Review: Vishay Z foil resistors as Secondary Ohm Standards
Hello Volt-Nuts,
The experiments and the following discussion with Vishay took nearly one
year, until now I am able to present a final result.
Further details about the measurements and the setup will be given later.
I now have a personal, thorough understanding of Vishays Z-foil
technology and its capabilities.
This can also be found between the following lines.
'The specification for the hermetically sealed, oil filled metal foil Z
resistors (VHP202ZT) imply a usage as a secondary Ohm standard:
Longterm stability of 2ppm/6 years and a low TCR of 0.05ppm/K between
0..60°C typically,
vanishing @ 25°C, given by a parabola form of resistance over
temperature: http://www.vishaypg.com/docs/63120/hzseries.pdf
I ordered 5 EA 9k9998 +/- 10ppm, to be trimmed to nominal 10000.00 Ohm
+/1 ppm by low ohmic thin film resistors.
Vishay always performs measurements on VHP types, i.e. absolute
resistance and TCR.
So I also ordered the measurement protocol for a small additional charge.
The complete batch of resistors were measured by Vishay at 25°C to a
precision of about +/- 1ppm, each one 3 times.
After delivery, I compared each resistor of the ensemble against each
other by a HP3458A.
The initial setup was sufficiently stable (order of 1 ppm) to determine
the relative resistance deviation, and to fine tune the trimming
resistors to the desired precision level.
It became apparent during these first measurements, that the setup, and
the resistors were not as stable over temperature, as needed for sub ppm
level accuracy, and as expected.
First, the measurement setup (i.e. environmental conditions, HP3458A and
resistor assembly) was greatly improved to get a verifiable transfer
accuracy of < 0.2ppm over one hour, although the HP3458A is not
specified in this regard.
Each resistor was assembled into an aluminium bar, together with a
precision NTC.
The bar then is mounted into a small aluminium box, which carries Au
plated CuTe plugs to provide 4 pole measurement for the resistor, and 2
plugs for the NTC and case grounding.
The different sources of instablities, especially thermally induced
ones, could then be separated, quantified and reduced.
One resistor out of the ensemble was thermally cycled from 25°C to about
0°C, back to 25°C, and then to 60°C and back.
As the resistance value at 25°C was exactly reproduced every time to
within <0.2ppm, obviously no thermal hysteresis occurred.
During the measurement, the HP3458A was frequently checked by another
VHP202 being at constant temperature.
This demonstrated that the instrument stayed stable to within 0.2ppm
transfer accuracy.
The differential TCR (dR/dT) of the DUT over this temperature range was
calculated to be between -0.5 .. -2.6 ppm/K.
That is an order of magnitude above the expected, "typical" TCR in the
Vishay specification.
Also, the upwardly directed parabolum of R vs. T could not be detected,
which should be typical for the Z foil technology,
according to: "Zero TCR Foil Resistor Ten Fold Improvement in
Temperature Coefficient", R. Goldstein, J. Szwarc @ Visahy.
Coming from the same batch, all resistors have the same T.C.; this was
checked briefly for another DUT.
This excludes the possibility to improve TCR by a serial/parallel
assembly of 4 resistors.
I complained at Vishay PG about the unexpected behaviour.
Vishay had made TCR measurements at -55 and +125 before delivery and
repeated this on one resistor I had resent them. They found a TCR of
-0.4..-0.6ppm/K.
Although they found a similar result, my complaint was completely rejected.
They claimed, that this TCR is well within specification.
Although most of the datasheets about Z-foil resistors and several other
publications about the Z-foil technology imply a more optimistic
picture, only in Table 1 of the specification, a max. upper limit
('spread') of +/- 2.2ppm/K TCR over the complete temperature range is
specified. This is a barn door, 10 times bigger than the typical values
over the complete temperature range.
Additionally, Vishay defines the TCR as a "Chord Slope", i.e. as
(Rx-R25)/(Tx-T25), not as a differential one.
Recalculation of my results then gave a TCR of -0.5 .. -1ppm/K, still
very high.
Also, the parabolum with vanishing TCR at 25°C, (or at any other center
temperature) is "typical" only, and does not have to occur on real
components.
