First, humidity sensor classification and feeling wet
characteristic
The humidity sensor, divides into the resistance type and the
electric capacity type two kinds, the product fundamental mode all for
湿材?forms the feeling in the substrate spreading feeling 湿膜.
In the air steam adsorption after the feeling 湿材? the
part impedance, the dielectric constant has the very big change, thus
makes the wet sensitive part.
The domestic and foreign various factories humidity sensor
product level one, the quality price all does not differ in a big way,
how does the user choose the performance price compared to the most
superior to be ideal product to really have the certain difficulty,
needs to make the thorough understanding in this aspect. The
humidity sensor has the following characteristic:
1st, precision and long-term stability
The humidity sensor precision should achieve ±2%~±5%rh, could
not achieve this level very difficult achievement measurement
appliance use, the humidity sensor to have to achieve ±2%~±3%rh the
precision is quite difficult, in the usual product material produces
the characteristic is (20 ?±10 ? and in the pure gas surveys in
the normal temperature. In actual use, as a result of dust,
greasy dirt and noxious gas influence, period of revolution one long,
can have the aging, the precision drops, the humidity sensor precision
level must unify its long-term stability to judge, in general, the
long-term stability and the service life are affects the humidity
sensor quality the prime question, the year drifting quantity control
in the 1%RH level product very few, generally all about ±2%, even is
higher.
2nd, humidity sensor temperature coefficient
Wet sensitive part besides to environment humidity sensitive,
extremely is also sensitive to the temperature, its temperature
coefficient generally in 0.2~0.8%RH/ ?scope, moreover some wet
sensitive parts under the different relative humidity, its temperature
coefficient also have the difference. Warm floats the
non-linearity, this need adds the temperature compensation type on the
electric circuit. Uses monolithic integrated circuit software to
compensate, or without the temperature compensation humidity sensor is
could not guarantee the entire warm scope precision, the humidity
sensor warm floated the curve linearization directly to affect the
effect which compensated, non-linear warm floated often cannot
compensate a better effect, only had uses the hardware temperature to
follow the nature to compensate only then can really obtain
compensates the effect. The humidity sensor work temperature
range also is the important parameter. Most wet sensitive parts
with difficulty in 40 ?above normal works.
3rd, humidity sensor power supply
When the metal oxide compound ceramics, the high polymer and the
lithium chloride and so on the wet sensitive material exerts the DC
voltage, can cause the performance change, even expires, therefore
this kind of humidity sensor cannot use the DC voltage or have the
direct current ingredient the alternating voltage. Must be the
alternating current power supply.
4th, interchangeability
At present, the humidity sensor generally has the
interchangeable bad phenomenon, the identical model sensor cannot
exchange, seriously has affected the use effect, for the service, the
debugging increased the difficulty, some factories have made all sorts
in this aspect diligently, (but interchangeability very was still bad)
has obtained a better effect.
5th, humidity adjustment
The adjustment humidity must be much more difficult than
the correction temperature. The temperature demarcates often
uses a standard thermometer to make the standard then, but the
humidity demarcates the standard comparatively to be difficult to
realize, does the wet-bulb thermometer and some common indicator type
hygrometers is cannot use for to do demarcates, the precision is
unable to guarantee, because its request environmental condition is
extremely strict, the ordinary circumstances, (should better under
humidity environment suitable condition) when lacks the consummation
the examination equipment, passes the commonly used simple saturated
salt brine 检? and surveys its temperature.
Second, makes the preliminary judgement to the humidity sensor
performance several methods
Actually demarcates in the difficult situation in the
humidity sensor, may carry on the humidity sensor performance
judgement and the inspection through some simple methods.
1st, the uniform determination, the identical type, the
identical factory humidity sensor product is best time purchases above
two, more more explains the question, puts in the same place circular
telegram comparison examination output value, under relatively stable
condition, observation test uniformity. If further examines, may
be separated period of time records in 24h, in one day generally all
has high, center, the low 3 kind of humidities and the temperature
conditions, may comprehensively observe the product the uniformity and
the stability, including temperature compensation characteristic.
2nd, breathes as when warming the hands with the mouth or uses
other to add the wet method to add wet to the sensor, observes its
sensitivity, the duplication, rises wet to escape the wet performance,
as well as resolution, product highest measuring range and so on.
3rd, makes the box to the product and closes the box two kind of
situations the tests. Compared with is whether consistent,
observes its thermal reaction situation.
4th, (according to instruction booklet standard) carries on the
test to the product in the high temperature condition and the low
temperature condition, and restores to the normal state under before
the examination and the experimental recording makes the comparison,
the examination product temperature compatibility, and observation
product uniform situation.
The product performance finally needs to rest on the quality
testing department regular complete examination method. Uses the
saturated salt brine to do demarcates, also may use the famous brand
goods to do compared to examines, the product also should carry on in
the long-term use process to demarcate for a long time can
comprehensively judge the humidity sensor the quality.
Third, to market in humidity sensor product several analyses
In the domestic market appeared many domestic and foreign
humidities sensor product, the electric capacity type wet sensitive
part comparatively saw, the feeling 湿材?type mainly was the high
polymer, the lithium chloride and the metal oxide compound.
The electric capacity type wet sensitive part merit lies in the
speed of response quickly, the volume small, linear good, is stabler,
the overseas some products also have the elevated temperature service
performance. But achieved the above performance the product many
is the overseas name brand, the price is all more expensive. In
the market sells some electric capacity type wet sensitive part low
price products, often could not achieve above level, linearity,
uniformity and duplication all not really ideal, below 30%RH, the
80%RH above feeling wet section distortion is serious. Some
products use the monolithic integrated circuit to compensate the
revision, causes the humidity to appear the "step to leap the" nature
jumping, causes the precision to reduce, has the uniformity to be bad,
the linear bad shortcoming. Regardless of the high scale or the
low scale electric capacity type wet sensitive part, the long-term
stability is not all ideal, the most long-term uses drifting is
serious, the wet sensitive electric capacity accommodates the value
change is the pF level, 1%RH change insufficient 0.5pF, Rong Zhi
drifting change often causes several dozens RH% error, the majority
electric capacity type wet sensitive part does not have the
performance which under 40 ?above temperatures works, often expires
and the damage.
The electric capacity type wet sensitive part corrosion
resistance ability also comparatively is short of, often to
environment purity request higher, some products also have
illumination to expire, phenomenon and so on static electricity
expiration, the metal oxide compound for the ceramic wet sensitive
resistance, has the wet sensitive electric capacity same merit, but
under the dust environment, the ceramic pore is sealed off the part to
be able to expire, often uses the circular telegram dust removal the
method to process, but the effect insufficiently is ideal, also cannot
use under the flammable explosive environment, the aluminum oxide
feeling 湿材?is unable naturally to overcome its surface texture
"to get older" the weakness, the impedance is unstable, Metal
oxygen ceramics wet sensitive resistance also similar existence
long-term stability bad weakness.
The lithium chloride wet sensitive resistance, has the most
prominent merit is the long-term stability greatly strengthened,
therefore through the strict craft manufacture, makes the measuring
appliance and the sensor product may achieve a higher precision, the
stability is strong is the product has the good linearity, the
accuracy and the uniformity, is the long-term service life reliable
guarantee. The lithium chloride wet sensitive part long-term
stability other feelings 湿材?still were unable to substitute.
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