1–2 Getting Started
1.1 Introduction to the Instrument
The SIM923 Pt RTD Monitor monitors up to four (4) resistive tem-
perature detectors (RTDs) using a stable 1 mA DC current excitation.
1.1.1 Overview
The SIM923 uses a ratiometric design, where the excitation current
is passed through both an internal, stable reference resistor and the
user’s resistive thermometer. The instrument calculates the ratio
of the voltage across the user’s resistor to the voltage across the
reference resistor, multiplied by the (known) value of the reference
resistor, to obtain the user result.
Each channel has an independent current source and reference re-
sistor, so the excitation to the user’s sensor remains steady as the
SIM923 cycles between measurements. Disabling a channel switches
that channel’s current source off.
A precision 24–bit analog-to-digital converter is cycled between the
channels with a rate of 4 conversions per second. With all channels
enabled, a complete cycle is completed every second; disabling some
channels yields a corresponding increase in overall cycle rate.
1.2 Front-Panel Operation
The front panel of the SIM923(see Figure 1.1) provides a simple op-
erator interface.
1.2.1 Excitation
The four channels of the SIM923 Pt RTD Monitor can be indepen-
dently enabled or disabled from the front panel. Disabling a channel
turns offthe 1 mA excitation current to that sensor, and speeds the
readout rate for the remaining (enabled) channels.
To toggle the excitation for a channel, first briefly press [Excitation].
One of the four channel displays will appear highlighted, indicating
which channel is selected. Briefly tap [Excitation] to advance the
highlighted selection until the desired channel is selected. Then
depress and hold [Excitation] until the display switches between the
numerical value and the word .
1.2.2 Units
The SIM923 displays the sensor results either as resistance (in ohms)
or temperature (in kelvin). Pressing [Units] toggles between these
two modes; the active units are indicated by the illuminated Kor Ω.
SIM923 Pt RTD Monitor
