Humidity sensor ideas?

[J. Michael Milner]

I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
I've missed something, there doesn't seem to be a nice physical property
that can convert humidity to a resistance as in most sensors. The
application is indoor/outdoor humidity sensing for control of my furnace's
humidifier. I'd rather roll my own before I go to the hvac group for a
standalone commercial solution.

 

[John Popelish]

I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
I've missed something, there doesn't seem to be a nice physical property
that can convert humidity to a resistance as in most sensors. The
application is indoor/outdoor humidity sensing for control of my furnace's
humidifier. I'd rather roll my own before I go to the hvac group for a
standalone commercial solution.

THe lowest cost sensors I have seen have a variable capacitance versus
relative humidity. But I doubt you'll find one for $5.

Digikey sells this one for about $10.
http://rocky.digikey.com/WebLib/Humirel/HS1101.pdf

 

[John Fields]

I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
I've missed something, there doesn't seem to be a nice physical property
that can convert humidity to a resistance as in most sensors. The
application is indoor/outdoor humidity sensing for control of my furnace's
humidifier. I'd rather roll my own before I go to the hvac group for a
standalone commercial solution.

 

[Lewin A.R.W. Edwards]

I've missed something, there doesn't seem to be a nice physical property

that can convert humidity to a resistance as in most sensors. The

There are - or used to be - dual thermometer/hygrometer units made which
contained a spiral bimetallic strip driving a needle for the temp
measurement and a similar strip made of some fibrous paperlike white
substance used to drive the hygrometer needle. Actually, now I come to
think of it, it may have been a strip of papery substance (expands when
moisture adsorbed) glued to a metal strip (doesn't expand).

Acquire such a hygrometer, point a cheap $5 webcam at it, and use simple
edge detection techniques to work out the needle's angle. Hint: take the
image in grayscale, consider each scanline as representing a continuous
function, and take an arithmetic derivative of this function to obtain
an "edginess" plot for the image. Establish a threshhold for what you
will truly consider an edge and convert to a 1-bit representation using
this threshhold.

Scan in an arc around the outside of the hub of the hygrometer's needle
until you hit an edge. Move out and scan the arc again. Repeat a few
times until you can extrapolate the line of the needle. Should be no
more than a weekend's effort in programming, as you can fix the camera's
aim directly on the needle.

 

[Jan Panteltje]

There are - or used to be - dual thermometer/hygrometer units made which
contained a spiral bimetallic strip driving a needle for the temp
measurement and a similar strip made of some fibrous paperlike white
substance used to drive the hygrometer needle. Actually, now I come to
think of it, it may have been a strip of papery substance (expands when
moisture adsorbed) glued to a metal strip (doesn't expand).

Acquire such a hygrometer, point a cheap $5 webcam at it, and use simple
edge detection techniques to work out the needle's angle. Hint: take the
image in grayscale, consider each scanline as representing a continuous
function, and take an arithmetic derivative of this function to obtain
an "edginess" plot for the image. Establish a threshhold for what you
will truly consider an edge and convert to a 1-bit representation using
this threshhold.

Scan in an arc around the outside of the hub of the hygrometer's needle
until you hit an edge. Move out and scan the arc again. Repeat a few
times until you can extrapolate the line of the needle. Should be no
more than a weekend's effort in programming, as you can fix the camera's
aim directly on the needle.

Yep, but these hygrometers (I have one), need calibration (salt water plastic bag),
every so many weeks google for 'hygrometer calibration'.
Difficult to do remotely.
I like you video processing knowledge.
JP

 

[Jim Thompson]

I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
I've missed something, there doesn't seem to be a nice physical property
that can convert humidity to a resistance as in most sensors. The
application is indoor/outdoor humidity sensing for control of my furnace's
humidifier. I'd rather roll my own before I go to the hvac group for a
standalone commercial solution.

Complex, but very accurate...

Use a Peltier cooler to chill a mirror.

Observe with a light beam when you get condensation (monitoring mirror
temperature)

This gets you dew point

Calculate humidity from dew point and ambient temperature

...Jim Thompson

 

[John Popelish]

Complex, but very accurate...

