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'[EE]: How to measure low cap (120pF)'
2002\04\02@170900 by Diego Sierra

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Hi!

I have a humidity sensor from Philips (H1
ftp://ftp.iac.es/out/dsg/H1_Humidity.pdf), it's a device that changes
its capacity value from 110pF to 140pF. I found its datasheet not on
Philips web, where I cannot find either an application note about it. On
its electrical characteristics is talks about a frequency range which
have no meaning for me.

How could be this device conected into a circuit?

How to measure such a low capacity value?, with an oscilator (555)?, if
the capacity is so low, could the parasitic capacities easily changes
the measured value?

Thanks in advance,
Diego.

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2002\04\02@184708 by Dave Dilatush

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Diego Sierra wrote...

>I have a humidity sensor from Philips (H1
>ftp://ftp.iac.es/out/dsg/H1_Humidity.pdf), it's a device that changes
>its capacity value from 110pF to 140pF. I found its datasheet not on
>Philips web, where I cannot find either an application note about it. On
>its electrical characteristics is talks about a frequency range which
>have no meaning for me.
>
>How could be this device conected into a circuit?

Linear Technology, Inc. has some nice application notes covering
various signal-conditioning tasks.  Here are two that discuss
processing capacitive sensors:

http://www.linear-tech.com/pdf/an3.pdf has a couple of circuits
on pages 7 and 8 that deal with capacitive humidity sensors
similar to the one you have.  These circuits use their LTC1043
switched-capacitor instrumentation building block IC plus some
opamps.

http://www.linear-tech.com/pdf/an87.pdf on pages 87-88 shows
another pair of circuits, also using the LTC1043, for processing
capacitive sensors.  (NOTE: the diagrams show wrong pin numbers
for the opamps in the circuit; the "+" and "-" inputs are labeled
correctly but their pin numbers are reversed.)

>How to measure such a low capacity value?, with an oscilator (555)?, if
>the capacity is so low, could the parasitic capacities easily changes
>the measured value?

Parasitic capacitance is definitely a problem, and the circuits
in AN87 (see above) are designed to minimize the effects of
parasitics.

One thing to remember with capacitive humidity sensors is you
need to keep DC away from the sensor, so it sees only an AC
signal.  The circuits in AN3 show how to do this, and the
technique shown there (putting a large capacitor in series with
the sensor and a large resistor in parallel with it) should be
copied if you use the circuit in AN87 for your humidity sensor.

Hope this helps a bit...

Dave

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2002\04\02@195852 by Herbert Graf

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{Quote hidden}

       I would think the easiest way to measure it would be a DMM with
capacitance, most go that low, although you may have to compensate for the
test leads. TTYL

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2002\04\02@205554 by Augusto de Conto

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sorry, but what is a DDM?
----- Original Message -----
From: "Herbert Graf" <mailinglistspamKILLspamFARCITE.NET>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Tuesday, April 02, 2002 9:59 PM
Subject: Re: [EE]: How to measure low cap (120pF)


> > {Original Message removed}

2002\04\02@210011 by Randy Glenn

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DMM = Digital Multimeter

-Randy Glenn

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{Original Message removed}

2002\04\02@211224 by Spehro Pefhany

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At 11:07 PM 4/2/02 +0100, you wrote:

>How to measure such a low capacity value?, with an oscilator (555)?, if
>the capacity is so low, could the parasitic capacities easily changes
>the measured value?

Yes, you can use a 555 (with a larger cap in series with the humidity
sensor. Then you just need to measure the frequency with your PIC.

The change (40pF) is relatively large, and when mounted on a PCB
the changes in parasitic capacitance should not be too bad.

More of a problem with the Philips part is calibration.. you have to
procure saturated salt solutions like these:

RH @25°C
Lithium chloride                LiCl            11.3%
Potassium acetate               KC2H3O2         22.5%
Magnesium chloride              MgCl2           32.8%
Potassium carbonate             K2CO3           43.2%
Magnesium nitrate               Mg(NO3)2        52.9%
Sodium chloride                 NaCl            75.3%
Potassium chloride              KCl             84.3%
Potassium nitrate               KNO3            93.6%
Potassium sulphate              K2SO4           97.3%

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\04\02@235252 by Sean H. Breheny

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Hi Spehro,

Why do you have to prepare a range of solutions of different salts in order to calibrate a Humidity sensor?!

Sean

At 09:26 PM 4/2/02 -0500, you wrote:
More of a problem with the Philips part is calibration.. you have to
{Quote hidden}

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2002\04\03@012855 by Vasile Surducan

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Maybe with a climatic chamber and a psichometer will be easyest...

About small capacitance measurement, domestic scales are using an 8...10
pF variation for a 0 to 1000g measurement. Without any external circuit !
The sensor is tied directly to microcontroller from gnd ( what I wish to
find what chip is ! ) and a 300k resistor to VCC.

best, Vasile


On Tue, 2 Apr 2002, Sean H. Breheny wrote:

{Quote hidden}

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2002\04\03@075601 by Olin Lathrop

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> I have a humidity sensor from Philips (H1
> ftp://ftp.iac.es/out/dsg/H1_Humidity.pdf), it's a device that changes
> its capacity value from 110pF to 140pF. I found its datasheet not on
> Philips web, where I cannot find either an application note about it. On
> its electrical characteristics is talks about a frequency range which
> have no meaning for me.

One way to measure a capacitor is to use it in an oscillator and measure the
resulting frequency.

> How could be this device conected into a circuit?

Lots of ways.  I'm not familiar with this particular device, but there must
be app notes that answer exactly this question.

> How to measure such a low capacity value?, with an oscilator (555)?, if
> the capacity is so low, could the parasitic capacities easily changes
> the measured value?

