[time-nuts] HP5061B Versus HP5071 Cesium Line Frequencies

Bob kb8tq kb8tq at n1k.org
Sun Jun 4 14:46:03 EDT 2017


I think you have thermistors and thermocouples a bit mixed up. You can get
quite substantial output voltages from a thermistor bridge….


> 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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