Conclusion:
The ensemble of 5 precision resistors represents a median standard
resistance of 10k +/- 1ppm, if thermally stabilized to 25.00°C precisely.
The 3458A in combination with this stable resistor ensemble achieves a
repeatability / transfer stability of <0.2ppm over 1 hour and an
absolute stability of < 1ppm over days for the HP3458A alone.
The longterm stability of the ensemble should be less than 2ppm/6 years,
which greatly improves the mediocre longterm stability of the internal
Ohm reference of the HP3458A.
This is to be checked during the coming years.
A goal which could not be achieved, is the usage of the VHP202Z
resistors as secondary Ohm standards, as the TCR of roughly -0.5ppm/K is
unexpectedly too high.'
Best regards - Frank
Frank,
I enjoyed reading your post. I am new to the topic of 'standards' but I am
wondering how you built your 'environmentally stable' environment for the
3458A and what mechanisms you used to create, measure and log this? I look
forward to your next post and the details of that.
Thanks,
Joe
-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On
Behalf Of Frank Stellmach
Sent: Saturday, November 19, 2011 9:57 AM
To: volt-nuts@febo.com
Subject: [volt-nuts] Review: Vishay Z foil resistors as Secondary
OhmStandards
Hello Volt-Nuts,
The experiments and the following discussion with Vishay took nearly one
year, until now I am able to present a final result.
Further details about the measurements and the setup will be given later. I
now have a personal, thorough understanding of Vishays Z-foil
technology and its capabilities.
This can also be found between the following lines.
'The specification for the hermetically sealed, oil filled metal foil Z
resistors (VHP202ZT) imply a usage as a secondary Ohm standard: Longterm
stability of 2ppm/6 years and a low TCR of 0.05ppm/K between
0..60°C typically,
vanishing @ 25°C, given by a parabola form of resistance over
temperature: http://www.vishaypg.com/docs/63120/hzseries.pdf
I ordered 5 EA 9k9998 +/- 10ppm, to be trimmed to nominal 10000.00 Ohm
+/1 ppm by low ohmic thin film resistors.
Vishay always performs measurements on VHP types, i.e. absolute
resistance and TCR.
So I also ordered the measurement protocol for a small additional charge.
The complete batch of resistors were measured by Vishay at 25°C to a
precision of about +/- 1ppm, each one 3 times.
After delivery, I compared each resistor of the ensemble against each
other by a HP3458A.
The initial setup was sufficiently stable (order of 1 ppm) to determine
the relative resistance deviation, and to fine tune the trimming
resistors to the desired precision level.
It became apparent during these first measurements, that the setup, and
the resistors were not as stable over temperature, as needed for sub ppm
level accuracy, and as expected.
First, the measurement setup (i.e. environmental conditions, HP3458A and
resistor assembly) was greatly improved to get a verifiable transfer
accuracy of < 0.2ppm over one hour, although the HP3458A is not
specified in this regard.
Each resistor was assembled into an aluminium bar, together with a
precision NTC.
The bar then is mounted into a small aluminium box, which carries Au
plated CuTe plugs to provide 4 pole measurement for the resistor, and 2
plugs for the NTC and case grounding.
The different sources of instablities, especially thermally induced
ones, could then be separated, quantified and reduced.
One resistor out of the ensemble was thermally cycled from 25°C to about
0°C, back to 25°C, and then to 60°C and back.
As the resistance value at 25°C was exactly reproduced every time to
within <0.2ppm, obviously no thermal hysteresis occurred.
During the measurement, the HP3458A was frequently checked by another
VHP202 being at constant temperature.
This demonstrated that the instrument stayed stable to within 0.2ppm
transfer accuracy.
The differential TCR (dR/dT) of the DUT over this temperature range was
calculated to be between -0.5 .. -2.6 ppm/K.
That is an order of magnitude above the expected, "typical" TCR in the
Vishay specification.
Also, the upwardly directed parabolum of R vs. T could not be detected,
which should be typical for the Z foil technology,
according to: "Zero TCR Foil Resistor Ten Fold Improvement in
Temperature Coefficient", R. Goldstein, J. Szwarc @ Visahy.
Coming from the same batch, all resistors have the same T.C.; this was
checked briefly for another DUT.
