What is a thermocouple?
Let's split the word: 'thermo and couple'. This means that it has to do with temperature and a couple (two).
In the 19th century Mr Seebeck discovered that there was an electrical
current flowing through a couple of metal wires (of different material)
when they where joined at the two ends and if the two ends where at different temperature. This
meant that if you could measure the current and the temperature of one end,
you could calculate the temperature at the other end.
In earlier times the temperature
of the first end (the reference junction) was kept at melting ice temperature (called the cold junction).
Nowadays we measure the temperature of the reference junction electronically, do some
calculations and display the result.
Two things on thermocouples must be said:
first of all depends the relation between electrical current and temperature difference
on the different metals used in the two wires and secondly it is absolutely non linear.
From the first remark follows that there are different types of thermocouples (for different applications and ranges).
So make sure that you have a thermocouple and an instrument for the same thermocouple type.
Out of the second remark we learn that the mentionned calculations can compensate only partially with the linearity errors,
which restricts the accuracy.
What is a Pt100?
As for the thermocouple, we split the word: Pt and 100. Pt is the chemical symbol for Platinum,
100 is the resistance in Ohm of the Pt100 at 0 °C. The resistance changes (fearly linear)
with temperature are: 0.385 Ohm/°C for European and 0.392 Ohm/°C for American elements.
So, by measuring the resistance we can calculate the temperature.
Never use a European Pt100 with an American instrument unless you know that it is set for the
right conversion factor (usually called 'alpha'). In english a Pt100 is also called a RTD element,
meaning Resistance Temperature Detector.
What is the difference between 2, 3 and 4 wire Pt100 sensors?
An RTD or Pt100 sensor is connected with two, three or four wires to the measuring device.
From What is a Pt100? we learned that we are in fact measuring resistance
to determine the temperature. Now when measuring the resistance of the sensing element, we also measure
the resistance of the leads and cables used. This gives an error! To compensate for this, the three wire
type (bridge) is used, giving enough accuracy in most industrial applications. Even better accuracy is possible
with a four wire Pt100 (laboratory applications). Our Pt100 panel mounted indicators have an offset compensation
when using two wire sensors.
What thermometer should I use: thermocouple or Pt100 (RTD) type?
This depends on your application and it is not possibke to give a straightforward answer.
Most important things to be considered are: temperature to be measured, accuracy and response time.
If the temperature you're interested in is above 650°C, use a thermocouple. You can find Pt100 sensors
going up to 850°C, but only at a lesser accuracy.
If you want high accuracy, for example better then 0.5°C between 0 and 100°C, go for the Pt100.
If you want a fast measurement, e.g. when using portable instruments, use a thermocouple sensor.
What is the relation between the different temperature units?
°C = degrees Celcius, °F = degrees Fahrenheit, K = Kelvin
°C = 5/9(°F - 32) = K - 273.15
°F = 9/5*°C + 32 = 9/5*K - 459.67
K = °C + 273.15 = 5/9*°F + 255.37
Example: To convert 280 K to °F, use the formula °F = 9/5*K-459.67
This gives 9/5*280-459.67 or: 280 K = 44.33 °F
What is absolute, relative, gauge, differential, line, over- and underpressure?
It has all to do with the reference we are taking to measure the pressure against.
Easiest way to understand is to consider a modern pressure sensor. Normally a diaphragm is used
that bends under the pressure applied. By measuring this deflection, we can calculate the pressure that is applied.
The pressure we want to measure is put on one side of the diaphragm, the front side. On the back side of the diaphragm
is another pressure: the reference pressure.
If the reference pressure is the atmospheric pressure, we call it a relative or gauge pressure sensor.
A good example where a relative pressure sensor is necessary is a level measurement. When you
install a pressure sensor in the bottom of a liquid tank (with the top of the tank open to air),
the sensor would see the following pressur on the front side of the diaphragm:
Atmospheric pressure (on the liquid) + Hydrostatic pressure (pressure given by the liquid = spec.gravity * height * gravity).
If it is a relative pressure sensor, the reference side pressure would be:
Atmospheric pressure (on the referenc side of the sensor)
Because the both atmospheric pressures will only be marginally different, the diaphragm will only deflect
under the influence of the hydrostatic component and if gravity (g) and specific gravity remains constant,
the sensor will output a signal proportional to the height of the liquid.
Over- and under pressure (vacuum) are (in theory) no correct definitions, but they are often used
terms to describe a pressure which is over or under atmospheric pressure. Because
atmospheric pressure is around one bar, your under pressure or vacuum can't be larger then
one bar.
If the reference pressure is absolute vacuüm (no pressure), it is called an absolute sensor. A good example
being a barometer.
If the reference pressure is another (measured) pressure, the sensor is called a differential pressure sensor.
A typical application is in controlling a fan. To see if the ventilator is working, you could bring
the pressure before the fan to the front side of the sensor and the pressur behind the fan to the
reference side of the diaphragm. The output signal will depend on how hard the ventilator is blowing.