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Technical Note (National Research Council of Canada. Division of Building Research), 1961-03-01
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The Measurement of Ground Temperatures Using a Modified Electronic Temperature Indicator
Johnston, G. H.
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No.
DIVISION OF BUILDING RESEARCH
330
e
'fE
C
1HI N II CAlL
NOTlE
NOT FOR PUBLICATION FOR INTERNAL USE
PREPARED BY G. H. Johnston CHECKED BY HED and KRS APPROVED BY NBH DATE March 1961 PREPARED FQR General Distribution
SUBJECT THE MEASUREMENT OF GROUND TEMPERATURES USING A MODIFIED ELECTRONIC TEMPERATURE INDICATOR
Ground temperatures have generally been measured, particularly in more remote areas, by means of a portable
precision potentiometer (described in Technical Note 329). The external resistance of thermocouple circuits having long lengths of wire can seriously affect the sensitiVity of this instrument, however, and it must therefore be used with
caution in such cases (1). The use of a reference ice bath
which is not maintained at a temperature of 32°F exactly can introduce further errors in temperature measurement. In addition, observer error can have an appreciable effect on ground temperature observations particularly when the various adjustments necessary must be made to the potentiometer under adverse weather conditions such as low air temperatures, cold winds and rain or snow, or when the observer is not fully familiar with the operation of the instrument.
To offset these problems the use of an electronic temperature indicator has been adopted by the Division of Building Research of the National Research Council. It appears to have certain advantages that would improve the accuracy and reliability of ground temperature observations.
It is relatively simple to operate, reqUiring few adjustments by the observer; although basically a laboratory instrument it is fairly rugged in construction and can bensed in the field after certain modifications are made. The external resistance of the thermocouple circuit has no effect on the
(1) Refer to Technical Note 329, page 3
sensitivity of this instrument within the size and length range of the thermocouple wire normally used for ground temperature work. It does require a portable power source, however, when used at isolated locations, and therefore the total weight of this equipment (about 200 Ib) is substantially greater than the hand-carried potentiometer (30 Ib). It is considered, however, that the improved reliability of the results justify its use in cases where a large number of readings have to be taken.
ThiS note describes one type of temperature indicator which has been modified by the Division of Building Research for field use for the measurement of ground temperatures.
The procedures to be followed in the operation of this instru-ment are also outlined in detail.
SPEEDOMAX TEMPERATURE INDICATOR I. DESCRIPrION OF EQUIIMENT
A. Temperature Indicator
This temperature measuring instrument is a Leeds
and Northrup Speedomax Type G, Model D, double range temperature indicator (i.e., non-recording).
The indicator is essentially a thermocouple potentio-meter containing a dry cell and a standard cell but instead of an ice bath reference junction it has, incorporated in the electronic circuit, a reference junction the temperature of
which is compensated for automatically. This reference junction is dependent on the air temperature immediately surrounding
the indicator mechanism. Temperatures (OF) are read directly on a double range,_drum-type scale mounted in the front of the instrument. Temperatures from +45°F to +164°F are read on the red or upper scale and temperatures from -80°F to 60°F are read on the black or lower scale. To change from one range to the other a switch is located on the left side of the observation window. A push-button switch on the right side of the window permits easy standardization of the potentiometer current.
The indicator has been mounted in a special insulated box to prevent damage to its component parts when used during Gold weather and to protect it when being transported. A hinged door has been fitted to the front of the insulated box to allow access to the indicator mechanism. This door SHOULD NOT be opened when the instrument is being used during cold weather so that a constant temperature can be maintained within the indicator. Should the temperature within the indicator case
3
-change appreciably or rapidly, then the accuracy of the observations can be affected.
Sufficient heat is generated by the amplifier unit in the indicator to maintain a proper operating temperature
(between.60oF and 100°F) for the indicator mechanism. A 25-watt bulb which can be turned on to assist in maintaining a temperature within the range just noted has been fixed to the inside of the metal case. The operating temperature
should not be allowed to exceed 100°F, however, for the accuracy of the results can be affected. should the temperature rise above 1000 then the instrument should be allowed to cool off
by Shutting down the generator or by opening the door on the protective box. The light bulb should not be left on when readings are being taken. A thermocouple point has been placed inside the indicator case so that the operating temperature of the instrument can be measuredo The ｣ｬ｡ｭｰｾ
type connector should always be connected to the thermocouple point terminals, mounted outside the indicator box, except, of course, when readings are being taken on a thermocouple installation - at which time the clamp-type connector is connected to the switch box terminals.
An observation window in the front of the door of the insulated box permits temperature readings to be taken without opening the door. In addition, two controls have been mounted on the observation window so that the potentio-meter current may be standardized and the temperature reading scale range changed without opening the door.
THIS INDICATOR IS AN EXPENSIVE PRECISION INSTRUMENT, DIFFICULT AND COSTLY TO REPLACE, AND MUST BE HANDLED WITH CARE. WHEN NOT IN USE, IT MUST BE PLACED IN HEATED STORAGE.
