Temperature measurement is one of the most fundamental measurements in almost any electronic, mechanical, or electromechanical system. However, when dealing with a military system, combat vehicle, propulsion system, ruggedized electronic assembly, or equipment operating in harsh environmental conditions, selecting the right Temperature Sensor becomes considerably more complex.

It is not enough to select a sensing element capable of measuring the required temperature.

Other factors must also be considered, including the temperature range, accuracy and measurement characteristics, response time, mechanical design of the probe, thread type, housing materials, wiring, electrical connector, and the way the complete sensor assembly integrates into the system.

In some cases, a standard off-the-shelf temperature sensor simply does not meet the requirements. This is where a Custom Temperature Sensor, designed around the specific requirements of the system, becomes necessary.

What Is a Temperature Sensor for Military Applications?

The term Military Temperature Sensor does not necessarily describe a sensing technology that is fundamentally different from those used in industrial applications.

Military systems can use well-established temperature sensing technologies such as:

• NTC Thermistor
• RTD – Resistance Temperature Detector
• PT100 / PT1000
• Thermocouple
• Other temperature sensing technologies depending on the application

The main difference often lies in everything surrounding the sensing element.

The sensor must become an integral mechanical and electrical part of the system in which it is installed. Therefore, it is important to consider not only what measures the temperature, but also how the sensor is installed, how it connects to the system, and what environmental conditions the complete sensor assembly must withstand.

For example, a temperature sensor installed in a propulsion system or mechanical assembly may require a specific mounting thread, defined insertion depth, metal housing, rugged connector, and a wide operating temperature range.

It is also important to distinguish between a sensor designed for a military application and a sensor that has actually been qualified or tested to a specific military standard. The use of a military-grade connector or rugged construction alone does not constitute qualification to a MIL-STD requirement.

The Real Challenge: The Sensor Is an Assembly, Not Just a Sensing Element

When an engineer starts looking for a temperature sensor, it is easy to begin with specifications such as:

10kΩ NTC, 30kΩ NTC, PT100, or PT1000.

But the sensing element is only one part of the product.

A complete temperature sensor may include the sensing element, probe, metal housing, insulation materials, potting compound, internal wiring, mounting thread, and electrical connector.

Each of these components can affect how well the sensor fits the application.

A good example is a sensor based on a 30kΩ NTC Thermistor. In a development carried out by Variohm Eurosensor, a TDK 30k Thermistor, part number B57861S0303F040, was selected with a Beta value of B(25/100) = 3964K ±1% and an NTC operating temperature range of -55°C to +155°C.

However, these electrical characteristics are only the starting point. The same sensing element was integrated into a custom brass housing with an M14×1.5 thread, Polytec TC437 potting compound, and a rugged Souriau connector as part of the complete sensor assembly.

This brings us to one of the most important parameters when working with an NTC Thermistor: the Resistance/Temperature (R/T) Curve.

How Does an NTC Thermistor Work?

NTC – Negative Temperature Coefficient is a temperature-dependent resistor whose resistance decreases as temperature increases.

This behavior can be clearly seen in the characteristics of the 30kΩ sensor: at 0°C, the resistance is approximately 95.5kΩ; at 25°C, it is 30kΩ; at 50°C, it decreases to approximately 10.95kΩ; and at 100°C, to approximately 2.07kΩ.

The controller or measurement system does not receive a direct “temperature” reading from the NTC. Instead, it measures the resistance – or a voltage that depends on that resistance – and converts the measured value into temperature according to the sensor characteristics.

One of the key parameters used to characterize an NTC is the Beta Value. This value describes the relationship between resistance and temperature and is used to calculate the Resistance/Temperature (R/T) Curve.

For example, the 30kΩ sensor in this case has a Beta value of:

B(25/100) = 3964K ±1%

This means that the Beta value is specified between the reference temperatures of 25°C and 100°C.

Therefore, when developing a replacement sensor or a Custom Temperature Sensor for an existing system, it is not enough to specify, “We need a 30kΩ NTC.” The Resistance/Temperature Curve must also match the requirements of the measurement system.

