The essential
An unamplified thermocouple signal is in the millivolt or microvolt range. In a measured example, about 1 mV of type K signal sat in about 30 mV of noise. An amplifier at the measurement point turns it into a high-level voltage, adds the cold junction compensation and, on some models, a linear output. Through a slip ring, it also keeps the temperature gradient across the contacts out of the reading. Five questions then choose the model: mounting, channels, thermocouple type, range and accuracy.
Why amplify at all?
Because the raw signal can be smaller than the noise it picks up on the way to the recorder. A thermocouple is a two-wire sensor whose voltage is proportional to the temperature difference between the measurement junction and the cold junction, and that voltage is in the millivolt range.
- In a measured example at 25 °C, an unamplified type K signal of about 1 mV sat in about 30 mV of noise: a signal-to-noise ratio of 1/30.
- Amplified, the same measurement gave about 125 mV for about 25 mV of noise: a ratio of 5, a 150-fold improvement.
- Cold junction compensation: every Michigan Scientific thermocouple amplifier provides it, and gives an absolute temperature.
- Linearity: the thermocouple output is non-linear with temperature; amplifiers with a linear output remove the polynomial conversion from post-processing.
Why does it matter with a slip ring?
Because a temperature gradient across the slip ring otherwise enters the measurement. The rings and brushes are not made of thermocouple alloys, so a temperature difference between the rotor and stator connections adds an error at those dissimilar-metal contacts. An amplifier on the rotating side holds the cold junction compensation and sends a high-level voltage through the slip ring, which the gradient cannot affect.
How will it be mounted?
On the part itself: most amplifiers are small enough to be held by a cable tie, epoxy or a single screw, including on a rotating part. The MicroTC weighs 3.9 g.
- AMP-TC units mount on the rotating side of an end-of-shaft slip ring.
- Short S-Series slip rings can have the thermocouple amplifier built into the rotor.
How many channels, and linear or not?
Amplifiers with a linear output are single channel. For several channels in one unit, the output is non-linear and converted with the polynomial equations supplied.
- Linear, one channel: MicroTC, AMP-TC-LIN.
- Non-linear: AMP-TC-M1 with 1 to 3 channels, AMP-TC with up to 9.
- Several single-channel units can share one power source and their signal ground.
Which thermocouple type and range?
The thermocouple type narrows the choice first; then the range that must be read without polynomial correction.
- MicroTC: type K only. Linear 5 mV/°C from −25 to 400 °C; −200 to 970 °C with post-processing polynomials.
- AMP-TC-LIN: types K and J. Linear 10 mV/°C, from −80 up to 1 000 °C.
- AMP-TC and AMP-TC-M1: types K, J, T and E. From −260 to 1 000 °C, approximately 10 mV/°C, linearized by polynomial.
What accuracy can you expect?
± 2 °C or ± 3 °C in normal ambient conditions, depending on the model. The worst case for any model is ± 5 °C, when the case temperature of the amplifier is too high or too low; each datasheet gives the limit.
To remember
- Amplify at the sensor, above all through a slip ring.
- One linear channel: MicroTC (type K) or AMP-TC-LIN (K, J).
- Several channels or types T and E: AMP-TC-M1 or AMP-TC, with the polynomial.
- ± 2 to ± 3 °C in normal conditions, ± 5 °C worst case.