Electric power to operate the indicator is supplied by a gas engine-driven generator (described below). A three-wire electric cord links the generator to the indicator by means of a Twist-Lok connector mounted on the indicator box. B. Power Generator
Electric power is supplied to the Speedomax temperature indicator by a Fairbanks-Morse Model 6P-36 generating set. The capacity of this set is 600 watts at 115 volts, ＶＰｾ｣ｹ｣ｬ･Ｌ 3600 r.p.mo (full load amps. -- 5.2). The generator is driven by a 4-cycle Briggs and Stratton Model 60431 air-cooled gas engine.
The generating set has been enclosed in a removable wooden box. This box has been provided to ensure warm engine
4
-operating ｴ･ｭｰ･ｲｾｴｵｲ･ｳ under winter conditionso A dial thermometer has been fitted to the protective box with ｳｴｾｭ
extending close to the engine so that its operating tempera-ture (about 90°F) can be determined o
The temperature inside the protective wooden box can be adjusted by opening and closing the sliding panels at each end of the box. A grounding cable has been attached to a motor mount to protect the generating set and the temperature indicatoro This grounding cable should be attached to a leg of the switch box or some other suitable ground 0
Both the temperature indicator and the generating set have been mounted on a common base with carrying handles so that the equipment can be moved as a unit. Both have been fastened to the base by means of spring fittings to cut down the effects of ｾｩ｢ｲ｡ｴｩｯｮｯ
II. PROCEDURES FOR MEASURING GROUND TEMPERATURES Storage
erature indicator contains several
Ao COLD WEATHER OPERATION (Air temperatures below 40°F) Preparaticns at the Office
20 Check to see that the Speedomax "Line" switch is in the "ON" position. Disconnect the power cable at the Speedomaxo (This has a twist-lock connector). Connect the indicator operating temperature lead to the main indicator terminalso 30 Check the following on the generator set:
(a) plug in the power cable (b) crankcase full of oil (c) fuel tank full of gas (d) fuel tank valve open
40 Start generator engine -- to start engine, close choke. When engine ｳｴ｡ｲｴｳｾ open choke gradually.
5.
Connect power cable from generator to Speedomax, after generator is running smoothlyo When the power is turned onthe indioator should at all times be oonneoted to a thermo-oouple iunotion and the range seleotor switoh set for the arireremfierature range since the indioator may otherwise
t to ｾ･ extreme erid of the soale ｯ｡ｵｳｬｮｾ damage to the
6. While the indioator is operating, oheok the standardization of the potentiometer ourrent by pressing the button in the
observation window and holding it in this position for a few seoondso If the ourrent is properly adjusted, the soale will not move. If this oondition does not exist, keep the button depressed until this oondition is obtained. Should the soale travel to either end before ooming to rest, release the button, wait a few seoonds and then prooeed again as indicated above o When the ourrent is properly adjusted, release the button. If the above oondition oannot be obtained, the dry oell has probably deteriorated and should be replaoed with a fresh oneo When the dry oell has been replaoed, the above prooedure should be
repeated (see also step 22).
7.
Cheok the generator engine temperature by observing the temperature on the dial thermometer. Adjust sliding panels of the generator box so that a temperature of about 80° to 100°F is maintained inside the box. Also oheok the operatingtemperature of the indioator to see that the temperature is at least 60°F - but not greater than 100°F.
8. When the observer is satisfied that the equipment is funotioning properly, it may be taken outside. The door of the insulated box enolosing the Speedomax should not be opened in the field when air temperatures are below
35°F:--9. Load the unit on a vehiole for transportation to the site. The generator set is to be left running while being transported suffioient hGat is thus generated by the Speedomax amplifier to prevent freezing of the indioator oomponents.
10. A oheok should be kept on the engine and indioator ｴ･ｭｰｾｲ｡ﾭ
tureswhen the unit is being transported - as outlined in step
70
Preparations at the Switoh Box
II. Cheok engine and indioator temperatures -- refer to step 1
above. The indioator temperature should not exoeed 100°F. Turn off light bulb, if not required to maintain a temperature between 65° and 100°F.
12e Set rotary switch pointer to first thermocouple point to be measurede
13e Attach the clamp-type connector to the terminals on the switch boxe
14e Make sure that a good contact bas been made at the switch box and that the indicator scale has been set for the correct range 0
TO TAKE A TEMPERATURE READING
150 Check standardization of the indicator current by pressing the button in the observation window (reter to step 6)0
16. Read the ground temperature (in OF), for the first switch point, directly on the indicator scale. The correct scale range can be chosen by throwing the switch on the face of the indicator (left side) by means of the brass handle mounted on the observation window. Then turn rotary switch to the next thermocouple pointe
11. Repeat step 16 for all switch pointse
18. Check standardization of indicator current (refer to step 6) after reading all switch points. Repeat first few readings --they should check within + 0.2°Fo
190 When all readings have been taken, disconnect the clamp-type connector from the switch box and attach it to the
terminals on the indicator as soon as possibleo Record the indicator operating temperature (which must not exceed lOOOFo) Be WARM WEATHER OPERATION (Air temperature above 40°F)
20. When air temperatures are above 40°F the generator need not be started until the equipment has been taken to the field installations, although it is advisable to check the operation of the equipment to make sure it is functioning properly before it is moved from storage (refer to steps 2 to 1)0 The
protective wooden box covering the generator set can be taken off by removing the hinge pinse The door on the insulated box enclosing the Speedomax indicator should be kept closed at all times when the unit is being transported but when readings are being taken at the thermocouple installations, the door can be left open.