Yes, so you have to take that into account.  If everything is well shielded,
then the parasitic capacitance is constant and will be compensated for in
overall system calibration.


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(978) 742-9014, http://www.embedinc.com

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2002\04\03@081110 by Spehro Pefhany

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At 11:51 PM 4/2/02 -0500, you wrote:
>Hi Spehro,
>
>Why do you have to prepare a range of solutions of different salts in
>order to calibrate a Humidity sensor?!

One or two might do, depending on the accuracy you need (and where you need
the accuracy to be best). One fellow I know makes these things
professionally and they have developed an automated system to calibrate the
sensors at a number of points to give better accuracy. In fact the
(non)linearity of the Philips parts is fairly predictable so two points
would not be bad. One could even be okay if you only care close to one
point (say you are *controlling* humidity at xx%, you only need to know it
accurately near that point. I thought I'd list the important points and
associated salts for the convenience of the OP.

Note that the tolerance on sensor sensitivity is +/-12.5% and the tolerance
on the zero value is +/-15% (of capacitance in both cases). Also there is a
temperature sensitivity of 0.1%RH/K.

More on the calibration issues in ASTM E 104.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\04\03@104557 by Sean H. Breheny

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Thanks for the quick reply. I probably should have phrased my question
better: what do salt solutions have to do with the calibration at all? What
do you do, spray a mist of the solution into the area where the sensor is?

I would have thought that calibration would simply involve using a second,
known good humidity sensor, but I certainly have not thought this through
much. Perhaps you are referring to a more basic way of calibrating that
does not require that you already have a humidity measuring instrument?

Thanks,

Sean

At 08:25 AM 4/3/02 -0500, you wrote:
{Quote hidden}

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2002\04\03@112634 by Spehro Pefhany

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At 10:42 AM 4/3/02 -0500, you wrote:
>Thanks for the quick reply. I probably should have phrased my question
>better: what do salt solutions have to do with the calibration at all? What
>do you do, spray a mist of the solution into the area where the sensor is?

Ah, okay. You can create the desired humidity with the salts in an airtight
container along with the sensor. Refer to ASTM 104 E (? I think) for the
exact techniques (textbooks should have it too).  Really accurate RH meters
are not that common, and of course they would have to be recalibrated
periodically as well.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\04\03@125234 by Thomas McGahee

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Build an oscillator using the sensor as the capacitance.
Use a PIC to measure the period. Convert period to Humidity using
math. For greater accuracy, measure multiple periods (say 10).

My CMETER PIC controlled capacitance meter is overkill for what
you need, but you might find some useful ideas contained therein
that you can use. CMETER will measure from .01 pf to over
16,000 microfarads and has auto-zeroing and the ability to
cancel out the effects of stray capacitance.

With a differential of 40 pf, CMETER will display 40.00

Fr. Thomas McGahee


{Original Message removed}

2002\04\03@140030 by Brandon Irwin

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Just wondering...
Wouldn't it be possible to measure the capacitance (most likely larger than
a couple pF) using an ac signal attached to a voltage divider, where the
capacitor you were measuring would be part of the divider? So as the
capacitance decreased, the signal level at the output would increase or
decrease, depending on placement of the cap in the divider.
Would this work?


Brandon Irwin

At 09:58 AM 4-3-2002, Thomas McGahee wrote:
{Quote hidden}

>{Original Message removed}

2002\04\03@152603 by Olin Lathrop

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> Wouldn't it be possible to measure the capacitance (most likely larger
than
> a couple pF) using an ac signal attached to a voltage divider, where the
> capacitor you were measuring would be part of the divider? So as the
> capacitance decreased, the signal level at the output would increase or
> decrease, depending on placement of the cap in the divider.
> Would this work?

Yep.  When you work out the math, you have to take into account the phase
relationship.  A capacitor is NOT just a frequency-variable resistor.


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(978) 742-9014, http://www.embedinc.com

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2002\04\03@163053 by mike

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A 555 is a pretty good, cheap oscillator for this. It also has the
advantage that one side of the cap is grounded. This allows a neat
trick : Connect the low side of the sensor and a reference cap to PIC pins. By
enabling and grounding each pin in turn, you can measure the frequency
for each of the caps and calculate the ratio. This eliminates almost
all potential errors - stray capacitance, voltage & temp fluctuations.
The accuracy is just dependent on the stability of the reference
capacitor.

On Wed, 3 Apr 2002 12:58:55 -0500, you wrote:

{Quote hidden}

>{Original Message removed}

2002\04\03@163101 by Douglas Butler

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> Just wondering...
> Wouldn't it be possible to measure the capacitance (most
> likely larger than
> a couple pF) using an ac signal attached to a voltage
> divider, where the
> capacitor you were measuring would be part of the divider? So as the
> capacitance decreased, the signal level at the output would
> increase or
> decrease, depending on placement of the cap in the divider.
> Would this work?
>
>
> Brandon Irwin

Back before microprocessors made frequency counting easy that was how it
was usually done.  Often it was a bridge setup where the divider was
composed of the unknown capacitor and a precision variable capacitor.
The variable capacitor was turned until the voltages were at a known
ratio, which meant the capacitors were at the inverse known ratio.

Sherpa Doug

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2002\04\04@020201 by Vasile Surducan

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On Wed, 3 Apr 2002, Thomas McGahee wrote:

> that you can use. CMETER will measure from .01 pf to over
> 16,000 microfarads and has auto-zeroing and the ability to
> cancel out the effects of stray capacitance.
>
 Even if he will use a pair of twisted wires to connect the .01pF to the
capacimeter [ grin] ?

best regards, Vasile

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