Rhodium-iron resistance thermometer with fused-silica coil frame

R h o d i u m - i r o n resistance thermometer with fused-silica coil frame O. Tamura and H. Sakurai National Research Laboratory of Metrology, 1-4, Umezono 1-chome, Tsukuba, Ibaraki 305, Japan

Received 28 February 1991 The dependences of the thermometric properties of a rhodium - iron (Rh - Fe) (mole fraction 0.5%) wire resistance thermometer on the annealing temperature between 7 0 0 and 9 0 0 ° C are investigated using fused-silica coil frames. The thermometers annealed at or above 8 0 0 ° C have similar t e m p e r a t u r e - resistance characteristics. These thermometers can be calibrated with an accuracy of 0.5 mK over the range 4 . 2 - 25 K using a reference temperature - resistance function and three calibration points. Rh - Fe wire thermometers from different origins can be calibrated with an accuracy of the order of 10 mK in the same way. The temperature rise of the present thermometers due to heating by the measuring current is in a reasonable range for thermometry with a millikelvin accuracy.

Keywords: thermometry; resistance thermometer; rhodium-iron

Since the rhodium- iron ( R h - Fe) (mole fraction 0.5 %) wire resistance thermometer was proposed as one of the most reliable low temperature thermometers ~, various types of R h - F e resistance thermometers including a thin film type have been used from millikelvin regions up to room temperature. Among these thermometers the Rusby type R h - F e thermometer2 has been the most stable commercially available one and has been used for maintaining temperature standards, especially below 20 K, where platinum resistance thermometers have no significant sensitivities. This thermometer has a structure similar to the Barber type platinum wire resistance thermometer, in which the coil of sensing wire is mounted inside glass tubes in a platinum sheath ~. The annealing treatment after coil-forming is known to be one of the most important factors in the construction of stable thermometers, especially thermometers with metal sensing wires. The effects of annealing on R h - F e thin film resistance thermometers have been reported 3, but there is only a limited amount of information available regarding R h - F e wire thermometers. An alternative type of structure for R h - F e wire thermometers is described and the effects of annealing on the thermometers are reported in this work. In the new thermometer the sensing wire is wound around a crossshaped frame machined from a fused-silica rod. Almost the same structure has been adopted for coil frames in some standard platinum resistance thermometers, the socalled Meyers type 4'5. Fused silica is adopted as the frame material to enable the sensing wire to be annealed at higher temperatures. How the annealing temperature influences the resistance is investigated. The thermometers fabricated in the present work are compared with commercial R h - F e thermometers with

respect to temperature-resistance characteristics and self-heating effects. A simple, reasonable calibration method is also proposed for cryogenic use of the thermometers.

Fabrication The original Meyers type thermometer used a mica coil frame. In the present thermometers fused silica is adopted as the frame material to enable the sensing elements to be annealed at temperatures above 600°C. Fused silica is more suitable than mica for annealing at high temperatures from the viewpoint of contamination of the sensing wire. The coil frame has a cross-shaped cross-section, as shown in Figure 1. Its width is 4.5 mm and its length 44 mm. This format is obtained by modifying the coil frame of a high temperature standard platinum resistance thermometer (HTSPRT) commercially available from Chino Corporation. The coil frame of the HTSPRT is similar to that previously reported 5. The

_J Figure 1 Structure of present Rh - F e (mole fraction 0. 5 %) wire thermometer. The sheath and the four leads are made of platinum and the coil frame is made of fused silica. The sheath has a diameter of 5 turn and length of 45 mm

0011 - 2 2 7 5 / 9 1 / 1 0 0 8 6 9 - 0 5 © 1991 B u t t e r w o r t h - Heinemann Ltd

Cryogenics 1991 Vol 31 October

869

Rh-Fe resistance thermometer: O. Tamura and H. Sakurai Table 1 Mass fractions of chemical impurities in the wire (measured in ppm except for Fe, according to optical emission and absorption spectrography by the wire supplier, Engelhard Industries Ltd) Pt 75 AI < 10 P b < 5 Si < 10 Rh dominant

