Instructions for Fabrication and Installation of Thermocouples for Measuring Ground Temperatures

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Publisher’s version / Version de l'éditeur:

Technical Note (National Research Council of Canada. Division of Building Research), 1952-02-04

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Eden, W. J.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

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SUBJECT _{Instructions for Fabrication and Installation}

of Thermocouples for Measuring Ground Temperatures

This note gives instructions for the fabrication and installation

of thermocouples suitable for measuring ground ｴ･ｭｰ･ｲ｡ｴｵｲ･ｳｾ If all

instructions are followed carefUlly, it should be possible to obtain

temperatures accurate to

±

1/2°F.

ｔｨ･ｲｭｑｯｯｾｰｬ･ Circuit

. Whenever two dissimilar metals are joined in a circuit, a small ･ｬ･｣ｾｲｯｭｯｴｩｶ･ foree, varying with temperature, is induced. Tempera-tures of the Fahrenheit scale are obtained by measuring the difference in electromotive force between a junction of known temperature, i.e.

a reference, ｪｵｮ｣ｴｾｯｮＬ and one ot unknown temperature. In the usual

thermo'couple circuit the referenc'e junction is simply a bath of .'ter and finely crushed ice which will maintain a constant temperatUre of

32°F.

In a circuit in which a switch is inserted, a junction will be

created at each switch point. If the switch is made of uniform

material and the temperature of each switch point is the same, this

effect will cancel out. Thus, for accurate readings, adequate

pr_-cautions must be taken to insulate the switches against uneven

temperatures. It must be assumed that the switches are of uni.form

material and no large error will be intro&uced by this assumption.

AlSO,· only copper leads ｳｨｯｾｬ､ be ｡ｴｾ｡｣ｨ･､ to the switch points,

since copper is likely to be similar to the material of the swi tah points.

Two circuit diagrams are enclosed with this note (Figs. 1 and 2).

Figure 1 issuitable ｷｾ･ the switch installation is within

50

feet

of tb.e thermocopple. Figure 2 should be used where long leads are

necessary. This circuit is preferred for two reasons:

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-·

.

2

-(1) it outs down resistance o£ the circuit, thus leads to more sensitive determinations with the potentiometer; (2) since constantan wire is much more expensive than

copper wire, the circuit will prove to be more eoonomieal.

box ,

The sWitch/£or either circuit can easily be made out o£ plyWood

and insulated with rockwoo1 as shown in the accompanying sketch

(Dwg. No. BR 198). The dimensions indicated are those

recom-mended ror an installation requiring rive lO-point switches. The

box must be constructed so that one end can be removed

to

install

the switches and such an assembly procedure is suggested by following the numbers on the bill or quantities on the sketch. The assembly is made in numerical order.

Construction of Thermocouples for Short Lead Installation

Copper-constantan 20-gauges duolex wire is recommended for

ground t mperature installations. Such wire has two conductors,

one copper and one constantan, being encased together in an outer covering of cotton impregnated with asphalt.

To oonstruct a thermocouple, first measure off the desired length of wire from the switch to the point where it is desired to measure the temperature, plus the distance from switch to

reference bath. Taking one end of the wire, strip back the

outer covering about one inch, and strip about

3/4

inch of the

Vinyl plastic covering from each of the conductors. Clean each

of the conductors with a fine emery cloth, then twist them

tightly together for about

3/8

inch, cutting off any jagged ends

of wir. D p the thermocouple thus formed into a soldering

nux

or paste. With a hot soldering iron, "sweat" solder between the twists

of the thermocouple. Then wash the thermocouple with aoetone to

remove any of the flUX, as it is corrosive. If resin is used

in-stead of corrosive flUX, washing in acetone can be omitted. To

campI te the thermocouple, fit a short piece of l/8-inch plastic tubing (called 'spaghetti' by eleotricians) over the junction, and

dip it in electrical insulating enamel. Two or three dippings

are required, allowing the glyptol to harden fully after each dipping.

As each length o:f thermocQuple wire i8 cut it should be

numbered. A small numbered flag made of masking tape, attached

near the nd of the wire, will be suitable ror this purpose.

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..

corresponding points on the switches should also be numbered with

a pencil. This is done as follows: turn the switch pointer as

far as possible in counter-clockwise position; label the position indicated "off"; do not attach any leads to this point as an "off" position is necessary on each switch; going in a clockwise

direction, each successive point may be marked according to the numbering system on the thermocouples.

Beference Junction

For the circuit shown for the sbort lead installation, the reterence junction thermocouples are constructed 10 the same manner a s the thermocouple of unknown tempera ture, but on the

opposite end of the wire. All the reference junctions may be

bound together with electrician's tape and inserted as a unit in

the reference bath. A suitable bath can easily be made of a

thermos flask filled with finely crushed ice and water. A hole

can be bored in the cork of the flask.