This excludes the possibility to improve TCR by a serial/parallel
assembly of 4 resistors.
I complained at Vishay PG about the unexpected behaviour.
Vishay had made TCR measurements at -55 and +125 before delivery and
repeated this on one resistor I had resent them. They found a TCR of
-0.4..-0.6ppm/K.
Although they found a similar result, my complaint was completely rejected.
They claimed, that this TCR is well within specification.
Although most of the datasheets about Z-foil resistors and several other
publications about the Z-foil technology imply a more optimistic
picture, only in Table 1 of the specification, a max. upper limit
('spread') of +/- 2.2ppm/K TCR over the complete temperature range is
specified. This is a barn door, 10 times bigger than the typical values
over the complete temperature range.
Additionally, Vishay defines the TCR as a "Chord Slope", i.e. as
(Rx-R25)/(Tx-T25), not as a differential one.
Recalculation of my results then gave a TCR of -0.5 .. -1ppm/K, still
very high.
Also, the parabolum with vanishing TCR at 25°C, (or at any other center
temperature) is "typical" only, and does not have to occur on real
components.
Conclusion:
The ensemble of 5 precision resistors represents a median standard
resistance of 10k +/- 1ppm, if thermally stabilized to 25.00°C precisely.
The 3458A in combination with this stable resistor ensemble achieves a
repeatability / transfer stability of <0.2ppm over 1 hour and an
absolute stability of < 1ppm over days for the HP3458A alone.
The longterm stability of the ensemble should be less than 2ppm/6 years,
which greatly improves the mediocre longterm stability of the internal
Ohm reference of the HP3458A.
This is to be checked during the coming years.
A goal which could not be achieved, is the usage of the VHP202Z
resistors as secondary Ohm standards, as the TCR of roughly -0.5ppm/K is
unexpectedly too high.'
Best regards - Frank
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
A goal which could not be achieved, is the usage of the VHP202Z
resistors as secondary Ohm standards, as the TCR of roughly -0.5ppm/K is
unexpectedly too high.'
Best regards - Frank
Hello Frank,
yes: reading between the lines of a datasheet is very difficult
because they are made from salesmen which want to sell their stuff
and have to be "better" than competition.
For understanding the datasheet values you have to understand how they are
specified.
In most cases the "box method" is used for specifying values over
temperature.
In the box method the value is measured at 3 temperatures: (to save
cost/measurement time against a continuous curve).
At 25 degrees, at minimum operating temperature (e.g. -55 degrees) and at
maximum temperature (e.g. 125 degrees).
Maximum and Minimum of the 3 values give now the "spread" of the
measurement.
So for TC calculation (RMax-RMin) / RNom / (TMax-TMin) is calculated.
For the Datasheet the TC is even halfed by writing +/- TC = +/- (RMax-RMin)
/ 2 / RNom / (TMax-TMin).
The only thing that you can say from TC specification is that the difference
between minimum and maximum resistance is 2 * (+/-)TC (including "spread") *
(TMax-TMin) over the temperature range.
There is no assumption to make over the actual slope at a specific
temperature.
Although typical you will measure double TC for the "slope" as in the +/-
specification for TC due to the box method.
And: the only thing you can expect from the same "Lot" is that the resistors
behave mostly equal.
So for a voltage divider you can expect that the division factor will be
nearly constant over temperature.
By the way: from where did you order the parts: from England or from Teltow?
With best regards
Andreas
Frank,
Unfortunately, your results closely match my findings on a batch of four
VHP202ZT's that I bought almost two years ago with
the same purpose in mind.
While the initial accuracy and the stability with respect to thermal
hysteresis are both excellent, I too found TC's that are nowhere near
the 'typical' value specified by Vishay. Neither could I find any trace
of the parabola curve.
Very similar results were found for a group of 10 Z201's (the
non-hermetic version of the VHP202).
Having since acquired an SR104, the actual 'need to build' has gone, but
I must say I was (and still am) severely disappointed
with these findings. Although, as an experienced electronics engineer, I
should know better than to follow 'typical' values on
a datasheet, I feel there is a certain element of false advertising
involved here. I find it very hard to swallow that, out of 14 units
tested, not a single one comes even close to the performance we're led
to believe they are capable of.