III. MAINTENANCE OF EQUIPMENT
Ae Speedomax Temperature Indicator
operating the Speedomax, little maintenance of the instrument should be required for a period of at least six months.
Certain checks can be carried out, however, (as outlined in the folloWing paragraphs) if the instrument seems to be malfunctioning.
22. Replace Dry Cell
Warning that the dry cell must be renewed is given when a small red flag appears at the right side of the indicator
scale. When the red signal arm moves out so that it can be seen at the right edge of the observation window, the dry cell has to be replaced by a new one. After the new cell is installed, the current of the potentiometer circuit should be adjusted as outlined in the following section.
23. Check and Adjust (Standardize) Current of Potentiometer Circuit
With the indicator operating, press the button on the observation window and hold it in this position for a few seconds. If the current is properly adjusted, the scale will not move. If this condition does not exist, keep the button depressed until this condition is obtained. Should the scale travel to either end before it comes to rest, release the
button, wait a few seconds, and then again proceed as indicated above. When the current is properly adjusted, release the
button.
If the above condition cannot be obtained, the dry cell has probably deteriorated and should be replaced with a fresh one.
24. Check Amplifier
If the indicator does not perform properly, it may be caused by a faulty tube in the amplifier. Under such conditions, it is suggested that the four tubes be checked and that any weak ones be replaced with new ones. If tube checking equipment is not available, then all tubes should be replaced with the spares supplied. In addition, the two fuses (one 3A and one 3/16A) should be checked and replaced if necessary. .
If the new tubes and/or fuses do not remedy the faulty operation, the convertor in the amplifier should then be replaced with the spare one supplied.
25. If the faulty operation of the indicator has still not been rectified, then a collect telegram outlining, as briefly but clearly as possible the trouble occurring should be sent
to: G. H. Johnston, Division of Building Research, National Research Council, Ottawa.
On no account should the indicator mechanism be touched in an attempt to locate the trouble unless someone familiar with this type of equipment is available.
B. Fairbanks-Morse Generator Set
26. With proper care and maintenance, the gas engine and the generator should operate for a considerable time without any major troubles occurring. Special oil for the gas engine has been supplied.
IV. TROUBLE-SHOOTING
FAULTY OPERATION OF SPEEDOMAX INDICATOR INDICATED BY (a) no movement of the temperature scale ｾ that
is, the scale runs to one end and remains there? OR (b) continuous oscillation of the temperature scale over a range of perhaps several degrees. POSSIBLE CAUSES
I. Temperature scale on wrong range.
2. Loose power supply cable connections at generator or at indicator.
3.
"Deadflswitch point - that is, no thermocouple is connected to that switch point OR, the thermocouple connected to that switch point has somehow become disconnected. Check this by turning to the next switch point. If a reading is obtained on this one, then omit further readings on the "deadlt
point unti.l it is repaired.
4.
Broken wire in the switch box lead cable. This will generally occur near the connector at either end of the cable due to the continual fleXing of the cable by the operator when it is being connected or dis-connected and when being drawn to or back from the switch box. This cause can be confirmed by haVing the person at the switch box Wiggle the cable back and forth just above the connector (when attached to the switch box terminals). If the wire is broken, some contact will be made by the broken ends of the wire so that the oscillation of the temperature scaleon the indicator will be dampened momentarily. Great care should be exercised during cold weather in
••
wires will become quite brittle and are ｴｨ･ｲ･ｾｯｲ･
more subject to damage o Care should also be taken to see that no kinks occur in the cable and that there is no undue ｾｬ･ｸｩｮｧ ｯｾ the cableo
50
Instrument not standardized o60 Dry cell weak -- indicated when a red ｾｬ｡ｧ appears on the right hand side ｯｾ the indicator observation window0 A new dry cell will be shipped by NRC every
2 or 3 months or sooner ｩｾ requested o
70
Loose or weak tUbes, ｾｵｳ･ｳ and/or convertor in the｡ｭｰｬｩｾｩ･ｲ - spare ones are supplied with the indicatoro
80 Generator not running smoothly - operation of the
indicator will be ｡ｾｦ･｣ｴ･､ ｩｾ the voltage from the generator drops below about 108 volts or rises above 125 voltso
90
Loose thermocouple connection inside the Speedomax indicatoro The duplex thermocouple wire runningｾｲｯｭ the clamp-type connector terminates inside the instrument at a panel boardo The screws ｨｯｬ､ｾｮｧ the copper and constantan wires to the panel board may have loosened due to vibrationo These should be reconnected with the copper to the positive post and the constantan to the negative posto
