Campbell CS500 Instruction Manual - 6.  Long Lead Lengths

CS500 Temperature and Relative Humidity Probe

8

;Measure the CS500 relative humidity.

;

02:  Volt (SE) (P1)

1:

1

Reps

2:

5

2500 mV Slow Range

;CR500 (2500 mV); CR23X (1000 mV); 21X,

CR7 (5000 mV)

3:

4

SE Channel

;Brown wire (SE 4), Green wire (G)

4:

3

Loc [ RH_pct    ]

5:

.1

Mult

;See Table 3 for alternate multipliers

6:

0

Offset

;Limit the maximum relative humidity to 100%.

;

03:  If (X<=>F) (P89)

1:

3

X Loc [ RH_pct    ]

2:

3

>=

3:

100

F

4:

30

Then Do

04:  Z=F (P30)

1:

100

F

2:

0

Exponent of 10

3:

3

Z Loc [ RH_pct    ]

05:  End (P95)

6.  Long Lead Lengths

Long lead lengths cause errors in the measured temperature and relative

humidity.  The approximate error in temperature and relative humidity is

0.35

°C and 0.35% per 100 feet of cable length, respectively.

When long lead lengths are required and the above errors in temperature and

relative humidity are unacceptable, use the HMP45C temperature and humidity

probe.

Understanding the following details are not required for the general operation

of the CS500 with Campbell Scientific’s dataloggers.  The signal reference and

the power ground (black) are the same lead in the CS500.  When the CS500

temperature and relative humidity are measured, both the signal reference and

power ground are connected to ground at the datalogger.  The signal

reference/power ground lead serves as the return path for 12 V.  There will be a

voltage drop along this lead because the wire itself has resistance.  The CS500

draws approximately 2 mA when it is powered.  The wire used in the CS500

(P/N 9720) has resistance of 17.5 

Ω/1000 feet.  Using Ohm’s law, the voltage

drop (V

d

), along the signal reference/power ground lead, is given by Eq. (1).

V

I

R

d

= ∗

=

∗

=

 mA   

 ft 

 mV   

 ft 

2

17 5

1000

35

1000

.

Ω

(1)