User Manual
Dual Input Type J/K Digital Thermometer
Model 421502
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Introduction
Thank you for selecting the Extech 421502 Digital Thermometer. The 421502 features
dual thermocouple inputs, T1, T2, and T1 – T2 displays, MIN-MAX-AVG recording,
selectable units of measure, high/low alarms, automatic power OFF, and data hold. This
professional meter, with proper care, will provide years of safe reliable service.
Meter Description
1. Thermocouple Input Jacks (T1 and T2)
2. LCD Display. The primary temperature display is the
readout in large digits on the top row. The secondary
displays are shown on the bottom row (smaller digits)
and include a temperature reading (lower left) and the
real time clock (lower right). Temperature readings can
be set to show T1, T2, or T1 minus T2 temperatures
3. Dual-function control buttons. Primary function printed
on buttons and secondary function printed above
buttons (numerical keypad and ENTER button)
Note: The battery compartment (not pictured) is located on
back of meter. The meter’s protective jacket must be
removed to gain access to the compartment.
Operation
Meter Power
Ensure that a fresh 9V battery is installed (rear compartment) before powering the
meter. Press the power button (upper left button) to switch the meter ON or OFF. The
meter is equipped with an APO utility where it switches off automatically after 30
minutes of inactivity. If the SET or MIN-MAX-AVG mode is engaged, APO is disabled, and
the meter cannot be powered down manually. Exit these modes before attempting to
power down. Long press the “MIN-MAX” button to exit the MIN-MAX-AVG mode. To exit
the SET mode, use the “ENTER” button to step back to the normal operating mode.
Connecting Thermocouples
The thermocouples connect to the T1 and T2 input jacks on the top of the meter (observe
correct polarity). The meter accepts type J or K thermocouples, select the thermocouple
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type in the SET mode. A type-K thermocouple is supplied. Safety note: The supplied
thermocouple can only be used to measure temperature to 260oC (500oF). To measure
higher temperatures please obtain a thermocouple rated for higher temperature.
Temperature Units
Press the "oC/oF" button to toggle the temperature units. The meter recalls the selected
unit of measure when power is cycled.
Data Hold
Press the "HOLD" button to freeze the displayed reading. The "HOLD" indicator will
appear. Press "HOLD" again to return to normal operation. In MIN-MAX-AVG mode,
pressing the "HOLD" button pauses MIN-MAX-AVG recording (pressing "HOLD" again
resumes recording).
Primary and Secondary Display Modes (T1, T2, or T1 - T2)
Use the "T1 T2 / T1 - T2" button (the button with the number “4” overlay), to select the
mode for the meter's primary display (top, large digits). Press the "T1 T2 / T1 - T2" button
(the button with the minus sign [-] overlay) to select the mode for the meter's secondary
display (lower, smaller digits).
Choose "T1 or T2" to select thermocouple input 1 or 2 for display. Select "T1 - T2" to
display the difference between the two thermocouple measurements. In T1 - T2 mode, a
reading of zero indicates that both thermocouples are reading the same temperature.
Set the Thermocouple type
Choose Type-K or Type-J thermocouple using the “K/J” buttons. The “K/J” button with the
number “5” overlay applies to the primary display (large digits). The “K/J” button with the
“0” overlay applies to the secondary display (smaller digits). The selection must match
the inserted thermocouple type for accurate temperature measurements. The meter
remembers the selection when power is cycled.
Taking Temperature Measurements
The tips of thermocouple probes are used to sense temperature. Touch the tip of the
probe to an object’s surface to measure its temperature. You can also use a
thermocouple to measure the air temperature by placing the tip of the thermocouple in a
room, enclosure, or other area. Use care when placing the thermocouple; do not place it
in liquids or near live electrical circuitry.
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MIN-MAX-AVG mode (Primary Display)
1. Press the "MIN-MAX" button to access the mode and to start recording the
minimum, maximum, and average temperature readings
2. Use the “MIN-MAX” button to step through MIN, MAX, and AVG modes (the display
icons MIN, MAX, and AVG will change with each button press)
3. When a new MIN or MAX is detected, the previous MIN or MAX is replaced. An
audible beep alerts the user when a new MIN or MAX is recorded
4. Use the “HOLD” button to pause/resume recording
5. Long press the “MIN-MAX” button to exit and to reset the MIN-MAX-AVG memory
6. When recording is paused, the stored values are held in memory until MIN-MAX-
AVG mode is canceled or resumed
7. The Auto Power OFF utility is disabled, and the meter cannot be manually powered
OFF, in the MIN-MAX-AVG mode. Long press the “MIN-MAX” button to exit and
return to normal operation.
Relative (REL) Mode (Primary Display)
Relative mode allows the user to store a reference temperature for comparison with
subsequent measurements. The reference temperature can be automatically or manually
set (for manual entry, see the SET mode section). For automatic entry, the displayed
temperature is used as the reference value. Perform the steps below:
1. Press the “REL” button (the REL icon appears)
2. The currently displayed temperature becomes the reference value and
subsequent measurements represent the actual temperature minus the
reference temperature
3. Press the “REL” button to exit (the REL icon will switch OFF)
Real Time Clock and Elapsed Timer
The time display (bottom right) shows the real time clock. Refer to the SET mode section
to set the real time clock. The clock can also be used as an elapsed timer by resetting the
clock to zero. The clock allows you to see when MIN-MAX updates occur. Each time a new
MIN or MAX value is updated, the associated time is stored.
