Thermistor

The Thermistor instruction is used to make a bridge measurement and from that measurement, return temperature in degrees C or resistance in Ohms.

Syntax

Thermistor ( Dest, Reps, Range, UChan, ExmV, RevEx, SettlingTime, fN1, Therm_A [optional], Therm_B [optional], Therm_C [optional] )

Remarks

The Thermistor instruction takes advantage of the datalogger’s built in 5 kOhm resistor to make a bridge measurement. This instruction can be used to measure the thermistor for a vibrating wire separately from the VibratingWire instruction. A, B, and C are the Steinhart-Hart coefficients used to derive temperature in degrees C. These parameters are optional – if omitted, the output returned is in Ohms.

Parameters

Dest (Destination)

The Variable in which to store the results of the instruction. Right-click the parameter to display a list of defined variables.

If this instruction has a Repetitions parameter and it is greater than 1, the results are stored in an array with the variable name. The array must be dimensioned large enough to hold all of the values returned from all of the Reps.

Type: Variable or Array

Reps (Repetitions)

The number of repetitions for the measurement or instruction.

Type: Constant integer (or expression that evaluates as a constant).

For the Thermistor instruction, measurements are made on consecutive channels. If the Reps parameter is greater than 1, the Dest parameter must be a variable array.

Range

The voltage range for the measurement or input sensor. An alphanumeric code is entered. Right-click the parameter to display a list or enter the code directly.

Code	Description
mV5000	+5000 mV
mV1000	+1000 mV
mv200	+200 mV
Autorange	Datalogger tests for and uses most suitable range.
AutorangeC	Autorange, checks for open input.
mV5000C	+5000 mV, checks for open input, sets excitation to ~8000 mV. Overrange values may go undetected; use code to detect overrange values.
mV1000C	+1000 mV, checks for open input, sets excitation to ~1250 mV.
mV200C	+200 mV, checks for open input, sets excitation to ~1250 mV.

Autorange increases the time to make the measurement. For signals that do not fluctuate too rapidly, AutoRange allows the datalogger to automatically choose the input voltage range. AutoRange results in two voltage measurements being performed. The first voltage measurement is done quickly at a first notch frequency (f_N1) of 50 kHz, the result of which determines the input voltage range to use for the second measurement. The second measurement is then performed using the range determined from the first measurement, along with the f_N1 chosen for the measurement instruction. Both measurements use the settling time chosen for the particular measurement instruction. Auto-ranging optimizes resolution but takes longer than a fixed range measurement because of the two-measurement sequence. The exception to this two-measurement sequence is if Reps are made on the same channel (Reps parameter is negative). In this instance, the test measurement is made only once. Subsequent repetitions are made with the delay between each measurement being the specified settling time. Autorange should not be used if fast measurements are required or if the analog signal could change significantly over the course of the measurement.

The C options check for an open connection on the analog input by applying a short overranging test signal to the analog input prior to making the measurement. For a voltage range of mv5000C, 8V is applied. For a voltage range of mv1000C or mv200C, 1.25V is applied. An open (broken) sensor will result in a measurement overrange, clearly indicating a sensor problem instead of returning a bad measured value caused by a floating input.

The open-detect option may not work properly in the presence of external leakage (< 1 MOhm) to ground, as the overrange test signal could discharge through the external leakage during the input settling time such that an overrange condition no longer exists. In this case the measurement settling time should be minimized to minimize the discharge time of the overrange test signal.

The open circuit detection (C option) can cause measurement errors for slow responding sensors, as such sensors may not have sufficient time to recover from the applied short duration (50 microseconds) test signal. For such sensors, increasing the measurement settling time beyond the default may be necessary for sufficient sensor recovery time. If measurement speed is critical with a slow responding sensor, then it may be preferable to not use the open detect (C option).

Type: Constant

DiffChan (Differential Channel)

Specifies the number of the universal terminal pair on which to make the first measurement. If the Reps parameter is greater than 1, additional measurements will be made on sequential channels.

