EN
36
WIKA operating instructions, model CTR3000
14191761.01 05/2019 EN/DE
6. Commissioning, operation
■
Probe type
Is the thermometer a Pt25 or Pt100?
■
Wiring
Will be the connection a 3-wire or 4-wire?
■
Keep warm current
The instrument has the facility to drive deselected resistors or PRTs from a constant DC current source. This allows them to
be kept at their normal working temperature, and so decrease settling time for each channel.
The keep warm current is user selectable as ON/OFF. If set on, the CTR3000 automatically adjusts the PRT current (from
1 mA for the 100 Ω reference) to 2 mA for the 25 Ω reference.
■
Int. resistor
The CTR3000 allows the choice between two internal reference resistors (25 Ω and 100 Ω) or the Auto-function. The 25 Ω
resistor should only be used when a 25 Ω PRT (or lower) is used. Use the 100 Ω reference resistor for all probes with R0
values above 25 Ω.
The menu allows the use of Auto selection. The auto selection looks at the R0 value of the probe. If R0 is below 50 Ω, the
25 Ω reference is selected; any R0 value equal to or above 50 Ω will use the 100 Ω reference.
The CTR3000 automatically adjusts the PRT current (from 1 mA for the 100 Ω reference) to 2 mA for the 25 Ω reference.
■
T
min
Probes may have a minimum temperature set for them (positive or negative). When the probe is assigned to a channel, the
probe temperature is checked against the set minimum and a range error (exclamation mark left beside the button
13
)
issued if the minimum temperature is exceed.
■
T
max
Probes may have a maximum temperature set for them (positive or negative). When the probe is measured and active on
a channel, the probe temperature is checked against the set maximum and a range error (exclamation mark left beside the
button
13
) issued if the maximum temperature is exceed.
■
Last cal date
Please enter the last calibration date of the thermometer.
If the system date = last cal date + 1 year then a pop up is coming up which reminds that the probe is due to recalibration.
The recalibration cycle depends mainly on the thermal stress on the thermometer. The calibration time can only
be estimated and is decided by the user. Therefore check the calibration system at the triple point or freezing
point of water on a regular basis (independent of the recalibration cycle). Recommendation is a calibration once a
year.
■
Conversion
PRTs and SPRTs can be calibrated individually (resistance-temperature characteristics determined) in order to achieve
low uncertainties. There are two algorithms used to represent the thermometer’s characteristic (the Callendar-van-Dusen
equation and the ITS-90 equations). The Callendar-van-Dusen equation was developed first and was used as the primary
conversion algorithm for all PRTs up until 1990. It is still applicable for PRTs and SPRT, although better uncertainty can be
achieved by using the ITS-90 equations (particularly important when using higher accuracy SPRTs).
In 1990, the International Temperature Scale was revised and a new set of equations were defined for converting the
resistance of a PRT to temperature. These equations (commonly referred to as ITS-90) comprise a nominal conversion that
represent the average conversion characteristic and deviation functions that provide the adjustment for the characteristics
of the individual thermometer. They are intended for use with high-purity platinum and provide a better fit than can be
achieved with the older Callendar-van-Dusen equation.
The ITS-90 equations are sometimes also used with industrial PRTs made using the lower sensitivity 0.00385 K-
1
wire and
can provide a small improvement in uncertainty compared with the Callendar-van-Dusen equation.
