smaller type to the right of the measurement. The possible units are degrees
Celsius (C), resistance in ohms (Ω), degrees Fahrenheit (F), Kelvin (K), and
degrees Rankine (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 lower part of the display has various functions depending on the selected
mode. It has smaller alphanumeric characters. 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 mode is easily changed using the MODE button. (See details on the vari
-
ous modes beginning with Section 7.)
The display has 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
button for three seconds (see Section 6.3 above). Note: The battery dis
-
charges more quickly when the backlight is used.
6.5 Probe
The probe is used to sense temperature. The probe attaches to the 1521 using a
Hart INFO-CON probe connector that plugs into the top of the instrument. The
probe connector must be properly programmed with the correct charac-
teristics of the probe for measurements to be accurate (see Section 6.6).
The 1521 can be used with various types of PRT and thermistor probes:
• ITS-90 calibrated 25Ω or 100Ω PRT
• IEC-751 or DIN-43760 PRT (RTD)
• Callendar-Van Dusen calibrated 100Ω PRT
•
YSI-400 series or equivalent 2252Ω thermistor
•
Steinhart-Hart thermistor polynomial; nominal R(25°C) 2kΩ to 100kΩ
See Section 7.8.8 for details on the various probe types and their programming.
The 1521 cannot be used with thermocouples.
The 1521 can be used with probes having two, three, or four wires. Three wires
allow partial compensation for wire resistance. The 1521 measures three-wire
probes using the following algorithm. It measures the sensor resistance, includ
-
ing the lead resistance in C2. The 1521 then directly measures the resistance in
the C1 lead of the sensor. The 1521 then subtracts the measured C1 lead resis
-
tance from the measured sensor resistance (including the C2 lead resistance) to
obtain the sensor resistance used to calculate temperature. Note: This algorithm
assumes that the 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 allow complete rejection of wire resistance and
should be used when the best accuracy is desired. The 1521 wire setting must
be set to match the actual number of wires of the probe (see Section 7.8.14).
1521 Handheld Thermometer Readout
User’s Guide
22
