Manual Update
SPRT Measurements and ITS-90 Conversions
The International Temperature Scale of 1990 (ITS–90) is defined in terms of specific fixed
points to which temperature values (T
temperature calibration points selected for ITS–90 with their corresponding descriptions.
HP 34420A Revision 2.0 Changes
) have been assigned. Table 1 shows the
90
The Standard Platinum Resistance Thermometer (
Table 1. ITS-90 Temperature Calibration Fixed Points
Temperature (T90) Type Element
-259.3467 °C
-248.5939
-218.7916
-189.3442
-38.8344
+0.01
29.7646
156.5985
231.928
419.527
660.323
961.78
°C
°C
°C
°C
°C
°C
°C
°C
°C
°C
°C
Triple Point
Triple Point
Triple Point
Triple Point
Triple Point
Triple Point
Melting Point
Freezing Point
Freezing Point
Freezing Point
Freezing Point
Freezing Point
(H
(Ne)
(O
(Ar)
(Hg)
(H
(Ga)
(In)
(Sn)
(Zn)
(Al)
(Ag)
2
2
2
)
)
O)
Hydrogen
Neon
Oxygen
Argon
Mercury
Water
Gallium
Indium
Tin
Zinc
Aluminum
Silver
SPRT) is the defining standard for
temperature interpolation between the fixed temperature calibration points listed above.
The sensing element of an
An
(+0.01
SPRT normally exhibits a nominal resistance (R
°C).
SPRT varies resistance with temperature in a known manner.
) of 25.5Ω at the triple point of water
0
The HP 34420A (starting with firmware revision 2.0) incorporates built-in support for
precision temperature measurements using
SPRT transducers. An SPRT resistance
measurement is performed using 1.0 mA of current and offset-compensation — this is a
low-frequency (approximately 5 Hz) ac measurement technique which removes thermal
offsets in the measurement.
Note: When making direct resistance measurements of an
SPRT, make sure you use the
HP 34420A’s low-power resistance mode (use the SENS:FRES:POW:LIM ON command).
1
Calibration values for an SPRT are normally expressed as a ratio, W(T90), of the probe
resistance at some temperature (T
triple point of water. An
SPRT must be constructed of pure platinum and must be strain free.
) divided by the nominal probe resistance (R0) at the
90
The finished probe must meet the following resistance ratio requirements to be acceptable for
ITS-90 calibration and use:
W(+29.7646
°C) ≥ 1.11807 and W(–38.8344 °C) ≤ 0.844235
This is equivalent to a requirement that the alpha coefficient (average normalized temperature
coefficient of resistance) meets the following: α ≥ 0.003986 Ω / Ω / °C from 0
A single
are generally usable over the range of approximately –250
SPRTs are generally usable over the range of approximately –200 °C to +660 °C. Often the outer
sheath of an
SPRT cannot be used over the entire ITS-90 temperature range. Capsule-type SPRTs
°C to +200 °C. Long-stem type
SPRT will limit its high-temperature measuring range due to leakage effects
°C to +100 °C.
shunting the resistive measuring element.
The ITS-90 standard defines reference functions and calibration deviation functions which
precisely describe an
functions are made up of 9th order and 15th order polynomials which describe the W(T
SPRT’s resistance variation with temperature. The ITS-90 reference
90
)
resistance ratio variation for temperatures above and below the triple point of water,
respectively. In addition, ITS-90 deviation function polynomials are used to correct, or
calibrate, a particular probe’s response over a specified sub-range of temperatures
(for example, from 0
°C to the freezing point of aluminum, +660.323 °C). The following
sections in this manual list the standard ITS-90 temperature sub-ranges and their
corresponding deviation function calibration coefficients.
The conversion routines built into the HP 34420A implement the ITS-90 temperature
conversion equations directly for sub-range 4 and sub-range 7, covering the calibrated
temperature range of –189.3442
°C to +660.323 °C. The conversion routines allow you
to directly enter the following calibration constants:
Table 2. HP 34420A SPRT Calibration Constants
Constant
R
0
A4, B
4
A7, B7, C
7
Description
Triple Point of Water Probe Resistance Value
Sub-Range 4 Calibration Coefficients
Sub-Range 7 Calibration Coefficients
Temperature can be displayed with units of
°C, °F, or K.
2
Calibration Coefficient Substitutions
You can substitute probe calibration data from other temperature sub-ranges for the
sub-range 4 and sub-range 7 calibration coefficients as noted in the following sections.
You can enter sub-range 2 or sub-range 3 calibration coefficients in place of sub-range 4
coefficients by applying the following substitutions (see also Table 3):
A
The calibrated measuring range will be limited to that of sub-range 4 (–189.3442
to +0.01
Table 3. ITS-90 Sub-Ranges for Temperatures Below the Triple Point (TP) of Water
= A2, B4 = B2 or A4 = A3, B4 = B3 (Ignore the “C” coefficients)
4
°C) regardless of which sub-range coefficients are used.
Temperature Range
Sub-Range 2: -248.5939 °C to +0.01 °C
Sub-Range 3: -218.7916
Sub-Range 4: -189.3442
°C to +0.01 °C
°C to +0.01 °C
Fixed Points
TP of Neon to TP of Water
TP of Oxygen to TP of Water
TP of Argon to TP of Water
Calibration Coefficients
A2, B2, C1, C2, C
A3, B3, C
A4, B
°C
3
1
4
You can enter sub-range 6 calibration coefficients in place of sub-range 7 coefficients by
applying the following substitutions (see also Table 4):
A
The calibrated measuring range will be limited to that of sub-range 7 (0
= A6, B7 = B6, C7 = C6 (Ignore the “D” coefficient)
7
°C to +660.323 °C).
For sub-ranges 8 and 9, use the following substitutions (see also Table 4):
A
= A8, B7 = B8 or A7 = A9, B7 = B9 (C7 = 0 for both)
7
For sub-ranges 10 and 11, use the following substitutions (see also Table 4):
A
= A10 or A7 = A11 (B7 = 0 and C7 = 0 for both)
7
The calibrated measuring range will be limited to that of the sub-range whose coefficients
you entered. For example, if you used sub-range 10 coefficients, then you would enter A
= 0, and C
B
7
+156.5985
= 0. The resulting calibrated measuring range would extend from 0 °C to
7
°C (the freezing point of Indium).
= A10,
7
3
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