One wonders what Vishay's definition of "typical" is.
- Rob.
Op 19-11-2011 16:57, Frank Stellmach schreef:
Hello Volt-Nuts,
The experiments and the following discussion with Vishay took nearly
one year, until now I am able to present a final result.
Further details about the measurements and the setup will be given later.
I now have a personal, thorough understanding of Vishays Z-foil
technology and its capabilities.
This can also be found between the following lines.
'The specification for the hermetically sealed, oil filled metal foil
Z resistors (VHP202ZT) imply a usage as a secondary Ohm standard:
Longterm stability of 2ppm/6 years and a low TCR of 0.05ppm/K between
0..60°C typically,
vanishing @ 25°C, given by a parabola form of resistance over
temperature: http://www.vishaypg.com/docs/63120/hzseries.pdf
I ordered 5 EA 9k9998 +/- 10ppm, to be trimmed to nominal 10000.00 Ohm
+/1 ppm by low ohmic thin film resistors.
Vishay always performs measurements on VHP types, i.e. absolute
resistance and TCR.
So I also ordered the measurement protocol for a small additional charge.
The complete batch of resistors were measured by Vishay at 25°C to a
precision of about +/- 1ppm, each one 3 times.
After delivery, I compared each resistor of the ensemble against each
other by a HP3458A.
The initial setup was sufficiently stable (order of 1 ppm) to
determine the relative resistance deviation, and to fine tune the
trimming resistors to the desired precision level.
It became apparent during these first measurements, that the setup,
and the resistors were not as stable over temperature, as needed for
sub ppm level accuracy, and as expected.
First, the measurement setup (i.e. environmental conditions, HP3458A
and resistor assembly) was greatly improved to get a verifiable
transfer accuracy of < 0.2ppm over one hour, although the HP3458A is
not specified in this regard.
Each resistor was assembled into an aluminium bar, together with a
precision NTC.
The bar then is mounted into a small aluminium box, which carries Au
plated CuTe plugs to provide 4 pole measurement for the resistor, and
2 plugs for the NTC and case grounding.
The different sources of instablities, especially thermally induced
ones, could then be separated, quantified and reduced.
One resistor out of the ensemble was thermally cycled from 25°C to
about 0°C, back to 25°C, and then to 60°C and back.
As the resistance value at 25°C was exactly reproduced every time to
within <0.2ppm, obviously no thermal hysteresis occurred.
During the measurement, the HP3458A was frequently checked by another
VHP202 being at constant temperature.
This demonstrated that the instrument stayed stable to within 0.2ppm
transfer accuracy.
The differential TCR (dR/dT) of the DUT over this temperature range
was calculated to be between -0.5 .. -2.6 ppm/K.
That is an order of magnitude above the expected, "typical" TCR in the
Vishay specification.
Also, the upwardly directed parabolum of R vs. T could not be
detected, which should be typical for the Z foil technology,
according to: "Zero TCR Foil Resistor Ten Fold Improvement in
Temperature Coefficient", R. Goldstein, J. Szwarc @ Visahy.
Coming from the same batch, all resistors have the same T.C.; this was
checked briefly for another DUT.
This excludes the possibility to improve TCR by a serial/parallel
assembly of 4 resistors.
I complained at Vishay PG about the unexpected behaviour.
Vishay had made TCR measurements at -55 and +125 before delivery and
repeated this on one resistor I had resent them. They found a TCR of
-0.4..-0.6ppm/K.
Although they found a similar result, my complaint was completely
rejected.
They claimed, that this TCR is well within specification.
Although most of the datasheets about Z-foil resistors and several
other publications about the Z-foil technology imply a more optimistic
picture, only in Table 1 of the specification, a max. upper limit
('spread') of +/- 2.2ppm/K TCR over the complete temperature range is
specified. This is a barn door, 10 times bigger than the typical
values over the complete temperature range.
Additionally, Vishay defines the TCR as a "Chord Slope", i.e. as
(Rx-R25)/(Tx-T25), not as a differential one.
Recalculation of my results then gave a TCR of -0.5 .. -1ppm/K, still
very high.
