[NLRS] piezoresistive transducers

[Dr. Gerald N. Johnson]

Not having any experience with the part, I did a google search. The Wiki 
told me that the biggest problem with precision of the piezoresistive 
transducer is temperature effects. That stands to reason with a slab of 
some semiconductor as the resistive element. Another IEEE article the 
abstract says they solved the temperature sensitivity in 1983. I'd think 
a bridge as is commonly used in metal strain gauges and load cells could 
have minimal temperature sensitivity by using two identical 
piezoresistive elements, one protected from strain and the other 
strained but thoroughly coupled for thermal conditions, e.g. always at 
the same temperature. That could be accomplished by proximity or by good 
unit temperature control. Indeed good temperature control might minimize 
the need for the temperature compensating element. Load cells often use 
four resistive elements in the bridge circuit for temperature and 
bending compensation.

Since the range for atmospheric pressure is small, but dependent on 
altitude, like 400 mb to 1200 mb and the maximum pressure isn't great it 
would be likely there is some mechanical leverage involved, like a large 
area diaphragm or true linkage to increase the strain applied to the 
element since I see elements offered to handle a few thousand psi and 
1200 mb isn't more than about 17 psi.

In the 2011/2012 Fisher Scientific catalog they offer a precision dial 
barometer with accuracy 1/2% of the reading. So 5mb at 1 atmosphere. 
Traceable to NIST. Their digital on the same page, also traceable is +/- 
5mb or .1477" Hg. Well all four they offer are Traceable and 3 of the 
four show .1477" Hg or 5 mb accuracy and the last one says .1477"Hg or 8 
mb. Something's not consistent there. The dial unit has resolution of 
1/2 mb and the digital units resolution on the display of 1 mb so a 
quarter mb accuracy would be impossible to prove. And at 1 bar, 1/4 mb 
accuracy is .025%. Beyond Fisher's lab grade apparati.

The most expensive of the Fisher units is $185 and it does graphing on 
demand.

I wonder about their calibration reference at 1/4 mb, that seems to be 
not much change in inches of mercury and I'm certain the classic mercury 
barometer has serious difficulties with temperature changing the length 
of the glass as well as the mercury column and the quality of the vacuum 
above the mercury, from a quality of manufacture standard and vapor 
pressure of the mercury. 1 mb is .0295" Hg at 32 degrees F according to 
my table of conversions. So 1/4 mb is .0074" Hg, possible to see, 
difficult to be that precise with the temperature sensitivities of the 
classic mercury barometer. A water barometer would have much more 
movement and being about 32 feet tall would have a lot more sensitivity 
to temperature, especially in the vapor pressure of the water in the 
vacuum. My next question would be what's their calibration reference and 
how close will NIST calibrate? By the Fisher catalog barometers, I 
wonder if 1/4 mb accuracy is realizable, or is just the electronics does 
1/4 mb and the advertisers have confused precision with resolution. Do 
they admit any effects of temperature on the accuracy?

As W0PFP used to say teaching electrical engineering students about 
experimental lab techniques, its easy to "Measure with micrometer, mark 
with chalk, and cut with axe, but the final result may not be nearly as 
precise as anticipated." Its easy to have a precisely consistent result 
that's wrong because the measuring tape stretched or the micrometer was 
misread.

73, Jerry, K0CQ

On 12/25/2012 6:04 PM, Curt Peterson wrote:
>
>
> Perhaps one of you electronic experts can help me. I am considering
> buying adigital barograph that uses a "piezoresistive transducer" to
> determine atmospheric pressure. The manufacture  -  Aquatech
> Scientific Instr.  LLC claims their barograph has an accuracy of +/-
> 0.25 mb (or =/- 0.007 " hg) atmospheric pressure. The Company seems
> legit. but the claimed accuracy is very high. Are piezoresistive
> transducers that good ? ? Please reply. Thanks.  KC0FQZ  Curt
> Peterson
> ______________________________________________________________
>