The Ni100 follows a quadratic relationship between resistance and temperature, described by:

R(T) = R₀ (1 + αT + βT²)

avec :

• R₀ = 100 Ω

• α = 6,18 × 10⁻³

• β = 1,4 × 10⁻⁵

• (valable entre −60 °C et +180 °C)

🔹 Example 1: calculation of resistance at 100 °C

R(100) = 100 × [1 + 6,18 × 10⁻³ × 100 + 1,4 × 10⁻⁵ × 100²]

R(100) = 100 × (1 + 0,618 + 0,14) = 100 × 1,758 = 175,8 Ω

✅ Result : at 100 °C, the resistance of Ni100 is approximately 175.8 Ω.

🔹 Example 2 : calculating the temperature from a measured resistance

We measure R=140Ω.

What is the corresponding temperature ?

We rearrange the formula :

βT² + αT + (1 − R/R₀) = 0

1,4 × 10⁻⁵ T² + 6,18 × 10⁻³ T + (1 − 140 / 100) = 0

1,4 × 10⁻⁵ T² + 6,18 × 10⁻³ T − 0,4 = 0

T = [−6,18 × 10⁻³ + √((6,18 × 10⁻³)² − 4 × 1,4 × 10⁻⁵ × (−0,4))] / (2 × 1,4 × 10⁻⁵)

T ≈ 61 °C

✅ Result : the corresponding temperature is approximately 61 °C.

🔹 Practical Remarks

• The Ni100 offers a greater variation in resistance per °C than platinum, which simplifies the R→T conversion.
• The equation can be implemented directly in a microcontroller without complex functions.
• Below −60 °C or above +180 °C, the relationship becomes nonlinear → prefer a Pt100.

The Pt200 generates a proportionally higher signal than the Pt100, which allows for reduced excitation current and limits self-heating.

It is perfectly suited for 3 or 4 wire differential configurations.

🔹 Typical assembly components

              +3.3 V / +5 V
                   │
       Current source (0.3 mA)
                   │
                [ Pt200 ]
       (2 power wires + 2 measurement wires)
           │                   │
           │                   │
  Diff. amplifier ───→ High-resolution ADC
                              │
                       [ Microcontroller ]
                  (Calculation T = f(R) + compensation)

🔹 Operating principle

1️⃣ The constant current flows through the Pt200.

→ At 0 °C: V = 200 Ω × 0.3 mA = 60 mV

→ At 100 °C: V = 277 Ω × 0.3 mA = 83 mV

2️⃣ The differential amplifier elevates the signal (gain ≈ 50 to 100).

→ Typical output: 3 to 8 V depending on the ADC configuration.

3️⃣ The microcontroller calculates the temperature using the Callendar–Van Dusen equation.

🔹 Best Practices

🧩 Recommended 4-wire wiring to eliminate line effects.

⚡ Reduce the excitation current to a maximum of 0.3 mA to avoid overheating.

💧 Use a sealed enclosure to protect the circuit board in humid environments.

🔄 Regularly calibrate at 0 °C and 100 °C.

🧲 Filter the amplifier input (RC 1 kΩ / 100 nF) to reject industrial noise.