Also, the parabolum with vanishing TCR at 25°C, (or at any other
center temperature) is "typical" only, and does not have to occur on
real components.
Conclusion:
The ensemble of 5 precision resistors represents a median standard
resistance of 10k +/- 1ppm, if thermally stabilized to 25.00°C precisely.
The 3458A in combination with this stable resistor ensemble achieves a
repeatability / transfer stability of <0.2ppm over 1 hour and an
absolute stability of < 1ppm over days for the HP3458A alone.
The longterm stability of the ensemble should be less than 2ppm/6
years, which greatly improves the mediocre longterm stability of the
internal Ohm reference of the HP3458A.
This is to be checked during the coming years.
A goal which could not be achieved, is the usage of the VHP202Z
resistors as secondary Ohm standards, as the TCR of roughly -0.5ppm/K
is unexpectedly too high.'
Best regards - Frank
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
--
Small Design
Zuiddijk 354
1505 HD Zaandam
The Netherlands
tel. +31 (0)75 77 11 740
fax. +31 (0)75 77 11 742
e-mail: rob.klein@smalldesign.nl
Frank Stellmach frank.stellmach@freenet.de writes:
Hello Volt-Nuts,
The experiments and the following discussion with Vishay took nearly
one year, until now I am able to present a final result.
Further details about the measurements and the setup will be given later.
I now have a personal, thorough understanding of Vishays Z-foil
technology and its capabilities.
This can also be found between the following lines.
[Snipped great report on Z-foils, thanks Frank...]
Hi All,
Are the VAR type "naked" z-foils good for anything? They seem targetted
at the audiophool crowd. I was wondering about a DIY hermetically sealed
assembly. For example could it be mounted in one of those big old
crystal packages and hand soldered for sealing? I have never tried to do
this but it seems feasible.
--
John Devereux
In message 87hb1xbzhm.fsf@devereux.me.uk, John Devereux writes:
Are the VAR type "naked" z-foils good for anything?
They are undoubtedly very profitable, but I'll bet you a beer that
nobody relibly can hear the difference in a competent double-blind
test.
http://www.cco.caltech.edu/~boyk/sdttest.htm
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Thanks Frank for a most wonderful report.
Would you be able to share what the cost of each VHP202ZT you ordered?
best wishes,
-marv
At 10:57 AM 11/19/2011, Frank Stellmach wrote:
Hello Volt-Nuts,
The experiments and the following discussion
with Vishay took nearly one year, until now I am
able to present a final result.
Further details about the measurements and the setup will be given later.
I now have a personal, thorough understanding of
Vishays Z-foil technology and its capabilities.
This can also be found between the following lines.
'The specification for the hermetically sealed,
oil filled metal foil Z resistors (VHP202ZT)
imply a usage as a secondary Ohm standard:
Longterm stability of 2ppm/6 years and a low TCR
of 0.05ppm/K between 0..60°C typically,
vanishing @ 25°C, given by a parabola form of
resistance over temperature: http://www.vishaypg.com/docs/63120/hzseries.pdf
I ordered 5 EA 9k9998 +/- 10ppm, to be trimmed
to nominal 10000.00 Ohm +/1 ppm by low ohmic thin film resistors.
Vishay always performs measurements on VHP
types, i.e. absolute resistance and TCR.
So I also ordered the measurement protocol for a small additional charge.
The complete batch of resistors were measured by
Vishay at 25°C to a precision of about +/- 1ppm, each one 3 times.
After delivery, I compared each resistor of the
ensemble against each other by a HP3458A.
The initial setup was sufficiently stable (order
of 1 ppm) to determine the relative resistance
deviation, and to fine tune the trimming
resistors to the desired precision level.
It became apparent during these first
measurements, that the setup, and the resistors
were not as stable over temperature, as needed
for sub ppm level accuracy, and as expected.
First, the measurement setup (i.e. environmental
conditions, HP3458A and resistor assembly) was
greatly improved to get a verifiable transfer
accuracy of < 0.2ppm over one hour, although the
HP3458A is not specified in this regard.
Each resistor was assembled into an aluminium
bar, together with a precision NTC.
