How to determine conductance corrections for ammonia and carbon dioxide

Conductance Corrections for Ammonia and Carbon Dioxide

The purpose of the conductivity determination is to obtain a measure of the solids present in the condensed steam sample as an indication of the degree of carryover of boiler water solids. To properly interpret the conductivity of a condensed steam or condensate sample, it is necessary that corrections be made for the conductivity due to any gases present. After the concentration of ammonia or carbon dioxide has been determined the correction factor applying to that concentration is subtracted from the observed conductance. The conductivity correction for ammonia in the range of 0 to 1.0 ppm and carbon dioxide in the range of 0 to 30 ppm. For  example, if a condensed steam sample contained 10 ppm carbon dioxide and 0.6 ppm ammonia as N, the conductivity correction would be 6.0 micromhos. This correction must be subtracted from the observed conductivity of the sample to obtain the conductivity due to the solids present.

Example

Specific conductance = 8.0 micromhos ammonia as N = 0.6 ppm. Free carbon dioxide as CO2 = 10 ppm.

Correction due to gases = 6.0 micromhos

Corrected specific conductance: (8.0 - 6.0) = 2.0 micromhos

Solids content of sample: 2.0 micromhos X 0.5 (average factor.) = 1.0 ppm solids.

Limitation of Test

The conductance method is rapid and accurate for determining solids of 1 ppm or more in steam condensate. However, where any appreciable quantity of ammonia or free carbon dioxide is  present in the steam, it is necessary to determine the concentration of these gases and make suitable correction, otherwise the dissolved solids contents calculated from the conductance reading would be erroneously high. In many cases for routine control only negligible amounts of ammonia or free carbon dioxide may be present and a check for these gases need be made only at infrequent intervals. Where a high percentage of contaminated makeup is used in a plant, it usually is necessary to make frequent checks for the ammonia and free carbon dioxide content of the condensed steam.