If the DiffChan number is entered as a negative value, all Reps are performed on the same channel (burst measurement). The CR6 burst measurement sampling frequency is determined by the f_N1 (first notch frequency) parameter, which can go up to a maximum of 93750 Hz (minimum sample interval of 10.667 uS). The SettlingTime parameter is used to delay once prior to beginning the burst. The total time prior to beginning the burst is the SettlingTime plus the ADC flush time (which is 810uS). The sample interval resolution is 1/93750Hz. The specified notch frequency will use the nearest multiple of (1/93750Hz) to get as close to the specified frequency as possible.

Code	Description
U1	Universal Terminals 1 and 2 (U1 = high/U2 = low)
U3	Universal Terminals 3 and 4 (U3 = high/U4 = low)
U5	Universal Terminals 5 and 6 (U5 = high/U6 = low)
U7	Universal Terminals 7 and 8 (U7 = high/U8 = low)
U9	Universal Terminals 9 and 10 (U9 = high/U10 = low)
U11	Universal Terminals 11 and 12 (U11 = high/U12 = low)

Type: Constant

ExmV (Excitation Voltage in mV)

The excitation, in millivolts, to apply to the excitation channel. The allowable range is ±2500 mV. For ExciteV() only, with DiffEx set to 1, the allowable range is ±5000 mV.

Type: Constant. For ExciteV() only, this parameter can also be a Variable.

RevEx (Reverse Excitation)

A constant is entered for the RevEx argument to determine whether the excitation should be reversed and applied to the sensor after the first measurement is made. False (or 0) = Do not reverse excitation ; True (or 1) = Reverse excitation and make a second measurement (requires more time to complete). Use of RevEx cancels out voltage offsets due to the sensor, wiring, and excitation circuitry, but does not compensate for operational input voltage errors.

Type: Constant

Settling Time

The settling time is the duration (in microseconds) to allow for signal settling after setting up a measurement (switching to the channel, setting the excitation) and before making the measurement. Minimum settling time is 20 microseconds; maximum settling time is 600 ms. If 0 is entered, a default of 500 microseconds is used.

Additional settling time may be necessary to allow the signal to settle with high resistance or long lead lengths (higher capacitance). The time it will take to make the measurement will include the measurement itself, the SettlingTime, f_N1, and whether or not parameters are set to remove voltage offset errors. Using either RevDiff or RevEx causes two SettlingTimes to occur per channel; four SettlingTimes will occur when using both RevDiff and RevEx.

Type: Constant (or expression that evaluates as a constant)

f_N1

Determines the lowest frequency that will be eliminated or notched out by the sinc filter. This filter notches out frequencies at integer multiples of f_N1 by averaging for a time equal to 1/f_N1; thus, lower f_N1 frequencies result in longer measurement times. Set f_N1 to the lowest frequency that should be filtered out. For example, setting f_N1 to 60 filters all multiples of 60 and above (60, 120, 180, etc.) but will not effectively filter lower frequencies; for example 30 Hz. Any value between 5 Hz and 93.75 kHz can be entered, but common options for filtering noise are:

Option	Description
15000	Performs a 0.0667 millisecond integration (for fast measurements)
60 (or _60Hz)	Performs a 16.67 millisecond integration (filters 60 Hz noise)
50 (or _50Hz)	Performs a 20 millisecond integration (filters 50 Hz noise)

Type: Constant

Optional Parameters

Therm_A, Therm_B, Therm_C

Optional parameters used to enter the Steinhart-Hart thermistor coefficients. If one is present they must all be specified. When these coefficients are entered, the thermistor value will be returned as degrees C. If the coefficients are not entered, the result will be returned as resistance in Ohms.

Note that for the VibratingWire instruction, Therm_fN1must be specified in order for the thermistor to be measured.

Type: Constant