[time-nuts] Fwd: HP5061B Versus HP5071 Cesium Line Frequencies

jimlux jimlux at earthlink.net
Sun Jun 4 07:47:26 EDT 2017


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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