NTC, RTD, or Thermocouple – Which Technology Is Right for the Application?

There is no single temperature sensing technology that is suitable for every system.

An NTC Thermistor provides a relatively large change in resistance as a function of temperature, can be integrated into compact designs, and is suitable for a wide range of measurement and control applications. However, its response is nonlinear, so the measurement system must use the appropriate R/T curve.

An RTD, particularly a PT100 or PT1000, is based on the change in resistance of a metallic sensing element as a function of temperature. RTDs are commonly used when stability, repeatability, and accurate measurement over a defined temperature range are required.

A Thermocouple, on the other hand, generates a voltage as a result of the temperature difference between junctions of dissimilar metals. It is particularly suitable for applications requiring very high temperature ranges or operating conditions where other sensing technologies may be less suitable.

The selection process should therefore not begin with the question “Which sensor is better?”, but rather with “What does the system need to measure, and under what conditions?”

There is another important consideration in an existing system: the electronics may already have been designed for a specific NTC curve. In such a case, changing from an NTC to a PT100 – or even to another NTC with different Resistance/Beta characteristics – may require changes to the electronics or software.

In Military Temperature Sensors, Packaging Is Just as Important as the Sensing Element

This is where the difference between an NTC component and a complete Temperature Sensor Assembly becomes particularly important.

It is possible to purchase a small Thermistor with excellent performance, but a reliable method is still required to install it at the actual temperature measurement point.

In a military system or equipment operating under harsh environmental conditions, several questions need to be addressed: How is the sensor mounted to the assembly? What insertion depth is required? What probe diameter is possible? What thread is already used in the system? How are the electrical connections protected? And which connector must interface with the system wiring harness?

The mechanical and electrical design of the sensor must address all of these interfaces together.

A good example can be seen in two sensors developed in different configurations. One uses a 1/8-27 NPT thread with a 10kΩ NTC, while the other uses an M14×1.5 thread with a 30kΩ NTC.

In other words, even when the sensing principle is similar, the mechanical interface, sensing element, and physical construction can be customized to meet different system requirements.

Why Is the Thread Type Important in a Temperature Sensor?

The thread is not simply a means of holding the sensor in place.

It defines how the sensor integrates into the existing assembly and can affect the geometry of the probe and its position relative to the area being measured.

For example, NPT – National Pipe Thread is a family of threads commonly used, among other applications, in process connections, while other systems may use metric threads such as M14×1.5.

In a Custom Temperature Sensor design, not only the thread but also the probe diameter, probe length, insertion depth, and housing design can be customized to meet the mechanical constraints of the application.

The Connector Is Part of the Temperature Sensor Design

Once the sensing element and mechanical design have been selected, another question remains – one that sometimes receives less attention than it deserves: How does the temperature sensor connect to the system?

Many industrial temperature sensors use a fixed cable or flying leads extending from the probe and connecting to a separate connector further along the harness. This approach is suitable for many applications, but a rugged system may require a more compact assembly in which the connector is an integral part of the sensor.

This approach can offer several advantages: the system wiring harness can connect directly to the sensor, external wiring and additional connection points can be reduced, installation and field replacement can be simplified, and the complete sensor can be designed as a single mechanical and electrical interface.

In the development described here, a Souriau 8D1-09SZ98PA connector was integrated directly into the sensor. According to the design specification, the connector has an operating temperature range of -65°C to +175°C. The two NTC leads are connected to Pins A and B, while Pin C is populated but electrically not connected.

This is an important consideration when designing a Custom Temperature Sensor: the pinout, connector type, and interface with the system wiring harness are all part of the sensor specification – not decisions that should be made only after selecting the sensing element.

MIL-DTL-38999 Connector in a Temperature Sensor – What Is the Advantage?

The MIL-DTL-38999 connector family is widely associated with aerospace, military, and rugged applications where a compact and reliable circular connector interface is required.

Integrating this type of interface directly into the sensor makes it possible to design a Temperature Sensor Assembly that fits naturally into wiring harnesses and system architectures already based on rugged circular connectors.

