and the connections within it;

automation continuously corrects undervaluation and raises it;

the need for additional energy to reach temperature increases significantly;

Severe wear/ageing of all connected systems (appliance, machine, furnace, insulation, etc.) due to excessive temperature;

the temperature is much too high for the product process – scart;

everything is ageing abnormally fast;

maintenance workers intervene more often, increasing the risk of damage to themselves, equipment and infrastructure – from excessive temperatures, rainy access roads, dangerous areas.

How much does neglecting the problem actually cost us?

The point of all this is that the sensor pays for itself. We should always ask ourselves how much it is costing us:

  1. Inadequate product quality, rejection, scrap …
  2. Complaints and client badwill.
  3. Energy costs due to +10, +50, +100 °C – lower costs with metrology.
  4. Congestion and troubleshooting.
  5. Accelerated ageing of the furnace.
  6. Frequent interventions by maintenance staff, working hours, emergency interventions, on-call, bad mood.
  7. Risk of damage when changing at high temperature.
  8. Reputation and loss of new orders.

And all because of non-control over the sensor and the process. DON’T BELIEVE WHAT THE ALARM OR MEASUREMENT EQUIPMENT SAYS – check, plan!

The sensor is ALIVE, it senses, it wears out and it ages. When it breaks down and stops working – by then it is long overdue.

Figure: Diagram of the SRIP Circular Economy

Today, metrology to lower costs

Today, we put this on your conscience. As those responsible for measurements and sensors, don’t look at them from a price point of view. The more inadequate the sensor, and it doesn’t even have to cost more, the bigger your bill will be later! That’s why SENSOR PAYS ITSELF. But it is essential to choose the right sensor, whatever the price, check it properly, calibrate it and replace it on time. This cost is so much lower that it is negligible compared to the consequences mentioned above. Some manufacturers have life cycles for temperature sensors, refurbishing, repairing, recycling their relevant parts.

It doesn’t have to be a temperature sensor or other product to be binned. It can have a secondary life, see more at link .

☏ Call us: +386 2 62 96 720

They are always available for opinions and explanations:

Aleksandra Lepenik, expert in temperature measurements

Measurement expert Zoran Lepenik

Aleksandra Lepenik, lecture “With sensors to energy efficiency”;
SIMER workshop: Metrology to lower costs, 19. 1. 2023

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