Thermocouples for Temperature Measurement: Economic and Robust Devices
Thermocouple stand out as cost-effective and robust instruments for measuring temperature. Their small size allows for a rapid response, making them particularly suitable for applications where quick reactions to temperature variations are essential. Type K thermocouples, with their expansive temperature range, find use in diverse fields such as cryogenics and jet engine exhaust analysis. The circuit described herein transforms thermocouple outputs into a direct voltage with a gradient of 10 mV °C-1. This conversion is precisely what’s required to facilitate the utilization of a digital multimeter (DMM) for temperature readouts. Alternatively, the interface output signal can be directed to a computer system for more advanced temperature recording purposes.
AD595A: Versatile Instrumentation Amplifier for Thermocouples
The AD595A, developed by Analog Devices, serves as a comprehensive instrumentation amplifier and thermocouple cold junction compensator integrated into a single chip. This monolithic solution incorporates an ice point reference and a precalibrated amplifier, generating a 10 mV ‘C-1 output directly from a thermocouple signal. Laser-trimmed specifically for type K (chromel-alumel) thermocouples, the AD595A offers a versatile solution for accurate temperature measurements.
Simplified Application of AD595A: Type K Thermocouple Connection
In this straightforward application, the AD595A is effortlessly employed. The Type K thermocouple connects to a designated socket, with resistor R1 strategically incorporated to prevent common-mode voltages induced in the thermocouple loop from being converted to the normal mode. Utilizing the alarm output +ALM (pin 12) of the AD595A, a low-current LED is driven in this setup. The +ALM output switches low when either or both thermocouple leads are interrupted. It’s important to note that the cold junction compensation, a feature of the AD595A, is impacted whenever the alarm circuit is activated. Consequently, any readings taken during alarm activation are considered invalid.
Compensating for Thermocouple Non-linearity: Transfer Function Consideration
Due to the non-linear relationship between the thermocouple output voltage and temperature (refer to the table), and considering the AD595A linearly amplifies the compensated signal, a specific transfer function must be applied. The formula for determining the actual output voltage is Uo = 247.3 (Uth + 0.011), where Uo represents the output of the AD595A and Uth is the thermocouple output in mV. Notably, both ANSI Type K and DIN NiCr-Ni thermocouples, composed of identical alloys, can be seamlessly utilized with the provided interface.
Constructing the Interface: Simple Design with Few Components
The interface construction is uncomplicated due to the involvement of very few parts. Notably, attention is drawn to the copper area beneath the converter chip, strategically implemented to enhance thermal contact between the thermocouple socket and the IC. Maintaining a low thermal resistance is crucial to ensuring the proper operation of the on-chip ice point reference.
Power Supply Flexibility: Symmetrical or Asymmetrical Options
The interface offers versatility in power supply options, accommodating both symmetrical and asymmetrical configurations. In the latter case, the ‘0’ and ‘ground’ terminals are interconnected and connected to the battery terminal. However, it’s essential to recognize that temperatures below 0 °C cannot be accurately measured when a single-ended supply is employed. Opting for a symmetrical supply, ideally powered by two 9-V batteries, opens up the full temperature range for measurement. The circuit’s current drain remains modest, not exceeding 1 mA when the thermocouple is connected and not surpassing 10 mA when the thermocouple is disconnected, activating the alarm LED.
- R1 = 10 kΩ
- R2 = 2.2 kΩ
- C1 : C3 = 4.7 μF, 25 V, radial
- C2 : C4 = 100 nF
- D1 = LS3369EH (low-current LED: red)
- D2: D3 = 1N400
- IC1 = AD595A (Analog Devices)
- K1 = special thermocouple type-K socket, e.g., RS Electronics 473-127.
- Plastic enclosure with battery compart-ment; approx. size 80x60x20mm