Pd < 10 Ag 40

Cu < 10 Mg < 10

Sn < 1 Cr < 5

Ca < 10 Fe0.28%

o

1.000

700 %

0.998

800 %

850 °C

o

~ 0.996 1 0 0 0 °C9 0 0 °C

main difference between the present coil frame and that described in Reference 5 is the pitch of the notches for supporting the sensing wire. Also, the coil frame is now machined from a single fused-silica rod. A R h - F e (mole fraction 0.5%) wire with a diameter of 50/zm was used as the sensing element. The analysis of chemical impurities from the supplier of the wire (Engelhard Industries Ltd) is given in Table 1. The 0.5 % mole fraction of iron is the same as in the commercial R h - F e thermometers. Four platinum wires were welded to the R h - F e wire as current and voltage leads. The sensing wire was wound bifilarly around the coil frame, as shown in Figure 1. The sensing part, which consists of the coil frame, the sensing wire and the four lead wires, was first inserted in a silica glass tube for cleaning and annealing treatment. The sensing part was washed with steam for = 50 h in the glass tube. The inside of the glass tube was evacuated with an oil-diffusion pump for > 8 h and then filled with pure helium gas at = 30 kPa. The part was then annealed as follows. The sensing part in the glass tube was heated up from room temperature at a rate of 50°C h-1 using a furnace and then held at constant temperature for 20, 30 or 60 min. It was then cooled down and pulled out of the furnace after the temperature fell back below 500°C. The inside of the glass tube was evacuated for several hours and then purged with helium gas at = 30 kPa. This purging process was repeated several times. The resistance of the sensing wire was then measured at the temperature of the triple point of water with an a.c. bridge (ASL model 17). These annealing processes were repeated until the resistance change during an annealing cycle became smaller than a few per cent. The sensing part was then pulled out of the silica glass tube and inserted in a platinum sheath with a diameter of 5 mm and length of 45 mm. The inside of the sheath was filled with pure helium gas at =30 kPa as the medium for heat exchange. The sensing element was sealed hermetically by keeping the electrical insulation between the four platinum leads, in a way similar to that used for capsule type standard platinum resistance thermometers. Effect

of annealing

on resistance

Annealing temperatures of 700, 800, 850 and 900°C were chosen, since data concerning annealing effects were available only for annealing temperatures of 7006 and 750°C t . Figure 2 shows the resistances at the triple point of water during the annealing treatment. After 1 h of annealing, the resistance of the sensing wire annealed at 1000°C became unstable as a thermometer. This is the same result as was previously described ~ and indicates

870

Cryogenics

1991

V o l 31 O c t o b e r

~

O. 994 I

0

60

I

I

120

180

2L(

Ti me/rni n Figure 2 Resistance changes of Rh - Fe wires at the temperature of the triple point of water during annealing treatment. The resistances were measured at 20, 30 or 60 min and are represented as the ratio to the initial resistance before annealing. The temperature value indicates the annealing temperature for each wire

that 1000°C is too high for the annealing temperature. Only during the initial phase of the annealing process did the resistances of the sensing elements annealed at 700-900°C change rapidly; they then become stable. Figure 2 indicates that 2 h is necessary and sufficient for the duration of annealing. After annealing and encapsulation, the resistances of the thermometers were measured from 3.5 up to 373 K with a d.c. current comparator bridge (Guildline model 9975). Temperatures were measured with a standard platinum resistance thermometer and a R h - F e resistance thermometer calibrated to the International Temperature Scale of 1990. The thermometers fabricated in the present work are hereafter referred to according to their annealing temperatures, as shown in Table 2. For the purpose of comparison, two R h - F e wire resistance thermometers manufactured by H. Tinsley & Co. Ltd, referred to as 22148 and B152, respectively, were also measured. The former is an original Rusby type thermometer I and the latter a fully commercial manufactured version. Figure 3 shows the results of the measurements of the temperature dependence of resistance, where W(T) is the ratio of the resistance at temperature T to the resistance at the temperature of the triple point of water. The present thermometers have characteristics similar to those

Table 2

List of thermometers tested and annealing temperatures

Annealing Resistance temperature at 273.16 K Thermometer (°C) (~2) Source NRLM700 NRLM800 NRLM850 NRLM900 NRLM 1000 22148 B152

700 800 850 900 1000 -

6 16 16 16 Broken 97 99

NRLM NRLM NRLM NRLM NRLM H. Tinsley & Co. Ltd H. Tinsley & Co. Ltd

R h - F e resistance thermometer: O. Tamura and H. Sakurai

resistance indicates that 700°C is not a high enough temperature to release the strain induced in the R h - F e wires during coil-forming. The independence of residual resistance with respect to annealing temperatures _>800°C indicates that 800°C is sufficient as the annealing temperature. Above this temperature the annealing effect is not improved significantly. Figure 5 shows the temperature dependence of the fractional thermometric sensitivities of the resistances, (dW/dT)/W.