The bar then is mounted into a small aluminium
box, which carries Au plated CuTe plugs to
provide 4 pole measurement for the resistor, and
2 plugs for the NTC and case grounding.
The different sources of instablities,
especially thermally induced ones, could then be
separated, quantified and reduced.
One resistor out of the ensemble was thermally
cycled from 25°C to about 0°C, back to 25°C, and then to 60°C and back.
As the resistance value at 25°C was exactly
reproduced every time to within <0.2ppm,
obviously no thermal hysteresis occurred.
During the measurement, the HP3458A was
frequently checked by another VHP202 being at constant temperature.
This demonstrated that the instrument stayed
stable to within 0.2ppm transfer accuracy.
The differential TCR (dR/dT) of the DUT over
this temperature range was calculated to be between -0.5 .. -2.6 ppm/K.
That is an order of magnitude above the
expected, "typical" TCR in the Vishay specification.
Also, the upwardly directed parabolum of R vs. T
could not be detected, which should be typical for the Z foil technology,
according to: "Zero TCR Foil Resistor Ten Fold
Improvement in Temperature Coefficient", R. Goldstein, J. Szwarc @ Visahy.
Coming from the same batch, all resistors have
the same T.C.; this was checked briefly for another DUT.
This excludes the possibility to improve TCR by
a serial/parallel assembly of 4 resistors.
I complained at Vishay PG about the unexpected behaviour.
Vishay had made TCR measurements at -55 and +125
before delivery and repeated this on one
resistor I had resent them. They found a TCR of -0.4..-0.6ppm/K.
Although they found a similar result, my complaint was completely rejected.
They claimed, that this TCR is well within specification.
Although most of the datasheets about Z-foil
resistors and several other publications about
the Z-foil technology imply a more optimistic
picture, only in Table 1 of the specification, a
max. upper limit ('spread') of +/- 2.2ppm/K TCR
over the complete temperature range is
specified. This is a barn door, 10 times bigger
than the typical values over the complete temperature range.
Additionally, Vishay defines the TCR as a "Chord
Slope", i.e. as (Rx-R25)/(Tx-T25), not as a differential one.
Recalculation of my results then gave a TCR of
-0.5 .. -1ppm/K, still very high.
Also, the parabolum with vanishing TCR at 25°C,
(or at any other center temperature) is
"typical" only, and does not have to occur on real components.
Conclusion:
The ensemble of 5 precision resistors represents
a median standard resistance of 10k +/- 1ppm, if
thermally stabilized to 25.00°C precisely.
The 3458A in combination with this stable
resistor ensemble achieves a repeatability /
transfer stability of <0.2ppm over 1 hour and an
absolute stability of < 1ppm over days for the HP3458A alone.
The longterm stability of the ensemble should be
less than 2ppm/6 years, which greatly improves
the mediocre longterm stability of the internal Ohm reference of the HP3458A.
This is to be checked during the coming years.
A goal which could not be achieved, is the usage
of the VHP202Z resistors as secondary Ohm
standards, as the TCR of roughly -0.5ppm/K is unexpectedly too high.'
Best regards - Frank
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
Marv,
For what it's worth, I paid almost 66 Euro's (around 96 US$) each. That
was late March 2010.
They are significantly cheaper in 0.005%; some 57 US$ as per quotation
December 2009.
Best regards,
Rob.
Op 21-11-2011 13:06, Marvin E. Gozum schreef:
Thanks Frank for a most wonderful report.
Would you be able to share what the cost of each VHP202ZT you ordered?
best wishes,
-marv
"Poul-Henning Kamp" phk@phk.freebsd.dk writes:
In message 87hb1xbzhm.fsf@devereux.me.uk, John Devereux writes:
Are the VAR type "naked" z-foils good for anything?
They are undoubtedly very profitable, but I'll bet you a beer that
nobody relibly can hear the difference in a competent double-blind
test.
Hi,
I know that :)
I was wondering if they were good for anything of volt-nut interest!
Specifically the hermetic sealing idea. I thought since they are not
encapsulated they might be free of humidity induced strain or some such.
Since then Frank posted the price which was less than I expected. Pity
about the tempco though. I guess they could be temperature stabilised?
Thanks,
--
John Devereux