Temperature Sensor in BMS

Question

EnSmart Technologies · Accepted Answer

### Human Analogy

Your body has a normal temperature of 37°C. Your brain monitors this continuously. When it rises — you sweat to cool down. When it drops — you shiver to generate heat. The body thermometer is your skin and internal sensors constantly feeding information to the brain.

A BMS temperature sensor does exactly this for a building.

### NTC vs PTC — The Most Important Sensor Fundamental

**The Founder's Note:**
*"NTC vs PTC — very important. NTC: when temperature increases, resistance decreases. PTC: when temperature increases, resistance increases."*

**NTC — Negative Temperature Coefficient:**

Think of it this way — the hotter it gets, the easier electricity flows through it. Resistance goes DOWN as temperature goes UP. Negative relationship.

```
NTC sensor at 0°C   →   High resistance (say 10,000 ohms)
NTC sensor at 25°C  →   Lower resistance (say 10,000/some factor)
NTC sensor at 50°C  →   Even lower resistance
```

Used for: general temperature sensing in BMS — room thermostats, duct sensors, pipe sensors.

**PTC — Positive Temperature Coefficient:**

Opposite. The hotter it gets, the harder electricity flows. Resistance goes UP as temperature goes UP. Positive relationship.

```
PTC at 0°C   →   Low resistance
PTC at 25°C  →   Higher resistance
PTC at 50°C  →   Even higher resistance
```

Used for: motor winding protection, overtemperature detection.

### Platinum Sensors — PT100 and PT1000

**The Founder's Note:**
*"PT = Platinum. Number = resistance at 0°C. PT100 → 100 ohms at 0°C. PT1000 → 1000 ohms at 0°C."*

**Why Platinum?**

Platinum is a precious metal. In sensing, it is used because its resistance changes very predictably and linearly with temperature. No surprises. No drift. Consistent for decades.

```
PT100:
  PT = Platinum
  100 = 100 ohms resistance at exactly 0°C
  At 100°C: approximately 138.5 ohms
  At -10°C: approximately 96.1 ohms
  Very precise — used in AHU supply air, chilled water pipe, pharma

PT1000:
  PT = Platinum
  1000 = 1000 ohms resistance at exactly 0°C
  10x more sensitive than PT100
  Better for long cable runs — small resistance changes easier to detect
  Used in precision applications
```

**2-Wire vs 4-Wire PT100:**

2-wire: simpler wiring, small error due to cable resistance (acceptable for short runs).
4-wire: eliminates cable resistance error completely — used in precision pharma and lab applications.

**Temperature Sensor Application in Buildings:**
```
Room thermostat sensor         →  Measures space temperature for FCU/AHU control
AHU supply air sensor (SAT)    →  Controls chilled water valve via PID
AHU return air sensor (RAT)    →  Monitors return air conditions
Chilled water pipe sensor      →  Monitors chiller leaving water temperature
Condenser water sensor         →  Monitors cooling tower efficiency
Duct immersion sensor          →  Inside duct — measures air temperature
Outside air sensor             →  Measures ambient temperature for optimisation
```

**Common Mistakes:**
- Installing room sensor near a window — reads solar heat not room temperature
- Installing AHU duct sensor too close to supply fan — reads fan motor heat
- Using wrong sensor type for controller input type — NTC sensor on PT100 input = wrong reading
- Not accounting for self-heating error in high-accuracy applications

**Memory Hook:**
```
PT = Platinum (precious metal, precious accuracy)
100 = ohms at 0°C
Temperature rises → resistance changes predictably → controller knows temperature
```

Temperature Sensor in BMS

Human Analogy

NTC vs PTC — The Most Important Sensor Fundamental

Platinum Sensors — PT100 and PT1000

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