User Interface

Display Type:240x64 graphics TFT LCD with LED back-light.
1x font is 4mm, 2x is 8mm.

Number of Inputs Displayed: One to Four.

Keypad: Sealed Silicon Rubber.

Temperature Display: Six significant digits, auto-ranged.

Display Update Rate: 0.5 Seconds.

Display Units: K, C, F or native sensor units.

Display Resolution: User selectable to seven significant digits.

Input Channels

There are four input channels, each of which may be independently configured for any of the supported sensor type.

Sensor Connection: 4-wire differential. DIN-6 Connector.

Sensor Types: See Supported Sensor Section.

Thermocouple: External option. Up to four supported.

Sensor Selection: Front Panel or remote interface.

Input Configurations: See input specifications table.

Bridge Modes: Constant-Current or Constant-Voltage.

Bridge type: Ratiometric resistance bridge.

AC Excitation Frequency: 7.5Hz bipolar square wave.

Voltage Excitations: 10mV to 100uV.
Minimum excitation current is 50nA.

DC Offset: <30nA.

Sample Rate: 15Hz per channel, all sensor types.

Digital Resolution: 24 bits.

Measurement Accuracy: See input specifications table.

Measurement Drift: 15ppm/°C.

Isolation: Input channel circuits are electrically isolated from all other internal circuitry but not from each other.

Measurement Filter: 0.5, 1, 2, 4, 8, 16, 32 and 64 Seconds.

Calibration Curves: Built-in curves for industry standard sensors plus eight user curves with up to 200 entries each. Interpolation is performed using a Cubic Spline.

CalGen™: Calibration curve generator fits any Diode or resistor sensor curve at 1, 2 or 3 user specified temperature points.

Status Outputs

Audible and Visual Alarms: Independent audible remote and visual alarms.

Relays: Two dry-contact relays. N.O. contacts available. Ratings are 125VAC @ 10A. Maximum switching power: 150W.

Status reported via Remote Interface: Sensor fault, Heater over temperature fault.

Control Outputs

Number of Independent Control Loops: Four.

Control Input: Any sensor input.

Loop Update Rate: 15Hz per loop.

Isolation: Control loop circuitry is electrically isolated from all other internal circuitry.

Control Type: PID table, Enhanced PID, Ramp or Manual.

Autotune: Minimum bandwidth PID loop design.

PID Tables: Six user PID tables available for storage of setpoint and heater range vs. PID and heater range. 16 entries/table.

Set-point Accuracy: Six+ significant digits.

Fault Monitors: Control loops are disconnected upon detection of a control sensor fault or excessive internal temperature.

Over Temperature Disconnect: Heater may be relay disconnected from user equipment when a specified temperature is exceeded on any selected input.

A unique feature of the Model 24C is the use of a ratiometric AC resistance bridge to measure all types of sensors.

The AC excitation used with all resistor sensors (including Platinum RTDs) is a 7.5Hz bipolar square wave. This effectively eliminates DC offset errors including the thermal EMF induced offsets that often occur in cryogenic systems.

Positive Temperature Coefficient (PTC) resistor sensors including Platinum and Rhodium-Iron RTDs use the resistance bridge in it's constant-current, AC operating mode. Platinum RTD sensors use a built-in DIN standard calibration curve that has been extended to 14K for cryogenic use.

The Model 24C's resistance bridge is used in a constant-voltage AC mode to provide robust support for the Negative Temperature Coefficient (NTC) sensors commonly used by cryogenic applications. They include Ruthenium-oxide, Carbon-Glass, Cernox™, Carbon Ceramic, Germanium and several others. Since these sensors have a negative temperature coefficient, the use of a constant-voltage measurement method will reduce, rather than increase, power dissipation in the sensor as temperature decreases. By maintaining the lowest possible power level, sensor self-heating is minimized and useful temperature range is greatly increased.