1.5

1.0

0.5 J

0.0

'

'

'

100

0

B152

'

200 Temperature/%

300

400

Temperature dependence of resistance of the thermometers below 100°C. Resistances are represented by the ratio, W, of the resistance, R, to that at the temperature of the triple point of water [W = R/R(O.01°C)] Figure 3

0.14

.

.

.

.

,

.

.

.

.

,

•

.

0.12

0.o8

0.06

~

~l-f

.

O0

NRLM800

~ •

22148

#~f

'

[3152

.

.

.

~

10

.

.

.

.

'

20

.

.

.

.

method

A~ In

+ 9

k=0

NRLM900

•, ' - ~ /

calibration

As pointed out before 2, more than 10 temperature reference points are necessary to fit a function to the temperature- resistance dependence of the R h - Fe thermometers to give an accuracy within a few millikelvin. But only a few temperature fixed points are available below 25 K. Therefore the calibration method is often the most troublesome factor for users of R h - Fe thermometers. One solution is to use a deviation function with a few reference points as described below. An expression with the form 2

W=

0.10 f~-~.~

Simple

30

Temperature/K Figure 4 Temperature dependence of resistance of the thermometers below 30 K [W = R/R(O.01°C)]

is fitted to the measured W(T) values of thermometer NRLM850 using the least mean square method. The function obtained in this way is selected as a reference function, Wr~f(T), for approximating the W(T) characteristics of the other thermometers. The coefficients a0 and al in the deviation function AW(T) = ao + al W~ef(T)

from H. Tinsley & Co. Ltd above 30 K. Figure 4 shows the dependence of resistance on temperature more precisely below 30 K. At temperatures below 30 K, both the difference from the commercial thermometers and the variation between thermometers with different annealing temperatures become more apparent. The thermometers annealed at 800, 850 and 900°C have nearly the same temperature-resistance relations. Only the thermometer annealed at 700°C shows a different pattern, its residual resistance being higher than that of the other thermometers. Its relatively high residual

0.10

A

F--

+ ---o--~ ~ '

.~ ~ "k~ ' ~

NRLM700 NRLM800 NRLM850 NRLM900

o

.

.

.

.

~

10

.

.

.

.

~

20

22148

.

.

.

.

30

Tompemture/K Figure 5

Fractional thermometric sensitivity of resistance of the

thermometers as a function of temperature [(dW/dT)/W]

Wnt(T) = ao + (al + 1)Wref(T) The residual deviation, W(T) - Wnt(T), is converted to the equivalent errors in the temperature measurement based on Wnt(T) and is shown in Figure 6 for each thermometer. For more precise calibrations AW(T) is assumed to have the form AW(T) = ao + at

0.05

0.0(

are determined by equating A W to the difference between the measured W(T) of each thermometer and Wref(T) at two reference points, 4.2 and 24.5 K. In this way the measured W(T) values of each thermometer are approximated by

Wref(T)

"[-

a: ln(T/K + 9)

The coefficients a0, al and a2 are determined from the measured W at three reference points, 4.2, 13.8 and 24.5 K. Then the errors in temperature measurement based on Weir(T) are as shown in Figure 7. Even using a two-point calibration, Wf~t(T) describes the W - T characteristics with an accuracy better than 5 mK for the present thermometers annealed at 800, 850 and 900°C; indeed better than 0.5 mK for the thermometers annealed at 850 and 900°C. But the characteristics of the present thermometer annealed at 700°C cannot be approximated so accurately by this

Cryogenics

1 9 9 1 V o l 31 O c t o b e r

871

R h - F e resistance thermometer: O. Tamura and H. Sakurai

0.0010

---o---

0.04

A

0.02

t~

NRLM800 NRLM850 NRLM900

~

¢-

O~

=,

0.00

-0.02

O~ 03

,

0

,

,

,

i

,

10

,

,

,

i

Temperature/K

.

.

.

.

20

30

0

.

~

0

.

.

.

.

~

10

.

.

.

.

~

20

.

.

.

.

~

30

'

TemperatureJK

Figure 6 Errors in temperature measurement using a reference function, Wref(T), and a deviation function calibrated at t w o reference points of temperature, 4.2 and 24.5 K. Each symbol represents the same thermometer as in Figure 5

Figure 8

Temperature rise of the thermometers due to heating by measuring current. The current is 0.3 mA for B152 and 1 mA for the NRLM thermometers

batches. The three reference temperatures can be realized easily, for example using the vapour pressure point of helium with normal pressure, the triple point of equilibrium hydrogen and the triple point of neon.

0.01

0.00

Self-heating

~q

-0.01

-0.02

i

.

