{"id":1952,"date":"2025-04-14T11:48:34","date_gmt":"2025-04-14T11:48:34","guid":{"rendered":"https:\/\/webprojects.cloud\/wordpress\/splatco\/?post_type=spl_knowledgebase&#038;p=1952"},"modified":"2025-06-10T08:52:33","modified_gmt":"2025-06-10T08:52:33","slug":"ph-measuring-analysis","status":"publish","type":"spl_knowledgebase","link":"https:\/\/webprojects.cloud\/wordpress\/splatco\/knowledgebase\/product-documentation\/product-documentation-xwire-peripherals\/sx10507-ph-measurement\/ph-measuring-analysis\/","title":{"rendered":"pH measuring analysis"},"content":{"rendered":"\n<h6 class=\"wp-block-heading\">Introduction<\/h6>\n\n\n\n<p>pH probes provide a voltage that is a direct relation to the solution&#8217;s pH and temperature:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>pH probe&#8217;s output is measured in mV.<\/li>\n\n\n\n<li>the result is always mV = 0 for a pH = 7, for all temperatures.<\/li>\n\n\n\n<li>The reading is linear in relation to the solution&#8217;s acidity.<\/li>\n\n\n\n<li>The slope of the reading is linear in relation to the temperature.<\/li>\n\n\n\n<li>at 0 degC the slope is 54.20 mV\/pH<\/li>\n\n\n\n<li>at 25 degC the slope is 59.16 mV\/pH<\/li>\n\n\n\n<li>at 100 degC the slope is 74.04 mV\/pH<\/li>\n<\/ul>\n\n\n\n<h6 class=\"wp-block-heading\">Calculating pH<\/h6>\n\n\n\n<p>If\u00a0<a href=\"https:\/\/webprojects.cloud\/wordpress\/splatco\/knowledgebase\/glossary-of-terms\/#o3808\">X<\/a>\u00a0is the slope of the pH reading for a given temperature, then the pH is given by:<\/p>\n\n\n\n<p>pH = 7.0 &#8211; V\/X With V the measured voltage of the probe in mV.<\/p>\n\n\n\n<p>And X is given by:<\/p>\n\n\n\n<p>X = 0.1984 * T + 54.20 With T in degrees Celcius.<\/p>\n\n\n\n<h6 class=\"wp-block-heading\">Measuring temperature<\/h6>\n\n\n\n<p>Temperature is measured using an\u00a0<a href=\"https:\/\/webprojects.cloud\/wordpress\/splatco\/knowledgebase\/glossary-of-terms\/#o1842\">RTD<\/a>\u00a0(Resistance Thermal Device). Its resistance is given by the following quadratic equation:<\/p>\n\n\n\n<p>Rt = R0*( 1 + A*T + B*T*T)<\/p>\n\n\n\n<p>With:<\/p>\n\n\n\n<p>R0 = 1000<\/p>\n\n\n\n<p>A = 0.0039083<\/p>\n\n\n\n<p>B = -0.0000005775<\/p>\n\n\n\n<p>Using a linear approximation for estimating the temperature using the thermal resistance.<\/p>\n\n\n\n<p>Rt = 1000 * ( 1 + 0.0039083 * T ) Which gives;<\/p>\n\n\n\n<p>T = ( Rt &#8211; 1000 ) \/ 3.9083<\/p>\n\n\n\n<p>The max temperature approximation error is 1.5 degrees C.<\/p>\n\n\n\n<h6 class=\"wp-block-heading\">Sensitivity of the result to the different parameters<\/h6>\n\n\n\n<p>The biggest variation of the pH in relation to the measured voltage will be where X is the smallest, i.e at 0 degrees Celcius:<\/p>\n\n\n\n<p>A variation of the measured voltage by 1mV will change the pH value by at most 0.000018<\/p>\n\n\n\n<p>The biggest variation of the pH in relation to the measured temperature will be where X is again the smallest, i.e between 0 and 1 degree Celcius, and at the highest measured voltages, i.e -500mV<\/p>\n\n\n\n<p>A variation of the measured temperature by 1.5 degree Celcius will change the pH value at most by 0.06<\/p>\n\n\n\n<h6 class=\"wp-block-heading\">Calibrating a pH probe and electronics<\/h6>\n\n\n\n<p>The electronics and probe will introduce errors in the form of drift of the reference and slopes. To correct this, calibration is needed and is done in the following manner:<\/p>\n\n\n\n<p>On a regular basis, the system will be calibrated using solutions at known pH.<\/p>\n\n\n\n<p>As the solutions are at a known pH, the expected measured valued are known, and therefor can be used in comparison with the actual measured values to infer a calibration correction.<\/p>\n\n\n\n<p>Method:<\/p>\n\n\n\n<p>The correction will be a linear function: transforming a measured voltage into an expected voltage.<\/p>\n\n\n\n<p>V = a * v + b With V = expected voltage and v = measured voltage.<\/p>\n\n\n\n<p>The two buffers used will have a pH of 7.0 and 4.0, the current temperature is important for calculating the expected voltage for the pH = 4 solution.<\/p>\n\n\n\n<p>If vph4 and vph7 are the measured values at the current temperature tcur , then we have:<\/p>\n\n\n\n<p>a * vph7 + b = 0<\/p>\n\n\n\n<p>a * vph4 + b = 3.0 * ( 0.1984 * tcur + 54.20)<\/p>\n\n\n\n<p>a = 3.0 * ( 0.1984 * tcur + 54.20) \/ (vph4 &#8211; vph7)<\/p>\n\n\n\n<p>b = &#8211; a * vph7<\/p>\n\n\n\n<p>Note: once a and b are calculated, then the expected voltage in the range -600 mV to 700 mV will be calculated using the formula: V = a*v + b, with v being the measured value, no matter what the electronic circuit or measuring range or number system is used.<\/p>\n\n\n\n<h6 class=\"wp-block-heading\">Effectively calculating pH<\/h6>\n\n\n\n<p>It is assumed that the following data is known:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>a calibration data<\/li>\n\n\n\n<li>b calibration data<\/li>\n\n\n\n<li>Rt measured resistance off temperature probe.<\/li>\n\n\n\n<li>Vp measured voltage off pH probe.<\/li>\n<\/ul>\n\n\n\n<p>1st step: calculate theoretical voltage of the probe.<\/p>\n\n\n\n<p>V = a * Vp + b<\/p>\n\n\n\n<p>2nd step: calculate temperature.<\/p>\n\n\n\n<p>T = ( Rt &#8211; 1000 ) \/ 3.9083<\/p>\n\n\n\n<p>3rd step: calculate temperature correction<\/p>\n\n\n\n<p>X = 0.1984 * T + 54.20<\/p>\n\n\n\n<p>4th step: calculate pH<\/p>\n\n\n\n<p>pH = 7.0 &#8211; V\/X<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Introduction pH probes provide a voltage that is a direct relation to the solution&#8217;s pH and temperature: Calculating pH If\u00a0X\u00a0is the slope of the pH&#8230;<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":1946,"menu_order":6,"template":"","class_list":["post-1952","spl_knowledgebase","type-spl_knowledgebase","status-publish","hentry"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v25.6 - 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