Wiring the Switches for Shert Lead Installation

For the circuit shown, the selector switches are inserted in

the copper branch of the wire at whatever length is required

from the ｲＮｦＮｲ･ｾ･･ junction end. At this point, strip about

three inches of tne outer oovering from the duplex wire, taking care not to injure the inner plastic ooating on the conductors. The constantan conductor will be covered in a red plastic and

the copper in a green or blue covering. Cut the copper wir in

the middle of the three-ineh stripped patch and strip back the

plastic covering. Before connecting the copper leads to the

SWitch, it will be necessary to pull the leads through the hole

in the removable end of the swtieh box. No solder the copper

lead from the junction of unknown temperature to the top row of the switch points and the copper lead from the referenoe junction

to the lower row of switoh points directly belGw. Repeat this

process for each thermocouple.

If more than one switch is required for the installation, the switches are put in series by joining the instrument points of the switches with the copper wire.

Finally, the two leads to the instrument may lead from the instrument points on any switch to bind post connections on the switch box or. if desired, directly to the instrument itself. The instrument leads can be any good grade of 20-gauge copper wire.

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-..4...

When all leads have been soldered to the switches, the assembly of the switoh box may be completed.

Long Lead Circuit

For this circuit, the procedure for making thermocouples is exactly as previously described, but in this case the length of duplex wire for each thermocouple will be from the junction box to the point where it is desired to measure the temperature. The reference junction in this case is an entirely separate

length of duplex wire. It is recommended to use a larger

dia-meter wire for this reference junction to cut down the

resis-tance. Sixteen-gauge duplex wire is suggested for this lead.

The junctions in the junction box are formed by splicing the

copper lead wire to the copper constantan duplex wire. To for.m

this splice, strip back the covering from each wire for 1 1/2

inches. Clean each wire as before with an emery oloth. Now

s14.p a lnegth of "spaghetti" over one of the wires long enough

to cover the completed splice. Next cross the wires in the

middle of the bared section. Wind one wire aroWld the other

1 1/2 times at long spacing, then

5

times at short spacing.

Next "sweat" the solder well into the splice and remove any

of the soldering flux with acetone. To complete the joint,

slip the "spaghetti" over the splice and tape it in place. Follow this procedure for the compensating junction as well.

When all the splices have been finiShed, the junction box is sealed with insulating oompouna for the junction boxes. This compound is in a solid state and must be heated with a

blow torch, then poured into the box in a liquid state. After

the box has been tilled with the insulating compound, the cover may be placed on the box.

The junction bex must remain at a constant temperature

while the readings are being taken. For this reason, the box

should be buried at least three feet deep in the soil.

For the circuit for the long lead installation, both lead wires are copper, thus the ends of the leads may be attached

directly to the proper switch terminal. In this circuit, only

the positive lead frQm the switch runs to the instrument. The

negative one leads back to the compensating junction in the buried junction box. then to the reference junction, back again to the junction box then finally to the negative terminal of the potentiometer.

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-5-Installing the Thermocouples

This is simply a matter ｯｾ leading the thermocouple to

the desired position and fixing it in that position. It may

be necessary to staple the thermocouple tQ a small wooden ｾｴｩ｣ｫＮ

ｉｾ this is the case, it is advisable to attach the thermocouple

to stick with insulated staples which will not damage the coating on the conductors.

List of Materials and Equipment to Make Short Lead Installation

1. 20-gauge copper-constantan duplex wire

2. Two pole rotary switches, with good quality terminals

3. Portable precision potentiometer. Range: 0 to

10 millivolts

4.

Thermos bottle or other suitable vessel for a

reference bath

5. Electric soldering iron

6.

Solder, soldering paste or resin

7.

Acetone (if soldering paste is used)

8.

Electrical inSUlating enamel such as glyptol

9.

l/8-inch plastic tubing ("spaghetti")

10. Friction tape

11. Electrician1s staples (if thermocouples are to

be fixed to wooden sticks)

12. 3/8-inch plywood (as outlined in bill of quantities

on sketoh of switch box)

1).... No.7, 3/4-inch wood screws

14

Small quantity of rock wool insulation

15. Two binding posts (if desired)

Additional Materials and Equipment to Make Long Lead Installation Metal junction box with suitable tight-fitting lid

A supply of an insulating oompound for junotioB;'boxes

such as Canada Wire and Cables "Ozite - B"

20-gauge copper wire, with vinyl plastic covering l6-gauge copper-oonstantan duplex wire

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-6-Notes on Taking Readings

Instructions tor reading the potentiometer will be

supplied by the manufacturer or supplier ot the instrument.

One precaution should be caretully tollowed: it is im-perative that the switch box should not be subjected to any change in temperature 'while the readings are in progress.

I t it is necessary to leave the switch box out-at-doors, then

arrangements must be made to do the SWitching outside as well. If such is the case, a large waterproof box should be con-structed to house the switch box.

Since a potentiometer contains a small cell filled with distilled water, this instrument should never be subjected

to freezing temperatures. In the case where the switch box

is outside, it may be neoessary to read the potentiometer in

a warm vehicle. In this case, readings will be rather

difficult to obtain and this method should only be used as a last resort.

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