• Ben Owen-

THERMOCOUPLINGS For Measuring Cylinder Head Temperature mechanically adjusts the needle to read ambient temperature. These gauges, therefore, read accurately under all temperature conditions. To determine

By Al ROSS. EAA 113653

Rimrock, AZ

which type you have, check the gauge

rials, junctions can be devised that will produce enough current to drive a sensitive meter. This provides a reasonably accurate instrument that works without any external power. For high temperatures Chromel-Alumel is used; for engine temperatures Copper-Constantan or Iron-Constantan are used. The most

tions; one under the spark plug and one at the gauge end of the thermocouple wire. The voltage generated is a function of the temperature difference between these two junctions. It is vital, therefore, that the cool junction, the "other" end of the thermocouple wire, be inside the cabin, preferably at the gauge. This means that the thermocouple wire portion of the lead must be long enough to reach from the cylinder through the firewall into the cabin. To make a junction, simply twist together the two wires which comprise the thermocouple wire and silver solder or braze the end. A spark plug size lug may be made from a piece of .032 or .040 copper sheet. This replaces the usual spark plug washer. Braze the wires to the lug. Non-thermocouple wire may be used to extend the length, as long as the same type wire (copper) is used in each lead. The reason this can be done is that the error induced by the connection on one lead will be cancelled by the connection to the other lead. Obviously these two leads must be of the same length. This is true for the tin on the solder lugs, the brass screws, solder or any other metals used to complete the circuit. The net result is that the current through the gauge is the same as if

commonly used is Iron-Constantan with

there were no additional wire, except

two wires, one coated red and the other coated with white insulation. When these dissimilar metals are heated the junction between them generates a voltage that is proportional to temperature so as the heat goes up, the voltage goes up and this can easily be read on a gauge. It should be noted there are two junc-

for the small additional resistance added by the wire. A few words about cylinder head temperature instruments are in order. There are two commonly used types: temperature compensated and uncompen-

The original cylinder head temperature gauge in my KR-2 included only one probe. I decided I could make probes for the other cylinders and save a few bucks. I put probes on each cylinder and added a selector switch. This installation has performed well for over a year with only one broken wire due to replacing spark plugs. The insulation on thermocouple wire will eventually deteriorate and necessitate replacement of the wire. This article will assist you in constructing replacement probes or sensors for additional cylinders. When two unlike metals touch, a junction is formed which generates a voltage. By carefully selecting mate-

sated. The temperature compensated instrument is made with bimetal springs

which

react

to

temperature

and

when the airplane and engine are at the same temperature. A compensated gauge will read actual temperature, while an uncompensated gauge will read at the bottom of the scale. Uncompensated gauges are accurate only at their design temperatures, usually 75 degrees F. For example, if the temperature in the cabin is 95 degrees, then the difference of 20 degrees must be

added to the uncompensated gauge reading. Thus, if the cylinder head temperature is reading 350 degrees, the cabin is reading 95 degrees, add 20 degrees to make an actual 370 degrees. If you want to add probes for all cylinders and a switch, obtain the following materials:

1. Red-White (Iron-Constantan) Thermocouple Wire - Enough to reach from each cylinder to the instrument

panel. 2. Rotary Switch - Double pole, 4position (or 6-position if required).

3. An accurate digital multimeter for reading fractions of an ohm (even a good analog meter is not suitable for this purpose). 4. An accurate thermometer to 400 degrees F. A calibrated candy thermometer will do. 5. Cooking oil (it doesn't splatter, smell or burn).

6. Small pan to heat the oil. 7. Hot plate or understanding wife with stove. 8. Number 18 or 20 insulated copper wire. Twin conductor is preferred.

9. Spark plug sized lugs, one for each cylinder.

10. Miscellaneous - solder, brazing tools, terminal lugs, sleeving, etc. Cut a 6 foot length of thermocouple wire and carefully measure the resistance of each lead individually. Add the SPORT AVIATION 65

FIGURE 1

two values together and divide by 6 (feet) to obtain the ohms per foot. Write this down for future reference. Be sure to take into account the resistance of the meter leads, if necessary. Avoid measuring through a thermocouple, as the junction voltage will produce inaccurate readings. Remember, the more accurate you can be in all your measurements, the more accurate your gauge will be. Now form a junction at one end of the thermocouple wire by twisting and brazing a short (1/8") segment of the two wires. Connect the other end to the

gauge terminals and calibrate as follows: Immerse the junction in oil heated to 400 degrees F. Let the oil temperature stabilize and stir well before taking the reading. Take care while handling hot oil. Make a set-up which will support the thermometer and wire rigidly. Burns by hot oil are very painful. Keep the gauge at room temperature, preferably 75 degrees F. If the gauge reading is too high, splice on a couple of feet of thermocouple wire and try again. If the gauge reading is too low, indicating too much resistance, cut off a few inches of thermocouple wire and try again. Con66 JULY 1990

tinue until the gauge reads correctly. As the gauge readings approach a few degrees of the desired reading, cut only an inch or so at a time. You may want to take readings as the oil cools to check for linearity. If your gauge was made for Iron-Constantan, readings will be accurate throughout its range. If the resulting wire is long enough to reach from the most distant cylinder to the instrument panel, you are ready to install the switch. The thermocouple wire length (thermocouple to switch) must be adjusted to accommodate the added resistance of the copper wire between the switch and the gauge. Cut this length of copper 2-conductor wire and measure its total resistance (both wires). You must now remove that amount of resistance from the thermocouple lead. Since we know the ohms per foot, it is a simple matter to divide the measured ohms (copper wire) by the known ohms per foot (thermocouple wire) to obtain the number of feet to remove (probably a fraction). You may now cut the remaining thermocouple leads to match the just calibrated lead and braze them on the spark plug lugs. Connect the thermocouple leads and the gauge leads as shown in

Figure 1. If the thermocouple wire does not reach the firewall, then you must obtain larger diameter (lower resistance) thermocouple wire. If the thermocouple wire will not reach the control panel but is more than a foot into the cabin, proceed as follows: Determine the length of the wire required to reach from the end of the thermocouple wire to the switch and then on to the gauge. Cut a piece of 2-conductor copper wire to that length (plus one foot) and measure the resistance (both wires). Divide the measured ohms (copper) by the previously determined ohms per foot (thermocouple wire) to determine the number of feet to remove from the thermocouple wire. If that amount is less than the added foot of copper wire, shorten the thermocouple wire as determined and solder splice the copper wire. Use shrink sleeving. Construct sensors for the other cylinders using identical lengths of thermocouple and copper wire. The switch may be inserted into the copper or thermocouple wire at any point. Sources - Thermocouple wire - Anixter, Tempe, AZ, 602/968-7901. Switch - Radio Shack, No. 275-1386.