However, an important distinction must be made: using a connector from a military-specification family does not automatically mean that the complete sensor assembly is qualified to MIL-DTL-38999 or any other MIL-STD requirement. The specification and qualification of the connector must be distinguished from the qualification of the complete sensor assembly.

This distinction is particularly important in military applications. Requirements such as shock, vibration, sealing, temperature cycling, salt spray, or EMC must be evaluated according to the specific project requirements and the testing performed on the complete assembly.

The Temperature Range Is Determined by the Complete Sensor Assembly

One potential mistake when selecting a temperature sensor is to consider only the operating temperature range of the sensing element.

Even if the NTC itself is suitable for a particular temperature, this does not necessarily mean that every other component in the sensor assembly is suitable for the same range.

The 30kΩ development illustrates this clearly: the NTC element is specified for -55°C to +155°C, the connector for -65°C to +175°C, the Polytec TC437 potting compound for -55°C to +180°C, and the heat shrink for -75°C to +150°C.

Therefore, when defining the operating temperature range of a complete Temperature Sensor, the limitations of the entire assembly must be considered rather than simply adopting the temperature rating of the sensing element.

The same principle applies beyond temperature. The housing material, wiring, insulation materials, potting compound, and connector should all be selected as parts of one integrated system.

When an Off-the-Shelf Sensor Is Not Enough – Custom Temperature Sensor

This is perhaps the most important point for an engineer looking for a temperature sensor for a specialized system:

The system does not always need to be adapted to a sensor that already exists in a catalog. Sometimes, the sensor should be designed around the system.

A requirement may begin with a simple statement:

“I need a 30kΩ NTC.”

But once the system requirements are fully defined, that requirement may become:

30kΩ NTC + defined Beta value + required R/T curve + M14×1.5 thread + specific probe geometry + metal housing + potting + suitable wiring + rugged connector + defined pinout.

At that point, you are no longer looking for a Thermistor.

You are looking for a Custom Temperature Sensor Assembly.

In a development carried out jointly by Amironic and Variohm Eurosensor, these requirements were translated into a dedicated sensor assembly based on a TDK 30kΩ Thermistor, part number B57861S0303F040, with an M14×1.5 threaded housing and an integrated Souriau connector.

In parallel, another configuration was developed using a 10kΩ NTC with B(25/85) = 3977K ±0.5%, a 1/8-27 NPT thread, and a different probe geometry, while maintaining the same concept of an integrated Souriau connector.

These two examples illustrate an important principle: a Custom Sensor does not necessarily require reinventing the sensing technology. The innovation can lie in precisely integrating all the required components into a single product designed around the system.

What Parameters Should Be Defined When Developing a Custom Temperature Sensor?

When an off-the-shelf sensor does not meet the application requirements, it is important to start by defining the requirements of the system – rather than by selecting a part number.

The more precisely these requirements are defined, the easier it is to design a Custom Temperature Sensor that fits the system electrically, mechanically, and environmentally.

1. Temperature Range

The first question is what temperature range needs to be measured and under what environmental conditions the sensor itself must survive and operate.

These are not necessarily the same.

For example, the system may need to measure temperatures up to 120°C, while other parts of the sensor assembly may be exposed to different conditions. The sensing element, wiring, insulation materials, potting compound, housing, and connector should therefore be evaluated individually.

2. Sensing Element and Electrical Characteristics

If an NTC Thermistor is selected, the required parameters should include the nominal resistance at the reference temperature and the required Resistance/Temperature characteristics.

This is particularly important in an existing system. If the controller expects a 10kΩ NTC with a specific characteristic curve, replacing it with another 10kΩ NTC does not necessarily guarantee the same temperature readings across the entire operating range.

Parameters such as Resistance, Beta Value, Tolerance, and the R/T Curve should therefore form part of the sensor specification.

3. Thread and Mechanical Interface

The thread type, probe diameter, probe length, insertion depth, and geometric constraints of the installation area should all be defined.

In some applications, standard threads such as NPT or metric threads can be used. In others, a custom housing may be required to match an interface that already exists in the system.

