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Updated : 16/10/2016

GHowSAW Humidity Calibration

Introduction

The HIH-3610 Series humidity sensor delivers instrumentation-quality RH (Relative Humidity) sensing performance with a linear voltage output. Available in two lead spacing configurations, the RH sensor is a laser trimmed thermoset polymer capacitive sensing element with on-chip integrated signal conditioning. The sensing element's multilayer construction provides excellent resistance to application hazards such as wetting, dust, dirt, oils, and common environmental chemicals.

Vo = Vs (0.0062(Sensor RH) + 0.16)

Hence the transfer function:

RH = 161.28 (Vo / Vs) - 25.8064 @25C

Correcting for temperature:

True RH = (Sensor RH)/(1.0546-0.00216T), T in C

RH = (161.29 Vo / Vs -25.8064) / (1.0546 - 0.00216 T)

Note:

  1. Extended exposure to >90% RH causes a reversible shift of 3% RH.
  2. This sensor is light sensitive. For best results, shield the sensor from bright light.

Calibration

By itself, the HIH-3610 humidity sensor boasts accuracy of +/- 2% . Therefore, checking the calibration of the device is suggested, but not required.

Why Is Calibrating Humidity a Challenge?

You must contend with two issues to calibrate a humidity sensor or module. The first is the wide dynamic range the sensor must operate. Consider a typical range of 10% to 90%. At the low end of 10%, you have about 10,000 parts per billion of moisture whereas at the other end of 90%, you have approximately 200,000,000 parts per billion of moisture. That's a 20,000:1 range consequently it is quite difficult to establish an atmosphere of a specific humidity. The only method of achieving a specified relative humidity without laboratory facilities is to use saturated salt solutions.

The second issue is that humidity varies with temperature. When calibrating a humidity device, both the humidity and the temperature must be taken into consideration. For example, at room temperature and 50% RH, a 1 C change results in approximately a 3% RH change.

Calibration options

You have several options for calibrating your humidity sensor. The option you choose depends on how much effort you want to expend.

  1. "Out of the box" accuracy should be better than 10%, which is suitable for most applications and you can check this with a simple voltmeter reading.
  2. To achieve an accuracy of around 5%, you will have to do some calibration using saturated salt solutions. Either by calibrating again a known humidity meter or using the published relative humidity above saturated salt solutions (see table below)
  3. To achieve better than 5%, you will have to use a calibration laboratory and have your humidity sensor professionally calibrated.

Saturated salts for calibrating instruments

Relative humidity above saturated salt solutions at various temperatures

SaltTemperature °C
5.0 10.0 15.0 20.0 25.0
Lithium chloride 11.3 11.3 11.3 11.3 11.3
Magnesium chloride 33.6 33.5 33.3 33.1 32.8
Potassium carbonate 43.1 43.1 43.1 43.2 43.2
Sodium bromide 63.5 62.2 60.7 59.1 57.6
Sodium chloride 75.7 75.7 75.6 75.7 75.3
Potassium chloride 87.7 86.8 85.9 85.1 84.3
Potassium sulphate 98.5 98.2 97.9 97.6 97.3

Suitable dehydrating agents for calibration at zero RH

Baked molecular sieve with 3 micron holes is good and clean but its water content cannot be judged.

Baked calcium chloride is a widely used desiccant. An indicator is sometimes added to the salt when it is sold specially for use as a drying agent. It changes colour at too high an RH for calibration purposes.

Anhydrous copper sulfate is very good, and reusable by baking to a grey colour. It remains solid on absorption of water because it recrystallises to the monohydrate.

All these desiccants can be regenerated by heating at 150°C.

Phosphorus pentoxide comes closest to ensuring absolute dryness, but it rapidly acquires a syrupy surface film which prevents further absorption of water vapour. It is also very corrosive and nasty to handle. Not recommended for general use.

How to use saturated salts for calibrating instruments

The dry salt is spread about 3 mm deep in a shallow tray that occupies most of the bottom of an airtight box. Water is added to moisten the salt. Do not add more water than is needed to make the salt look damp. The instrument is then laid on a grid supported above the tray. Electronic sensors can be inserted through a hole in the box which is made reasonably airtight with a split rubber bung. Allow one hour for equilibration when the instrument is bulky. 20 minutes for small sensors.

Note that saturated salts that have equilibrium RH below ambient will continue to absorb water indefinitely and overflow. The 98% RH calibration can only be done in a room with constant temperature.

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