A K-type thermocouple is used with the Precision Wideband Controller to detect the temperature of the exhaust gas. But the reason is not to monitor the temperature for optimum mixture tuning. It turns out that to determine the gas composition of the exhaust requires knowledge of the exhaust gas temperature.

Do not confuse the exhaust temperature with the sensor heater temperature – even though close regulation of sensor head temperature is occurring, the actual temperature of the exhaust gas can be different. To measure the water-gas equilibrium, a thermocouple is mounted nearby the wideband UEGO sensor head. The controller supports the use of a K-type thermocouple, which are widely available. Finally, the Precision Wideband Controller will operate without an external thermocouple, but there will not be a correction of the water-gas equilibrium. For many applications this may be an acceptable situation. But, for applications requiring high repeatability, the fitting of a thermocouple should be considered, as is the monitoring of exhaust back pressure.

Using an analytical method for determining Lambda/AFR from generating a molecular balance of known intake hydrocarbon(s) (see the Bowling & Grippo paper on analytical lambda/AFR calculations), the numerical value of a ratio known as the “Water-Gas Equilibrium” is desired. The formulation of the water-gas equilibrium is the following:

where:

P_CO is the partial pressure of CO,
P_H2O is the partial pressure of H₂O,
P_CO2 is the partial pressure of CO₂,
P_H2 is the partial pressure of H₂,
K_p(t) is the water-gas equilibrium constant, which is a function of exhaust gas temperature (t) in Kelvins.

So, the value of K_p varies with exhaust gas temperature (for example, at 1700 °C the value of K_p is roughly 3.3) – this determines the ratio of partial pressures of CO, H₂O, CO₂, and H₂. Since the wideband lambda sensor is sensitive to both the partial pressure of CO and the partial pressure of H₂ (with different sensitivities), it is required to know the ratios of the partial pressure of these two gases in order to compute lambda ().

Note that the water-gas equilibrium is needed only for an oxygen-depleted mixture situation (i.e. a “rich” condition).

It is easy to generate the quantity (moles) of both CO and H₂ based on a given hydrocarbon composition, lambda (), and water-gas equilibrium ratio. For example, for a lambda value of = 0.5 and hydrocarbon ratio of 1.85 for H/C and 0 for O/C (unleaded pump gas) a table can be generated of CO and H₂ mole percentages vs. lambda for various water-gas equilibrium values (values can be verified by the use of Brettschneider):

Similar tables can be generated for other lambda values ( < 1.0). Also note that the table above lists the moles of each gas – the lambda sensor operates on partial pressure, so the quantities above need to be divided by the total moles of all gas constituents.

Do not confuse the exhaust temperature with the sensor heater temperature – even though close regulation of sensor head temperature is occurring, the actual temperature of the exhaust gas can be different. So, to measure the water-gas equilibrium, a thermocouple is mounted nearby the wideband UEGO sensor head. The controller supports the use of a K-type thermocouple, which are widely available.

Finally, the Precision Wideband Controller will operate without an external thermocouple, but there will not be a correction of the water-gas equilibrium. For many applications this may be an acceptable situation. But, for applications requiring high repeatability, the fitting of a thermocouple should be considered, as should the monitoring of exhaust back pressure.

The Type K thermocouple is the 'general purpose' thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. Thermocouples are available in the 95°C to 1260°C (200°F - 2300°F)range. For example, a K type thermocouple (which is made of a nickel-chromium/nickel-aluminum junction, called Chromel/Alumel) puts out a signal of 12.2 millivolts at 300°C. Sensitivity is approx 41uV/°C.

In particular, Chromel-Alumel makes a thermocouple that will withstand up to 2500°F (just a couple of hundred degrees below the melting point of iron). If you melt a “type K” couple in your exhaust system, you are sure to have more problems to worry about than a melted thermocouple.

A Type K-couple has a voltage coefficient of about 22 µV/°F (22 microvolts per degree Fahrenheit) so a 1500°F exhaust gas temperature should gives around 33 millivolts. Feed that 33 millivolts into a meter calibrated to show 1500°F at this voltage and voila, you have the exhaust gas temperature.

Note: There is an easy way to tell the difference between a J-thermocouple and a K-thermocouple. The J-thermocouple leads are usually white and red, while the K-thermocouple leads are usually yellow (+)(non-magnetic wire) and red (-)(magnetic wire). The voltage produced by the sensor has a definite polarity, and polarity must therefore be correct when connecting the thermocouple (and any extension wires) to the Precision Wideband Controller.

Note that the color coding may be different outside North America, see the Thermocouple Technical Reference for more details.

