The two legs of a thermocouple are each made of different material and each produce a voltage when exposed to a temperature gradient. Since the tip of the thermocouple is connected, we can measure the voltage of the whole circuit; e.g. the sum of the two voltages from each leg, and the total voltage is called thermocouple voltage, or from here on ‘voltage’. There are many different types of thermocouples (made from different combinations of materials each with their perks and boons, but here we concentrate on K- and S types. So the thermocouple is formed of a tip, and two separated legs. The tip is usually located at the point of interest and the two legs in ambient temperature. A compensation cable is something that produces exactly same voltage as thermocouple of the same type, but has lesser range towards high temperatures. Some thermocouple materials are expensive so sometimes thermocouples themselves are extended at the cold end with use of compensation cables. Polarity of connections is of utmost importance. Two wires that are of identical material exposed to a temperature gradient do generate identical and opposite voltage that always result to zero (when connected to a same circuit). A cold junction is considered to be the point where the dissimilarity ends and similarity begins in a circuit i.e. the thermocouple or thermocouple compensation wire ends, and two leads of identical material begin. Sometimes the aforementioned go all the way to the measurement instrument, but the temperature inside such instrument may be unstable or difficult to measure. In this example the measurement instrument is connected to the thermocouple circuit with identical leads. The thermovoltage generated is a function of temperature gradient between the legs and the tip, and the total temperature. A gradient of 50°C at room temperature produce different voltage than same gradient at 1000°C. This makes interpreting the voltage bit more complicated, as all lookup tables and conversion functions are based on the assumption that the cold junction is in an ice bath. This is inconvenient, so many devices have a way to know or estimate the ambient temperature and take it into account when calculating the tip temperature. The thermocouple conversion functions in Omega can take in the cold junction temperature as static number; user estimation, or as measured and calculated parameter (denoted as another expression) When the cold junction temperature is known, a reverse lookup is done with the thermocouple function to see how much voltage would such temperature produce. This voltage is then added to the measured thermocouple voltage, and the new total is fed to the conversion function and a corresponding temperature is received. The functions in Omega user interface to calculate temperatures from thermocouples are TCS(V; T) and TCK(V; T) for S- type and K type accordingly. V is the thermocouple voltage and T is the cold junction compensation temperature in °Celsius. If the measurement does not need to be accurate, the user can just enter for example 25 as the T parameter assuming the cold junction is in temperature controlled room with temperature always around 25°C. If the measurement needs to be accurate, the T can be an expression that calculates the cold junction temperature from a resistance measured from a thermistor (chip that has resistance as function of temperature) located right next to the cold junction, or the user can enter 0 and place the junction in ice bath. The TT2(R) function converts measured resistance to temperature in °C. Let us assume the user has set up Node 1 to measure the thermocouple voltage, and optionally Node 2 to measure the resistance of thermistor located at the cold junction. To calculate the generic temperature of the S-type thermocouple tip when the cold junction is not controlled, the user would use expression TCS($N1.MV,25) in the graph section to plot it or elsewhere where such expressions are accepted in the program. The $N1.MV signifies the measured voltage from node 1, and the 25 signifies the users best estimate of the temperature of the cold junction. Needlessly to say, when someone opens the windows in the laboratory, the result of this expression changes even if the temperature at the thermocouple tip remains the same. So to guard against this and to get the actual accurate temperature one needs to place the thermistor (one provided with each instance of Omega) next to the cold junction, and set up a node to measure the resistance. In such case the T term would be replaced with TT2(R), and the R with $N2.M2 signifying the measured (2-wire) resistance of the measurement node 2. In total the expression would look like TCS($N1.MV, TT2($N2.M2))
The user needs a multimeter with multiple channels or two separate devices for the two measurements made. We assume the user has Keithley 2000 with 10 channel card fitted. The thermocouple is connected to channel 1 and the thermistor is connected to channel 8. The instrument has a physical button to select front or back terminals, since the card is in the back, back terminals option must be selected with the mechanical selector. Node is se set up to be voltage measurement with Keithley 2000. In the GPIB commands field before the user needs to enable the ‘channel close 1’ command, and in the GPIB commands after measurement field ‘open all channels’ command. Likewise for the Node 2 the user must select 2-wire resistance measurement on Keithley 2000, and close channel 8 and open it accordingly. The actual commands to open and close the internal channels are: :ROUT:CLOSE (@1) :ROUT:OPEN:ALL Where the 1 is the channel to be closed (i.e. connected).
Further details in the Omega manual.
