Precision Measurement Engineering, Inc.

Discussing Calibration

CONDUCTIVITY

The range of sensitivity for both accurate and fast conductivity channels can be changed. In the case of accurate conductivity, one of three ranges can be selected. Many ranges are possible for fast conductivity since ranges are set by installation of a custom resistor. In all cases, the lower end of the range remains fixed at 0.05 S/m; only the upper limit is adjustable. SCAMP allows scaling of channels whereby parts of the range are mapped onto the full  +/- 32768 integer range of the A/D.

Accurate conductivity is supplied with 3 ranges. These are selected by moving a jumper block located on the accurate conductivity circuit. This diagram shows this circuit and the jumper block positions that correspond to the three ranges. Fast conductivity is supplied with only 1 range. This is set by the value of the resistor connected in the 9 S/m position on the fast conductivity circuit. Other ranges are selected by removing this resistor and installing a replacement.

PME calibrates only the 0 to 9 S/m range for both accurate and fast conductivity channels. If a different range is selected, then the corresponding calibration coefficients must be modified or the SCAMP re-calibrated. The recommended approach is to select a new range resistor, then re-calibrate the SCAMP using an accurate independent reference. If this is not possible, then modify the calibration coefficients as described below.

The modification is as follows:

Definitions:

When the SCAMP is calibrated, a relation between conductivity, in engineering units, and channel output is developed for R,

                            C = C0  +  C1*CR  +  C2*CR^2  +  C3*CR^3

The conductivity circuit operates in a way that causes CR to be very nearly a linear function of  R. At a given conductivity, the relation between circuit outputs and the resistors is approximately,

                                                    CR' /CR = R'/R

Solving this for CR and substituting into the calibration equation above gives

            C = C0  +  C1*(CR'*R/R')  +  C2*(CR'*R/R')^2  +  C3*(CR'*R/R')^3

The new coefficients are identified from this form as,

                                                     C0' = C0

                                                     C1' = C1*(R/R')

                                                     C2' = C2*(R/R')^2

                                                     C3' = C3*(R/R')^3

giving,

                            C = C0'  +  C1'*(CR')  +  C2'*(CR')^2  +  C3'*(CR')^3

The accurate conductivity channel is supplied with three scaling resistors. The normal values for these resistors are:

In some cases, different resistors may be supplied for special ranges. The values are printed on the resistors next to the jumper. Please check these.

The fast conductivity channel is supplied with a single resistor that is chosen to match the actual conductivity cell in-use. You must measure this resistor and then calculate the correct resistor to use for the desired range.

The new range will not be as accurate as the calibrated range. The first major contributor is the 1% resistor tolerance. This can be avoided by removing the jumper and measuring the resistances involved directly with an accurate ohm meter. The second contribution results from the observation above that the CR, CR' relation is only approximately correct. Presently, the amount of error that this contributes is unknown, but is believed to be in the few percent range.