Use a Peltier cooler to chill a mirror.

Observe with a light beam when you get condensation (monitoring mirror
temperature)

This gets you dew point

Calculate humidity from dew point and ambient temperature

I like the hair detector. It is not very linear, but if you need to
control humidity over a narrow range, a hair wrapped around a shaft
and tensioned with the shaft connected to a vane that passes through a
light beam detector (from an old mouse) makes a pretty stable 1 bit
A/D converter that indicates humidity too high or too low. (deep
breath) If the beam diameter is large enough it provides an analog
signal with a narrow range of measurement.

 

[Lewin A.R.W. Edwards]

Yep, but these hygrometers (I have one), need calibration (salt water plastic bag),

every so many weeks google for 'hygrometer calibration'.

Interesting. I picked up a huge bundle of these devices once (they were
intended for automotive use, I think - judging from the way they were
housed). The manual just said not to immerse in water.

I like you video processing knowledge.

Well... I am working on my book this weekend, and I have just been
writing a section about simple machine vision techniques. My example
application was reading an analog clock The particular clock I have
chosen, it's a great little academic exercise because the second and
minute hands are the same length, just different thicknesses.

 

[Spehro Pefhany]

I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
I've missed something, there doesn't seem to be a nice physical property
that can convert humidity to a resistance as in most sensors. The
application is indoor/outdoor humidity sensing for control of my furnace's
humidifier. I'd rather roll my own before I go to the hvac group for a
standalone commercial solution.

One good way is wet-bulb dry-bulb. Use a wick to keep one sensor wet
from the humidifier reservoir and in air flow and the other sits in
ambient air. The relative humidity (and dew point, and absolute
humidity) is a known function of the two temperatures (see
psychrometric tables)

Best regards,
Spehro Pefhany

 

[Rich Webb]

I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
I've missed something, there doesn't seem to be a nice physical property
that can convert humidity to a resistance as in most sensors. The
application is indoor/outdoor humidity sensing for control of my furnace's
humidifier. I'd rather roll my own before I go to the hvac group for a
standalone commercial solution.

Newark has this one for about $7.50 in small quantity pricing:
http://www.newark.com/NewarkWebCommerce/newark/en_US/support/catalog/productDetail.jsp?id=39C2249
(manufacturer p/n HSP15P and Newark p/n 39C2249, in case the URL gets
munged.)

Data sheet at http://www.thermometrics.com/assets/images/hs1215p.pdf

 

[Barry Lennox]

I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
I've missed something, there doesn't seem to be a nice physical property
that can convert humidity to a resistance as in most sensors. The
application is indoor/outdoor humidity sensing for control of my furnace's
humidifier. I'd rather roll my own before I go to the hvac group for a
standalone commercial solution.

Philips make one for control of clothes driers, they are less than $5
in qty, but are hard to track down. I have used them successfully, and
have the Philips part number somewhere if you need that.

Or, Use a couple of thermistors, one dry and the other damp with a
wick. There's tables around with the temp diff vs humidity. You do
need to keep the wick wet, which may or may not be a problem.

And, other's have mentioned the human hair, I have not tried it, but
IIRC there was one in the Amateur Scientist section in Scientific
American several years back.

Barry Lennox

 

[Garrett Mace]

I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
I've missed something, there doesn't seem to be a nice physical property
that can convert humidity to a resistance as in most sensors. The
application is indoor/outdoor humidity sensing for control of my furnace's
humidifier. I'd rather roll my own before I go to the hvac group for a
standalone commercial solution.

A solution I have seen in furnace humidifer "humidistats" is a band of
cellophane, which stretches and contracts with varying humidity. I have no
idea what the exact expansion and contraction coefficients are, so you'd
have to experiment.

You need a fairly long strip in order to get a decent amount of usable
movement. I drew up a little diagram showing how a long cellophane strip can
be woven around rollers or smooth rods in order to get a large movement in
a small space. You can attach a linear potentiometer as I have shown, or a
rotary potentiometer on a lever. The spring will have to be chosen with
care, so as not to overstretch the cellophane. I don't know what length and
width of cellophane will be necessary in order to get the right amount of
movement, but a couple feet by an inch wide might get you some measurable
movement.

