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Humidity meters and their sensors.

these are all humidity measures

hygrometers are readily available on

eBay and if you just type in digital

humidity meter you'll find them you can

also get round dial ones as well just

with a standard air non-electronic

mechanism but if you buy a few of these

and they're cheap they're about 90 named

pence each including shipping which is

so commonly the case on eBay if you buy

a lord you'll find that if sat next to

each other there's a variation in them

if you're wondering how to actually

determine the accuracy of them here's

how you can calibrate them or at least

get an indication of how accurate they

are if you get a little tub like even a

cap drinks cap or something and then you

add some sodium chloride which is common

table salt and you add enough water

theoretically if you're doing this super

mega accurately like lab grade it'd be

pure chemicals and distilled water but

you know for a home use table salt and

tap water is ideal and if you mix them

too slowly improve put it in a sealed

bag then it's going to create humidity

in the bag of approximately 75% humidity

and in this case it's showing 1% above

that in fact I think it's very 75.5 and

it's unusual it really bit it's

consistent across a wide range of

temperatures that are all set low to

this humidity and I've got another set

up here I've got a sealed container and

it's worth mentioning you have to put

the put them in and just leave them for

about at least a day just to let the

humidity stabilize but in this one and

I've got two displaying I've got three

displaying 75% so they're good I've got

one displaying sympathy one displaying

74 and one displaying 77 and all I'd do

with that is the one displaying 77 or I

would just write on it - - to show up

you know or plus - breeches whichever

way you want to actually do it to show

how you know it relates to the actual

humidity the reference humidity then is

a good simple way to do it it's really

incredibly simple so let's take a look

inside one of these there's not really

an awful lot in them they usually take

two cells but the two cells are just in

parallel I just dare

try and get those cells out of it and

you can run it on just one cell but I

think if you're used to just for the

long battery run time and if you open it

up inside is the usual little

arrangement you get the circuit board

you've got an LCD display with the zebra

strip on Tet which is our sort of

alternating insulating and conducting

strip that just transfers these contacts

on this circuit board onto the the

transparent conductive layers on the LCD

panel you've got the usual blob chip and

then you get the two sensors the

temperature sensor here which is a

little bead thermistor and then the

humidity sensor now the humidity sensor

is quite interesting because let's see

where where is it that I've varied it

yes I've buried it

oh yeah I have actually but there is the

humidity sensor looks like this and it's

basically a ceramic plate and this is

actual one of the humidity sensors

I should mention you're not supposed to

touch them because the container

moisture absorbing surface and if you

contaminate it with the salt in your

fingers that will basically record it

will never read right again which that

one's going to do but that's a picture

of that actual sensor and just zoomed up

and you can see it's got these

conductive layers on the on the ceramic

substrate and then not so visible is the

fact that there's a sort of it's been

dipped and that area there it has a sort

of lacquer across it and that's a that

lacquer is a sort of conductive of all

the I guess a polymer they call it I

looked up a data sheet on it

and it uses a thin film polymer and it

it's basically it's salts and it absorb

the water and change the resistance note

I save change the resistance and you

have to be careful because there are

little modules being sold on eBay which

have these sensors and they're using an

op-amp to actually measure it and

they're just using the sensor as a

resistor as far as I can see which isn't

ideal because you can't really do that

and if you use these sensors as a

resistor

it's got moisture in it the it will

cause polarization actual metal from the

electrode will migrate into the the film

and change its ability to write to the

humidity properly so the correct way to

drive these is with AC and this

datasheet here for unfair no willow

technologies so as DC current should

never be applied to the EMD 4000

humidity sensor application of direct

current will polarize a sensor and cause

an irreversible shift only a symmetrical

EC excitation current should be applied

Amphenol recommends the application of a

low-level AC excitation 1 volt AC at 1

kilohertz typical to minimize self

heating effects so basically speaking

you're measuring the impedance but

actually it is kind of resistance but

you're measuring it with AC so that you

don't cause that migration of the

electrodes into the into the material

and I have one of these modules knocking

about I think I've got the version there

relay and it just doesn't read right at

all it just gradually just my grapes the

range just migrates continuing that is

what's actually happening there it's

changing the characteristics but these

little sensors are available online you

can buy packs them the cold on HR 202 l

humidity sensor but having said that you

can also get the if you type in humidity

and Arduino you'll all see did you also

see digital versions which just have a

plus 5 0 volt and data and they're

actually a better option because they

deal with all the process and they have

the little thing that generates the AC

signal inside and measures of humidity

but and they're interesting technology

I'm certainly it's good for keeping tabs

on the humidity in your home and making

sure that humidifiers are doing the

dehumidifier to say are doing their job

well give it a farz as well if you live

in a very dry area they're cheap and

once you've actually got a rough

calibration I mean this one is now at

75% which means it's pretty much spot-on

and then you know you at least know

where you stand if it's way off or not

and it's interesting to know I had one

of these that was just absolutely just

bonkers it was like 10% off and I opened

it and there was flux all over the

humidity sensor and I tried washing off

that really didn't work and I had to

pack these little sensors and I put one

in to replace and

actually went to virtually 1% accuracy

so they seem to be very very standard in

the way the manufacturer it's very

consistent manufacturing but the other

interesting little devices