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SET Mode
Manually Setting the Relative (REL) Reference Value
Note: Refer to the Relative Mode section for setting the temperature reference value
automatically, using the currently displayed temperature as the reference value.
To set the reference manually and to then use the Relative Mode, press the "SET" button;
the display will show dashes “ - - - -“. Enter a reference temperature using the numeric
buttons. You must use leading zeros; for example, enter 0050.0 to for a 50.0o reference
temperature. Once the reference value is entered, press the “ENTER” button four times.
To display the difference between the actual temperature and a manually entered
Relative value, press the REL key and then the SET key. The meter will now display actual
temperature minus the reference temperature. Press the “REL” button to exit the mode.
Setting the Real Time Clock/Elapsed Timer
Press the "SET" button and then the "ENTER" button. Set the Hours, Minutes, and
Seconds (HH:MM:SS) using the numeric buttons. When editing is complete, the clock will
start to run. Press the "ENTER" button three times to exit. Use the “SEC/MIN” button to
switch between Hours/Minutes (Min) and Minutes/Seconds (Sec) display modes.
Setting the Alarm High/Low Limits
Press the "SET" button and then press the "ENTER" button twice. Set the desired High
Alarm limit using the numeric buttons. You must enter the value using all 5 digits,
including leading zeros. Example: 25.0oC is entered as 0025.0.
Press the "ENTER" button once. Set the Low Alarm limit using the numeric buttons. You
must enter the value using all 5 digits, including leading zeros. Example: 5.0oC is entered
as 0005.0. Press “ENTER” to exit.
To activate the high/low alarms, press the "HI/LO Limits" button, the meter will beep, and
the audio icon will appear. The meter will produce a continuous tone when a high or low
alarm limit is reached. Press the “HI/LO Limits” button to silence the alarm (see Alarm
Operation section below for more information).
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Alarm Operation
Press the "HI/LO Limits" button to enable the alarm mode (the meter will beep, and the
audio icon will appear). Set the low and high alarm limits in the SET mode. When the
actual temperature reaches the low or high alarm limit a continuous tone will be
produced. To silence the alarm (and to disable the alarm mode), press the “HI/LO Limit”
button (the audio icon will disappear).
Battery Replacement
Replace the 9V battery when the low battery icon appears on the upper left or when the
meter will not power up. To replace the battery, remove the meter’s protective jacket
and remove the two screws that secure the rear battery compartment cover. Remove the
battery, install a new battery observing correct polarity, secure the battery compartment,
and replace the meter’s protective jacket.
Battery Safety: Please dispose of batteries responsibly and never dispose of batteries in a fire,
batteries may explode or leak. When storing the meter for 60 days or more, remove the battery
and store separately.
Never dispose of used batteries or rechargeable batteries in household waste.
As consumers, users are legally required to take used batteries to appropriate collection
sites, the retail store where the batteries were purchased, or wherever batteries are
sold.
Disposal: Do not dispose of this instrument in household waste. The user is obligated to take
end-of-life devices to a designated collection point for the disposal of electrical and electronic
equipment.
Calibration and Repair Services
FLIR Systems, Inc. offers calibration and repair services for the Extech brand products we sell. We
offer NIST traceable calibration for most of our products. Contact us for information on calibration
and repair availability, refer to the contact information below. Annual calibrations should be
performed to verify meter performance and accuracy. Product specifications are subject to change
without notice. Please visit our website for the most up-to-date product information:
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Specifications
Display
5-digit multifunction LCD
Battery Power
9V (NEDA 1604, IEC 6F22, or 006P)
Auto Power OFF (APO)
Meter switches OFF after approx. 30 mins. of inactivity
Battery life
160 hours typical with carbon zinc battery
Dimensions / Weight
192 x 91 x 52.5mm (7.5 x 3.6 x 2.1”) / 365g (11.7 oz.)
Type-J thermocouple range
-200 ~ 1050oC (-328 ~ 1922oF)*
Type-K thermocouple range
-200 ~ 1370oC (-328 ~ 2498oF)*
*Safety note: The supplied type-K thermocouple can only be used
to measure temperature to 260oC (500oF). To measure higher
temperatures please obtain a thermocouple rated for higher
temperature
Resolution
0.1oC (0.2oF)
Temperature units
Selectable oC / oF
Accuracy
± (0.05% rdg + 0.3oC) -50oC ~ 1370oC
± (0.05% rdg + 0.7oC) -50oC ~ -200oC
± (0.05% rdg + 0.6oF) -58oF ~ 2498oF
± (0.05% rdg + 1.4oF) -58oF ~ -328oF
Temperature Coefficient
0.1 times the applicable accuracy specifications per oC from 0oC ~
18oC and 28oC ~ 50oC (32oF ~ 64oF and 82oF ~ 122oF)
Water resistant housing
Gasket protected front panel
Input Protection
24VDC or 24VAC rms max. input on any combination of inputs
Measurement rate
One reading per second
Input connectors
Accepts standard miniature thermocouple connectors
Supplied thermocouple
Type-K with Teflon® insulation. Cable length: 4 ft. (1.2m)
Max. insulation temperature: 260oC (500oF)*
Accuracy: ± 2.2oC (4oF) or ± 0.75% of reading (whichever is greater)
*Safety note: The supplied thermocouple can only be used to
measure temperature to 260oC (500oF). To measure higher
temperatures please obtain a thermocouple rated for higher
temperature.
Operating conditions
0 to 50oC (32 to 122oF); less than 80% RH
Storage conditions
-20 to 60oC (-4 to 140oF); less than 70% RH
Alarm beeper volume
95dB
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