smallertype to the right of the measurement. The possible units are degrees
Celsius (C), resistance in ohms (Ω), degrees Fahrenheit(F), Kelvin (K), and
degreesRankine (R). The units can be easily changed by placing the display in
the units setting mode using the MODE button (see Section 7.5).
The lowerpart of the display has various functions depending on the selected
mode. It has smaller alphanumericcharacters. It can be used to display mini-
mum and maximum measurements, delta(x) measurements, or measurements
stored in memory.It is also used to view and set various operating parameters.
The modeis easily changed using the MODE button. (See details on the vari-
ous modes beginningwith Section 7.)
The displayhas a backlight that can be switched on for better viewing in dim
light.The backlight is switched on and off by pressing and holding the power
buttonfor three seconds (see Section 6.3 above). Note: The battery dis-
chargesmore quickly when the backlight is used.
6.5 Probe
The probe is used to sense temperature. The probe attachesto the 1521 using a
Hart INFO-CON probe connectorthat plugs into the top of the instrument. The
probeconnector must be properly programmed with the correct charac-
teristics of the probefor measurements to be accurate (see Section 6.6).
The 1521 can be used withvarious types of PRT and thermistor probes:
•ITS-90 calibrated 25Ωor 100ΩPRT
•IEC-751 or DIN-43760 PRT (RTD)
•Callendar-VanDusen calibrated 100ΩPRT
•YSI-400 series or equivalent 2252Ωthermistor
•Steinhart-Hart thermistor polynomial; nominal R(25°C) 2kΩto 100kΩ
See Section 7.8.8 for detailson thevarious probe typesand theirprogramming.
The 1521 cannotbe used with thermocouples.
The 1521 can be used withprobes having two, three, or four wires. Three wires
allowpartial compensation for wire resistance. The 1521 measures three-wire
probes using the followingalgorithm. It measures the sensor resistance, includ-
ing the leadresistance in C2. The 1521 then directly measures the resistance in
the C1 leadof the sensor. The 1521 then subtracts the measured C1 lead resis-
tancefrom themeasured sensor resistance (including the C2 lead resistance) to
obtain the sensor resistanceused to calcu latetemp erature. Note: This algorithm
assumes thatthe lead resistance of C1 and C2 are equal. If C1 and C2 have dif-
ferent resistance there will be an error in the calculated sensor resistance equal
to this difference. Four wires allowcom plete rejection of wire resistance and
should be used when the bestaccuracy is desired. The 1521 wire setting must
be set to match the actualnumber o f wires of the probe (see Section 7.8.14).
1521 Handheld Thermometer Readout
User’s Guide
22