4. Response Time and Measurement Location

Even a highly accurate sensor may not provide the required information if the sensing element is positioned incorrectly or responds too slowly to changes in temperature.

The probe diameter, thermal mass, housing material, insertion depth, and contact with the measured medium can all affect the Thermal Response.

It is therefore important to define whether the sensor is intended to measure the temperature of a liquid, oil, air, a metal surface, or another part of the system.

5. Connector and Pinout

In a new system, it may sometimes be possible to select the connector based on the sensor.

In an existing system, the situation is often reversed: the sensor must match the existing wiring harness and interface.

In such cases, the connector family, arrangement, contacts, and pinout can be defined as part of the sensor development according to the system requirements.

6. Environmental Conditions

Military and rugged applications may involve additional requirements such as temperature extremes, vibration, mechanical shock, humidity, fluids, dust, corrosion, or sealing.

It is important not to use the term Military Grade as though it were a standard in itself.

If a project requires compliance with a specific MIL-STD, it is necessary to define which standard, which test methods, and which test conditions apply – and then verify that the product has actually been tested accordingly.

Where Are Rugged Temperature Sensors Used?

This type of Temperature Sensor can be relevant wherever temperature must be measured within a mechanical or electromechanical assembly and the measurement must be reliably connected to the system controller.

In military systems, temperature measurement may be required in applications such as propulsion systems, engines, transmissions, cooling systems, oils and fluids, power electronics, power supplies, batteries, and mechanical assemblies.

However, the technology is not limited to the defense market.

The same principles of Rugged Temperature Sensor Design can also apply to special-purpose vehicles, industrial equipment, energy systems, mobile equipment, and other systems operating in demanding environments.

A Military Temperature Sensor Is Ultimately a System Integration Challenge

This is perhaps the most useful way to look at the subject.

When an engineer says:

“I need a temperature sensor.”

The question is not simply which Thermistor to select.

It is necessary to understand what needs to be measured, where it needs to be measured, how the sensor will be installed, how it will connect to the system, and what environmental conditions it must withstand.

Only once these requirements are clearly defined can the sensing element, mechanical design, and electrical interface be integrated into a single product.

For a Custom Temperature Sensor, the starting point may therefore be an existing sensor that does not fully meet the requirements, a mechanical drawing of the system, a specific electrical characteristic – or even a physical sample of an existing sensor that needs to be replaced or improved.

Case Study: Developing a Rugged Temperature Sensor with a MIL-DTL-38999 Interface

The Challenge – A Temperature Sensor for a Military System with a Defined Interface

In military and aerospace systems, selecting a temperature sensor does not end with choosing the sensing element. The sensor must integrate into the existing system both electrically and mechanically, connect to the appropriate wiring harness, and operate under the environmental conditions defined for the project.

In this case, the requirement was to develop a Custom Temperature Sensor combining an NTC sensing element with defined electrical characteristics, a compact mechanical design with a metric thread, and a rugged connector interface compatible with the system architecture.

The solution developed jointly by Amironic and Variohm Eurosensor is:

ETP-CS-SP-9×26-D38999-30K3964

A custom temperature sensor based on a 30kΩ NTC Thermistor, a metal housing with an M14×1.5 thread, and an integrated Souriau 8D connector.

Requirement 1 – Electrical Compatibility with the System

The starting point was the sensing element.

The selected solution uses a TDK B57861S0303F040 30kΩ NTC Thermistor, with:

B(25/100) = 3964K ±1%

and an NTC operating temperature range of -55°C to +155°C.

The specified R/T characteristics provide a resistance of 95.502kΩ at 0°C, 30kΩ at 25°C, 10.949kΩ at 50°C, 4.5237kΩ at 75°C, and 2.0725kΩ at 100°C.

This means the development goes beyond a general requirement for a “30k NTC.” It defines a specific Resistance/Temperature characteristic that can be matched to the system’s measurement electronics.

