Measuring Relative Humidity using a Honeywell HIH-3610 Sensor, 18X

copyright, Peter H Anderson, Baltimore, MD, Jan, '04

Introduction.

This application note discusses interfacing a Honeywell HIH-3610 humidity sensor with a PICAXE-18X.

The HIH-3610 is a three terminal single inline package (SIP) which provides a linear voltage output proportional to relative humidity. This voltage is read using a 10-bit A/D on the PICAXE-18X and the relative humidity is calculated. A Dallas DS18B20 is used to measure temperature and adjust the raw RH reading.

I developed this routine for measuring the relative humidity at a single location. However, the routine could be easily extended to multiple points with a HIH-3610 and a DS18B20 located at each location.

Detailed Discussion

The output of the HIH-3610 relative humidity sensor is; 

    Vad = Vref * (0.0062 * RH_raw + 0.16)
This may be arranged; 
    RH_raw = 161.29 * Vad / Vref - 25.80
or 
    RH_raw = 161.29 * ADVal / 1024 - 25.8
At first glance, this appeared formidable for a processor not having floating point capability. However, it turned out to be simpler than I had thought.

The above equation might be multiplied by 10; 

	RH_raw_10 = 1613 * ADVal / 1024 - 258
Note that the product of 1613 and ADVal may well overflow a word (65535 max) and thus I opted to use two words to store the product; 
   HighWord = 1613 ** ADval
   LowWord = 1613 * ADVal
The division by 1024 is then a matter of shifting this 32-bit quantity, ten places to the right; 
   ' RH_10 = LowWord / 1024 + Highword * 64 - 258

   RH10 = LowWord / 1024		' lowest six bits
   LowWord = Highword * 64  	' put highword in the high ten bits of lowword
   RH10 = RH10 + LowWord
   RH10 = RH10 - 258
The RH reading is then compensated for temperature by multiplying by; 
   1 / (1.0546 - 0.00216 * Tc)
This poses a real challenge. Using Excel, this factor was calculated for temperatures over the range of -40 to 120 degrees C. 

	X   Tc	TFactor

	0	-40 0.87
	1	-30	0.89
	2	-20 0.91
	3	-10 0.93
	4	0   0.95
	5	10  0.97
	6	20  0.99
	7	30  1.01
	8	40  1.03
	9	50  1.06
	10	60  1.08
	11	70  1.11
	12	80  1.13
	13	90  1.16
	14	100 1.19
	15	110 1.22
	16	120 1.26
The temperature is measured using a DS18B20 and divided by 10. If the temperature is negative, the index X is calculated as 4 - (TC/10) and if positive as 4 + (Tc/10). 
   If Tc < 128 Then Positive
     SignBit = 1 	' minus
     Tc = Tc ^ $ff + 1	' 2's compliment
     X = Tc / 10
     X = 4 - X
     GoTo Continue1

Positive:
   SignBit = 0
   X = Tc / 10
   X = X + 4

A lookup table is then used to map index X into the temperature compensation factor and the RH is multiplied by this factor; 

TempCorrection:
   Lookup X, (87, 89, 91, 93, 95, 97, 99, 101, 103, 106, 108, 110, 113, 116, 119, 122, 126), TFactor
            ' -40 -30 -20 -10 0   10  20   30   40   50   60   70   80   90   100  110  120

   If TFactor < 100 Then TempCorrection_1

   TFactor = TFactor % 100

   Dig = TFactor / 10
   RH10 = RH10  * Dig / 10  + RH10
   TFactor = TFactor % 10

   Dig = TFactor
   RH10 = RH10  * Dig / 100 + RH10

TempCorrection_2:
   Return

TempCorrection_1:
   Dig = TFactor / 10
   RH10 = RH10 * Dig / 10
   TFactor = TFactor % 10

   Dig = TFactor
   RH10 = RH10 * Dig / 100 + RH10
   GoTo TempCorrection_2

Program HIH3610.Bas 

' HIH3610.Bas
'
' Illustrates an interface with a Honeywell HIH-3610 humidity sensor and a DS18B20 to
' measure relative humidity.
'
' The program continually loops, displaying the temperature compensated RH and the temperature.
'
' HIH-3610 --------------------- AN2 (term 1)
'
' DS18B20 ---------------------- INPUT6 (term 15)
'
' Note that a 4.7K Ohm pull up to +5 is required on the DQ lead associated with the DS18B20.
'
' copyright, Peter H Anderson, Baltimore, MD, Jan, '04

   Symbol ADVal = W0
   Symbol HighWord = W1
   Symbol LowWord = W2
   Symbol RH10 = W3
   Symbol Whole = B0
   Symbol Fract = B1
   Symbol X = B0
   Symbol Dig = B1
   Symbol TFactor = B2
   Symbol Tc = B3
   Symbol SignBit = B4

Top:
   ReadADC10 2, ADVal		' perform A/D

   HighWord = 1613 ** ADval	' calculate RH
   LowWord = 1613 * ADVal
   RH10 = LowWord / 1024
   LowWord = Highword * 64
   RH10 = RH10 + LowWord
   RH10 = RH10 - 258

   ReadTemp 6, Tc		' read Tc
   If Tc < 128 Then Positive
     SignBit = 1 	' minus
     Tc = Tc ^ $ff + 1	' 2's compliment
     X = Tc / 10
     X = 4 - X
     GoTo Continue1

Positive:
   SignBit = 0
   X = Tc / 10
   X = X + 4

Continue1:
   GoSub TempCorrection		' compensate RH

   Whole = RH10 / 10		' display RH
   Fract = RH10 % 10

   If Whole > 99 Then OverRange
   If Whole > 127 Then UnderRange

Continue2:
   SerTxD (#Whole, ".", #Fract, "  ")

   If SignBit = 0 Then Continue3	' display temperature
     SerTxD ("-")

Continue3:
   SerTxD (#Tc, 13, 10)
   Wait 1
   GoTo Top

OverRange:
    Whole = 99
    Fract = 9
    GoTo Continue2

UnderRange:
    Whole = 0
    Fract = 0
    GoTo Continue2

TempCorrection:
   Lookup X, (87, 89, 91, 93, 95, 97, 99, 101, 103, 106, 108, 110, 113, 116, 119, 122, 126), TFactor
            ' -40 -30 -20 -10 0   10  20   30   40   50   60   70   80   90   100  110  120

   If TFactor < 100 Then TempCorrection_1

   TFactor = TFactor % 100

   Dig = TFactor / 10
   RH10 = RH10  * Dig / 10  + RH10
   TFactor = TFactor % 10

   Dig = TFactor
   RH10 = RH10  * Dig / 100 + RH10

TempCorrection_2:
   Return

TempCorrection_1:
   Dig = TFactor / 10
   RH10 = RH10 * Dig / 10
   TFactor = TFactor % 10

   Dig = TFactor
   RH10 = RH10 * Dig / 100 + RH10
   GoTo TempCorrection_2