Principles Of Measurement; Prt Measurement - Isotech TTI-7-R Operator's Handbook Manual

Introduction

1.3 Principles of measurement

1.3.1 PRT measurement

The TTI-7-R measures the voltage (V
sensor resistance (R
reference resistance (R
current. The voltages are in proportion to the resistances so the
thermometer resistance is derived from: R
This technique achieves immunity from slow moving time and
temperature drift in the electronics as it is not affected by voltage
measurement gain variations or current source fluctuations.
In the same way that AC resistance measurement eliminates thermal
EMFs, switched DC achieves a similar advantage. Switched DC
works by reversing the current flow on alternate measurement cycles
and taking the average value, thereby cancelling any thermal EMF
offsets from the measurement.
For PRTs, the relationship between resistance and temperature varies
slightly from one PRT to another. Therefore, no matter how accurately
the TTI-7-R measures the PRT resistance, if the relationship between
resistance and temperature for a particular PRT is not known,
accurate temperature measurement is not possible.
The TTI-7-R uses PRT calibration data to overcome this problem and
calculates temperature from temperature conversion functions stored
in internal memory. This method enables the TTI-7-R accurately to
convert resistance to temperature, uniquely for each PRT used. It is
very important therefore that a PRT is used on the correct and
properly configured input channel.
The system accuracy is a combination of the TTI-7-R accuracy in
measuring PRT resistance and the calibration uncertainty placed on
the PRTs by the calibrating laboratory. Using the TTI-7-R with PRT
type 935-14-61, this is ±0.020°C for temperatures from -80°C to
+350°C. See section 9.1 for PRT measurement performance details.
) developed across the unknown
t
) and the voltage (V
t s
) connected in series and passing the same
s
) across a stable internal
= R x V / V
t s t s
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