Thermocouples are relative measurement devices, so a known reference is needed in order to obtain absolute temperatures. A “cold-junction” is normally used – it can be a literal thermocouple at a temperature of 0°C, or more practically a synthesized reference (i.e. current) which simulates the cold junction.

Rather than measuring the temperature of the reference junction and computing its equivalent voltage as we did with software compensation, we could insert a battery to cancel the offset voltage of the reference junction. The combination of this hardware compensation voltage and the reference junction voltage is equal to that of a 0°C junction.

The compensation voltage, e, is a function of the temperature sensing resistor, RT. The voltage V is now referenced to 0°C, and may be read directly and converted to temperature by using the tables.

Another name for this circuit is the electronic ice point reference. These circuits are commercially available for use within circuits and with a wide variety of thermocouples. The Precision Wideband Controller uses the AD595 Thermocouple Amplifier.

To make either a K type thermocouple, buy thermocouple wire, NOT thermocouple extension wire. Buy "K type thermocouple wire." A lot more people will know what you are talking about than if you ask for Chromel or Alumel wire. Both types are available with the extension wire being somewhat cheaper. Also buy some ceramic insulators, such as the 1/4 round 6" long two hole type. Strip the wires and insert through the insulators. Twist the two wires together about 1/2" in length after they pass through the insulator. Using an oxy/act torch and a little borax flux, melt the ends of the two wires together in as small a blob as is possible. What you want is an "intimate contact" connection, not a jumbo weld. I also used thermocouple connectors between my probe and the wire. The key factor in thermocouples is an "intimate contact" and keeping the materials the same throughout the sensor's entire length. Do not use copper wire.

Using Thermocouples

Many measurement errors are caused by unintended thermocouple junctions. Any junction of two different metals will cause a junction. If you increase the length of the leads from your thermocouple, you must use the correct type of thermocouple extension wire (eg. K-type for K-type thermocouples).

Using ANY other type of wire will create a thermocouple junction. Connectors used must also be made of the correct thermocouple material, and correct polarity must be maintained.

To minimize thermal "shunting", and improve their response times, thermocouples are made of thin wire. This may result in the thermocouple having a high resistance, making it sensitive to noise. It can also cause errors due to the input impedance of the measuring instrument.

A typical "exposed-junction" thermocouple with 32 gauge wire (0.25mm diameter) has a resistance of approximately 15 ^ohms/_meter. Sometimes a thermocouple with thin leads or long leads is required. In this case, keep the thermocouple leads short and use thermocouple extension wire (which is much thicker, and has a much lower resistance) between the thermocouple and measuring instrument.

Decalibration is the gradual change in the composition of thermocouple wire. The usual cause is the diffusion of atmospheric particles into the metal at extreme operating temperatures. Another cause is impurities and chemicals from the insulation diffusing into the thermocouple wire. There isn't much you can do about this, but it is worth being aware that as your thermocouple ages, its response may change.

The output from a thermocouple is a small signal, and it is prone to electrical noise interference, especially in the relatively hostile environment under the hood (with all the spark plugs firing, etc.). Most measuring devices (such as the AD595 Thermocouple Amplifier) reject any common mode noise (signals that are the same on both wires) so noise can be minimized by twisting the cable to help ensure both wires pick up the same noise signal. If operating in an extremely noisy environment, (such as near a large motor) it is worthwhile considering using a screened extension cable. If noise pickup is suspected first switch off all suspect equipment and see if the reading changes.

Although the thermocouple's signal is very small, much larger voltages often exist at the input to the measuring instrument. These voltages can be caused either by inductive pick up (a problem around spark plug wires, etc.) or by grounded junctions.

A typical example of an grounded junction would be measuring the temperature of a hot water pipe with a non insulated thermocouple. If there are any poor earth connections a few volts may exist between the pipe and the earth of the measuring instrument. These signals are again common mode (the same in both thermocouple wires) so will not cause a problem with most instruments provided they are not too large.

For example, the AD595 Thermocouple Amplifier has a common mode input range of -4V to +4V. If the common mode voltage is greater than this then measurement errors will result. Common mode voltages can be minimized using the same cabling precautions outlined for noise, and also by using insulated thermocouples.

All thermocouples have some mass. Heating this mass will affect the temperature you are trying to measure. For example, if you are measuring the temperature of gas in a test tube, there are two potential problems:

• The first is that heat energy will travel up the thermocouple wire, then dissipate to the atmosphere. This reduces the temperature of the gas around the wires.
• A second problem can occur if the thermocouple is not completely immersed in the gas. Due to the cooler ambient air temperature on the wires, thermal conduction can cause the thermocouple junction to be at a different temperature from the gas itself.