[time-nuts] HP5061B Versus HP5071 Cesium Line Frequencies

Bob kb8tq kb8tq at n1k.org
Sun Jun 4 19:40:11 EDT 2017


A thermistor has *no* output unless it is in a circuit that biases it up. A thermocouple
is the one that has an output when no bias is present….

Take a 10K thermistor and a 10K resistor and put them in series.  You will get roughly Vcc / 2 at 25C
at the junction of the two parts.. The output will change about 1.5% per degree. With a 5V Vcc, that’s 
around 38 mV/C. 


> On Jun 4, 2017, at 4:49 PM, Donald E. Pauly <trojancowboy at gmail.com> wrote:
> I own several Fluke 52 stereo thermometers with K themocouples.  They
> run 40 μV/C°.  All thermistors have tiny outputs without op amps.
> They also suffer from self heating.  AD590 sensors give AT LEAST 15
> mV/C° without op amps.  If a regulated 3,000V supply is available they
> can give 2 V/C° into a 1 Watt 10 Meg resistor.
> πθ°μΩω±√·Γλ
> ---------- Forwarded message ----------
> From: Bob kb8tq <kb8tq at n1k.org>
> Date: Sun, Jun 4, 2017 at 11:46 AM
> Subject: Re: [time-nuts] HP5061B Versus HP5071 Cesium Line Frequencies
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Cc: "rward0 at aol.com" <rward0 at aol.com>, "Donald E. Pauly"
> <trojancowboy at gmail.com>
> Hi
> I think you have thermistors and thermocouples a bit mixed up. You can get
> quite substantial output voltages from a thermistor bridge….
> Bob
>> On Jun 4, 2017, at 11:44 AM, Donald E. Pauly <trojancowboy at gmail.com> wrote:
>> I stand by my remark that thermistors have been obsolete for over 40
>> years.  The only exception that I know of is cesium beam tubes that
>> must withstand a 350° C bakeout.  Thermistors are unstable and
>> manufactured with a witches brew straight out of MacBeth.  Their
>> output voltages are tiny and are they inconvenient to use at different
>> temperatures.
>> Where did you get the idea to use a 1 k load for an AD590?  If you run
>> it from a -5 V supply you can use a 15 k load to a +5V supply.  This
>> gives 15 V/C° output.  If you drive it from a 10 Meg impedance current
>> source, you get 30,000 V/ C°.  If I remember correctly, I drove a
>> power MOSFET heater gate directly in my prototype oven 20 years ago.
>> It would go from full off to full on in 1/15 ° C.  Noise is 1/25,000 °
>> C in a 1 cycle bandwidth.
>> The room temperature coefficient of an AT crystal is -100 ppb per
>> reference cut angle in minutes.  (-600 ppb/C° for standard crystal)
>> The practical limit in a crystal designed for room temperature is
>> about 0.1' cut accuracy or ±10 ppb/C°.  If you have access to an
>> atomic standard, you can use feed forward to get ±1 ppb/C°.  If the
>> temperature can be held to ±0.001° C, this is ±1 part per trillion.
>> This kind of accuracy has never been heard of.  Feed forward also
>> allows you to incorporate the components of the oscillator into the
>> thermal behavior.  It does no good to have a perfect crystal if the
>> oscillator components drift.
>> πθ°μΩω±√·Γλ
>> ---------- Forwarded message ----------
>> From: jimlux <jimlux at earthlink.net>
>> Date: Sun, Jun 4, 2017 at 4:47 AM
>> Subject: Re: [time-nuts] Fwd: HP5061B Versus HP5071 Cesium Line Frequencies
>> To: time-nuts at febo.com
>> On 6/3/17 9:56 PM, Donald E. Pauly wrote:
>>> It was only in the early 70s that Analog Devices invented the AD590
>>> solid state temperature sensor.  It made thermister bridges obsolete.
>> There is a difference between something like a platinum resistance
>> thermometer (PRT or RTD) and a thermistor, but they both are "measure
>> resistance to measure temperature" devices.
>> Yes, the AD590 is a useful part (I've got some in a device being
>> launched in August), but PRTs,thermistors, and thermocouples are still
>> widely used.
>> I don't know that the inherent precision (at room temperature)of the
>> various techniques is wildly different.  A 1mV/K signal (AD590 into a
>> 1k resistor) has to be measured to 0.1mV for 0.1 degree accuracy.
>> That's out of 300mV, so 1 part in 3000
>> A type E thermocouple is 1.495 mV at 25C and 1.801 at 30C, so about
>> 0.06 mV/K slope. Measure 0.006mV for 0.1 degree  (plus the "cold
>> junction" issue).  1 part in 250 measurement.
>> Modern RTDs all are 0.00385 ohm/ohm/degree at 25C.  Typically, you
>> have a 100 ohm device (although there are Pt1000s), so it's changing
>> 0.385 ohm/degree.  1 part in 3000
>> Checking the Omega catalog.. A 44007 has nominal 5k at 25C, and is
>> 4787 at 26C, so 1 part in 24.
>> Especially these days, with computers to deal with nonlinear
>> calibration curves, there's an awful lot of TCs and Thermistors in
>> use. The big advantage of the AD590 and PRT is that they are basically
>> linear over a convenient temperature range.
>> In a variety applications, other aspects of the measurement device are
>> important - ESD sensitivity, tolerance to wildly out of spec
>> temperature without damage, radiation effects etc.  Not an issue here,
>> but I'll note that the thermistor, PRT, and thermocouple are
>> essentially ESD immune. The AD590 most certainly is not.
>> If you go out and buy cheap industrial PID temperature controller it
>> will have input modes for various thermocouples and PRTs.  I suppose
>> there's probably some that take 1uA/K, but it's not something I would
>> expect.
>> So I wouldn't say thermistor bridges (or other temperature
>> measurements) are obsolete.
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