Requirement 2 – Mechanical Compatibility

The sensing element was integrated into a dedicated probe with a brass housing and:

M14×1.5-6g thread

The design defines a 14 mm diameter in the probe area, a 15 mm probe insertion length, and additional mechanical dimensions intended to enable direct integration into the assembly.

This allowed the sensor to be designed around the existing mechanical interface rather than requiring the system to be modified around an off-the-shelf sensor.

Requirement 3 – An Interface Suitable for Defense and Aerospace Systems

One of the key design decisions was to integrate a Souriau 8D1-09SZ98PA connector directly into the sensor body.

The SOURIAU 8D family is based on the MIL-DTL-38999 Series III connector specification, with the manufacturer also identifying compliance with major standards including MIL-DTL-38999 Series III and EN3645. This provides an interface from a connector family widely used in U.S. and European military and aerospace applications.

MIL-DTL-38999 Series III incorporates features suited to demanding environments, including screw coupling with a self-locking mechanism, contact protection, EMI shielding, and construction designed for vibration and rough handling.

For this particular sensor, the connector itself is specified for an operating temperature range of -65°C to +175°C.

Integrating the connector directly into the sensor allows the system wiring harness to connect directly to the sensor, eliminating the need for flying leads and an additional intermediate connector.

Requirement 4 – Designing the Complete Assembly for a Wide Temperature Range

The development considered more than just the operating temperature of the Thermistor.

The NTC element is specified for -55°C to +155°C, the connector for -65°C to +175°C, the Polytec TC437 potting compound for -55°C to +180°C, and the heat shrink for -75°C to +150°C.

This illustrates an essential principle in Rugged Temperature Sensor design: environmental performance is not determined solely by the sensing element, but by all components that make up the complete assembly.

The Result – A Temperature Sensor Designed Around the System

The ETP-CS-SP-9×26-D38999-30K3964 demonstrates the difference between purchasing a Thermistor and developing a complete Temperature Sensor Assembly.

Rather than selecting an off-the-shelf sensor and adapting the system around it, the development combined:

30kΩ NTC + defined B(25/100) characteristic + custom probe + M14×1.5 thread + metal housing + potting + defined pinout + SOURIAU 8D / D38999 interface.

The result is a solution designed around the electrical, mechanical, and interface requirements of the system, while using a connector family widely adopted in U.S. and European Defense and Aerospace applications.

Important Engineering Note

The use of a connector from the MIL-DTL-38999 Series III / EN3645 family does not, by itself, constitute qualification of the complete sensor assembly to these specifications or to any other military standard. Environmental qualification requirements – such as vibration, shock, sealing, or temperature cycling – must be defined and verified at the complete assembly level according to the specific project requirements.

Summary

Selecting a Temperature Sensor for a military system or rugged application involves much more than choosing an NTC, RTD, or Thermocouple. A temperature sensor is a complete assembly, which means the sensing element, Resistance/Temperature curve, temperature range, response time, probe design, thread, housing materials, wiring, potting, connector, and pinout must all be considered together.

In an existing system, many of the mechanical and electrical requirements may already be fixed. In such cases, a Custom Temperature Sensor makes it possible to design the sensor around the system – rather than modifying the system to accommodate an off-the-shelf sensor.

The development of the ETP-CS-SP-9×26-D38999-30K3964 demonstrates this approach: a 30kΩ NTC with defined electrical characteristics, a custom probe and metal housing, an M14×1.5 thread, and an integrated Souriau 8D connector were combined into a single sensor assembly.

The same principle can be applied to different configurations depending on the application – changing the sensing element, R/T curve, thread, probe dimensions, housing materials, connector, or pinout while maintaining compatibility with the system interfaces and requirements.

In Defense, Aerospace, Military Vehicle, and other harsh-environment applications, this level of integration can be just as important as the accuracy of the sensing element itself.

The right question, therefore, is not simply:

“Which temperature sensor do I need?”

but rather:

“Which temperature sensor fits my system – electrically, mechanically, and environmentally?”

And when an off-the-shelf sensor cannot provide that fit, a Custom Temperature Sensor can be developed around the specific requirements of the application.