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calibrating ovens

时间:2013-11-30 11:47:48 作者:siluck_dnt 来源:ca 阅读:1400次
calibrating ovens
•• UNITED STATES DEPARTMENT OF THE INTERIOR •)
"• '• BUREAU OF RECLAMATION
USBR 1020-89
PROCEDURE FOR
CALIBRATING OVENS
INTRODUCTION
This procedure is under the jurisdiction of the Geotechnical Services Branch, code D-3760, Research and Laboratory Services
Division, Denver Office, Denver, Colorado. The procedure is issued under the fixed designation USBR 1020. The number immediately
following the designation indicates the year of acceptanceor the year of last revision.
1. Scope
1.1 This designation outlines the procedure for
calibrating ovens used for standard laboratory testing. This
calibration procedure is limited to ovens operating to a
maximum temperature of 300 °C. Ovens operating at
higher temperatures require special handling and, there-
fore, should be periodically checked by the manufacturer.
1.2 This procedure is to be used for periodic checks
of laboratory ovens. If stringent calibration tolerances are
required, thisprocedure should not be used and ASTM
designation: E 145 is to be consulted.
2. Applicable Documents
2.1 ASTM Standard:
E 1 ASTM Thermometers
E 145 Specification for Gravity-Convection and Forced-
Ventilation Ovens
3. Summary of Method
3.1 A thermometer is placed in an oven which is set
at the desired operating temperature. The temperature
reading of the thermometer is compared to the oven
thermostat reading. The appropriate thermostat adjust-
ment is determined.
4. Significance and Use
4.1 Accurate temperature control is important for
many soil testing procedures in order to obtain accurate
results.
4.2 Ovens are usedprimarily for determination of
moisture content in soils. The ovens should have minimum
inside dimensions of 3 feet wide by 2 feet deep by 2 feet
high. For most soils laboratory applicationovens should
be thermostatically controlled at 110___5 °C. Since it is
necessary that a uniform temperature be maintained
throughout the oven, the mechanical convection or forced-
draft ovens are recommended.
NOTE 1.-Ovens slightly smaller than the recommended size
may be used, but oven temperature varies significantly when
the door is opened, and calibration will be more difficult.
4.3 This calibration procedure is to be performed upon
receipt of the oven and annually thereafter.
5. Apparatus
5.1 Oven.-An oven, thermostatically controlled,
forced-draft or mechanical convection, horizontal air flow
type.
5.2 Thermometer.-Etched stem glass thermometer
with eye-hook at the end for hanging, 0 to 300 °C, 2 °C
divisions, mercury filled, conforming to the requirements
of ASTM E 1.
5.3 Gloves.-Asbestos, 1 pair, to be used whenever
handling equipment heated to elevated temperatures.
6. Precautions
6.1 Safety Precautions:
6.1.1 All electrical veires and cords are to be checked
for damage. If any damage is observed, it is to be repaired
before attempting to use the oven.
6.1.2 Care should be exercised in handling any
equipment that is heated to elevated temperatures. Gloves
are to be worn whenever handling such equipment or
materials.
7. Calibration and Standardization
7.1 Verify that the thermometer to be used for this
procedure has a certificate of inspection or calibration
verification from the manufacturer. If there is doubt as
to the accuracy of the thermometer, it should not be used
for this procedure; a thermometer with a verifiable accuracy
is to be obtained and used.
8. Conditioning
8.1 During the calibration procedure, the room
temperature should not vary more than _+10 °C.
9. Procedure
9.1 All dataare to be recorded on the "Oven
Calibration" form as shown on figure 1.
9.2 Locate and record the serial number, or any other
identifying markings, of the oven to be calibrated.
42USBR 1020
9.3 Remove any material (sample containers, pans,
etc.) fromthe oven.
9.4 Carefully hang the thermometer by its eye-hook
with a piece of wire as close to the center of the oven
chamber as possible. Make sure that the thermometer hangs
freely.
9.5 Determine the applicabletemperature at which the
oven will be calibrated and set the thermostat of the oven
to that temperature.
NOTE 2.-For most geotechnical purposes, the oven will be
calibrated at 110 °C. If calibration is desired at another
temperature, the procedure is similar. If a range of calibrated
temperatures is desired, the calibration should be performed in
intervals of 5 °C-over the desired range of temperatures.
9.6 Allow sufficient time for the oven temperature to
stabilize and record the oven thermostatreading.
9.7 Determine the thermometer reading and record
the value.
NOTE 3.-The oven temperature will drop drastically when
the oven door is opened. The thermometer reading should be
taken as quickly as possible once the door is opened.- •
9.9 Calculate the thermostat correction and record the
value.
9.10 If the thermometer reading is within +5 °C of
the thermostat reading, the calibration procedure is
completed. If the thermometer reading differs from the
thermostat reading by more than 5 o C, or if a more stringent
calibration is desired, see subparagraph 1.2.
9.11 Adjust the thermostat by the amount of the
thermostat correction.
9.12 Repeat subparagraphs 9.6 through9.11 until the
desired calibration accuracy has been achieved.
10. Calculations
10.1 Calculate the thermostat correction using the
following expression:
where:
Thermostat correction = (1) - (2)
(1) = oven thermostat reading
(2) = thermometer reading
11. Report
11.1 The report is to consist of a completed andchecked
"Oven Calibration" form (fig. 1).
11.2 All calculations are to show a checkmark.
43USBR 1020
7-23O6 (6-85)
Bureau of Reclamation
Oven Identification Model 1206 SN 36157
Calibrationperformed by Exomple
Calibrationcheckedby Exomple
OVEN CALIBRATION
Designation USBR 1020- 89
CalibrationTemperature [] °C
or Temperature Range 110 D o F
Date 3- 24 - 89
Date 3- 24-89
OVEN THERMOSTAT THERMOMETER THERMOSTAT
READING READING CORRECTION
(1) (2) (1) - (2)
II0 ° C 116 ° C -6° C
104oc II0°C 0
COMMENTS:
GPO 847 -97 I
Figure 1. - Oven calibration --example.
441400
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