,

,

,

i

,

10 20 Temperature/K

,

,

,

30

Figure 7

Errors in temperature measurement using a reference function, Wref(T), and a deviation function calibrated at three reference points of temperature, 4.2, 13.8 and 24.5 K. Each symbol represents the same thermometer as in Figure 5

method. This indicates that the present thermometers have quite similar characteristics if annealed at or above 800°C. The characteristics of the two Tinsley thermometers have larger discrepancies from Writ(T) using the two- or three-point calibration. The three-point calibration reduces the residual deviation to <0.5 mK for the present thermometers annealed at or above 800°C. The maximum residual deviation using this method is 13 mK for all thermometers, irrespective of their different origins. The three thermometers with larger residual deviation show similar behaviour in Figure 7. They are the present thermometer annealed at 700°C and the two Tinsley thermometers. The Tinsley thermometers are supposed to have been annealed at a temperature below 800°C after coil-forming2. These three thermometers have similar characteristics in spite of their different origins in terms of batch of wire used. These results suggest that annealing temperature as well as the difference in batch of wire used has a dominant effect on the temperature-resistance characteristics. In general the R h - F e thermometer can be used for thermometry with an accuracy better than ~ 10 mK using the calibration method described above, if a reference function such as Wref(T) is determined using a reasonable number of thermometers from different

872

Cryogenics 1991 Vol 31 October

The rise in temperature of the thermometer due to heating by the measuring current below 35 K is shown in Figure 8. For the present thermometers 1 mA was adopted as the measuring current and for the commercial thermometer 0.3 mA was used. The self-heating values were evaluated from the temperature increases per unit of power dissipated, which were calculated from the differences between the measured data at two measuring currents. All the thermometers tested here have almost the same conditions with respect to thermal contact to a temperature-regulated copper block. Under these conditions the temperature rise of the present thermometers due to self-heating was found to be reasonable in magnitude for thermometry with an accuracy of 1 mK. It should be noted for Figure 8 that the power dissipation per unit length of sensing wire in the present thermometers was about 11 times that in the commercial thermometer. Nevertheless self-heating in the present thermometers was less than half that in the commercial one.

Conclusions The dependence of the thermometric properties of R h - F e thermometers on annealing temperature between 700 and 900°C was determined using a fusedsilica coil frame. An annealing temperature of 800°C is necessary and sufficient to remove strains introduced into the sensing wires during coil-forming. The temperature-resistance characteristics are determined by the conditions of the annealing treatment as well as the difference in batch of the sensing wire. The present fabricated thermometers annealed at or above 800°C have very similar temperature-resistance characteristics. These thermometers can be used for thermometry with an accuracy better than 0.5 mK by calibrating the deviation from a reference function at only three reference points. Even other thermometers manufactured independently can be used for ther-

Rh-Fe resistance thermometer: O. Tamura and H. Sakurai

mometry with an accuracy of 13 mK using the same calibration procedure based on the same reference function. It is concluded that the construction of a reliable reference function for commercial R h - F e thermometers is necessary for those wishing to use the thermometers without a sufficient number of reference points. It is expected that a thermometer with a smaller selfheating effect and equal temperature sensitivity in voltage, compared with commercial R h - F e thermometers, can be obtained by adopting the present design of thermometer.

Acknowledgements The authors would like to thank Mr M. Shigeno and Mr S. Amano of the Technical Center at Chino Corporation for their help in the fabrication of the thermometers, that

is for their efforts in machining the coil frame from a fused-silica rod and wiring the sensing elements.

References I Rushy, R.L. Temperature: Its Measurement and Control in Science and Industry Vol 4 (Eds Rubin, L.G., Anderson, A.C., Janssen, J.E. and Cutkosky, R.D.) Instrument Society of America, Pittsburgh, USA (1972) 865 2 Rushy, R.L. Temperature: Its Measurement and Control in Science and Industry Vol 5 (Ed Schooley, J.F.) American Institute of Physics, New York, USA (1982) 829 3 Tamura, ~O. and Sakurai, H. Jpn J Appl Phys (1987) 26 L947 4 Meyers, C.H. Bureau of Standards J Res (1932) 9 807 5 Sawada, S. and Mochizuki, T. Temperature: Its Measurement and Control in Science and Industry Vol 4 (Eds Rubin, L.G., Anderson, A.C., Janssen, J.E. and Cutkosky, R.D.) Instrument Society of America, Pittsburgh, USA (1972) 919 6 Rusby, R.L. TemperatureMeasurementlnstituteofPhysics, London, UK (1975) 125

Cryogenics 1991 Vol 31 October

873