In this article, we discuss about How calibration is to be performed through decade resistance box for a conductivity transmitter.
Image courtesy : Emerson analytical
Theory :
Conductivity is the ability of any solution to pass a electric current. In any liquid current is carried by cation and anions.
Conductivity of any solution is depending on following factors:
The unit of conductivity is micro-Siemens per centimeter or µS/cm.
Need of conductivity measurement:
Calibration can be done by two methods.
Here we are discussing calibration with resistance decade box.
And consider Emerson make conductivity analyser or transmitter and model number is 1181T.
As we are discussing about calibration via a decade resistance box, we need to calculate the equivalent resistance which to be applied from a decade resistance box to the conductivity transmitter for the calibration purpose. As per our process temperature range, we have three different types of formulas for the resistance calculations.
The three formulas as per process temperature range are :
Resistance (conductivity) = Probe Constant X 1,000,000 / F.S. Conductivity
Resistance (conductivity) = 0.4 X Probe Constant X 1,000,000 / F.S. Conductivity
Resistance (conductivity) = 0.4 X Probe Constant X 1,000,000 / F.S. Conductivity
In our discussions, we considered sample temperature is in between 0 – 50 deg Celsius. so first formula is applied in our below example.
Suppose a conductivity transmitter calibration to be carried out which has range 0 – 100 µS/cm and cell constants 0.1
Solution:
For any transmitter, there are two minimum calibration checks will be there.
They are
Connect the decade box and power supply as shown in below figure.
Zero calibration :
For Zero calibration : remove/disconnect the decade resistance box and adjust the transmitter to 4 ma for Zero reading.
Span calibration :
Resistance (conductivity) = Probe Constant X 1,000,000 / F.S. Conductivity
= 0.1 X 1,000, 000 / 100 ohm = 1000 ohm = 1 kilo ohm
Middle point calibration :
we can also do a intermediate calibration check of the 4-20mA transmitter range.
Here we considered 12mA for the calibration purpose.
So again calculate the resistance for the middle point calibration
Resistance = 2 X full scale resistance conductivity
= 2 X 1000 ohm = 2 kilo ohm
Reference : Emerson analytical
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View Comments
This described method is not correct.
First of all it is assumed that the sample (process) temperature dependency alva is linear in particular temperature ranges. This may be acceptable valid for most of the water applications but certainly not for many other processes !
Then the cell constant K is also not explained how to choose the correct K value based on the measuring range of conductivity!
A far better full explanation is given by Yogogawa, who took it from earlier Electrofact.
Any comment, please contact me.
Mostly we measure the conductivity of water which is used in heat exchanger and in this article describe how to calibrate so k value is already given with sensor and transmitter
Which formula to be used if sample temp is in between 50 to 80 degree celcius?
which method is used to calibrate the toroidal conductivity analyzer?
Option A) with probe discounted ,connect decade resistance box across the input terminal , then dial a desire resistance.
B) with probe disconnected loop a wire toroidal , attached decade resistance box, then in desire resistance