Here's the drawing: http://www.macetech.com/hygrometer.pdf

I've also heard that magnetic tape can sometimes be affected by humidity in
this way. Loop a bunch of cassette tape around some nails in a plank for a
test setup, maybe?

 

[budgie]

I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
I've missed something, there doesn't seem to be a nice physical property
that can convert humidity to a resistance as in most sensors. The
application is indoor/outdoor humidity sensing for control of my furnace's
humidifier. I'd rather roll my own before I go to the hvac group for a
standalone commercial solution.

Most commercial units I've seen (eg Honeywell etc) use capacitance between
elements laid out in a grid on a plate or pcb, which is reasonably easy to
replicate. What you need is electronics to convert capacitance variation to
something readily received by your process controller. An oscillator based on
the cap, and a V-F converter is one path to consider.

Cap types are far better than "absorbtion" types (human hair etc) as the latter
require frequent recal to provide meaningful info.

 

[Paul Hovnanian P.E.]

I like the hair detector. It is not very linear, but if you need to
control humidity over a narrow range, a hair wrapped around a shaft
and tensioned with the shaft connected to a vane that passes through a
light beam detector (from an old mouse) makes a pretty stable 1 bit
A/D converter that indicates humidity too high or too low. (deep
breath) If the beam diameter is large enough it provides an analog
signal with a narrow range of measurement.

You're assuming that anyone on this group has any hair that they'd be
willing to part with (no pun intended).

 

[Don Lancaster]

Philips make one for control of clothes driers, they are less than $5
in qty, but are hard to track down. I have used them successfully, and
have the Philips part number somewhere if you need that.

Or, Use a couple of thermistors, one dry and the other damp with a
wick. There's tables around with the temp diff vs humidity. You do
need to keep the wick wet, which may or may not be a problem.

And, other's have mentioned the human hair, I have not tried it, but
IIRC there was one in the Amateur Scientist section in Scientific
American several years back.

Barry Lennox

Has to be UNWASHED human hair.
Shampooing trashes the effect.

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected] fax 847-574-1462

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[John Popelish]

John Popelish wrote:

You're assuming that anyone on this group has any hair that they'd be
willing to part with (no pun intended).

I have some beard hair that is almost a foot long. I lose a few every
time I zip up my jacket.

 

[Russell Shaw]

---
If you put a mechanical load on it, human hair stretches when it's damp
and tightens back up when it's dry, so you ought to be able to do
something with that. Google "human hair humidity sensor" for some more
info...

Well lemme guess...it's used in a "hair conditioner"...

 

[Spehro Pefhany]

Well lemme guess...it's used in a "hair conditioner"...

Hmm. The classic device was called the "horse hair hygrometer". Modern
humidistats use nylon, which is quite hygroscopic and changes
dimensions significantly when is absorbs moisture. You can consider
the slow rate of change (and hysteresis of the snap-switch) as a free
low-pass filter and timer; generally desirable when you're switching a
refrigeration apparatus as used in dehumidifiers.

Best regards,
Spehro Pefhany

 

[BFoelsch]

A very popular, smaller, more robust, and even more slow responding sensor
was the bi-wood element. Used in pneumatic systems. Imagine a bi-metal
thermostat made instead with two thin (.050 or so) strips of dissimilar wood
glued together.

Hmm. The classic device was called the "horse hair hygrometer". Modern
humidistats use nylon, which is quite hygroscopic and changes
dimensions significantly when is absorbs moisture. You can consider
the slow rate of change (and hysteresis of the snap-switch) as a free
low-pass filter and timer; generally desirable when you're switching a
refrigeration apparatus as used in dehumidifiers.

Best regards,
Spehro Pefhany

http://www.speff.com

 

[Dan]

what if you took (2) temp sensors, pumped a shot of water onto a cloth
covering one temp sensor and left the other one alone, but blew a fan over
both of them and recorded the temp of each. This is exactly how a sling
psychrometer works (in principle). I want to try this with 2 DS1620's one of
these days. Of course you have to know what the humidity really is to
calibrate them.

Dan