Friday, March 22, 2019

Webinar: pH Basics - Quick and Dirty Tips by Yokogawa

Yokogawa Webinar
March 28, 2019 2 PM Eastern

The pH electrode offers by far the greatest sensitivity and rangeability of any measurement. Making the most out of your pH readings requires proper mixing and control.

Join this webinar and learn how to monitor, troubleshoot, and maintain electrodes and control valves. Review control strategies for pH applications critical to product and water quality in the process industry.

In this webinar, you will learn:
  • Fundamental requirements for pH measurements
  • The importance of proper maintenance and calibration techniques
  • The value and use of equipment diagnostics.
Date: March 28, 2019
Time: 2:00 PM Eastern  
Duration: 1 Hour

Thursday, March 14, 2019

Process Weighing Systems Overview

High-accuracy, batch-processing tank
fully supported by KIS Weigh Modules.
In its simplest form, a weigh system consists of a vessel whose contents are to be monitored, load cells or weigh modules that generate a signal proportional to the vessel weight, and an electronic device to power, amplify, interpret and display the signal. However, the accuracy of such a system, while obviously a function of the instrumentation, is also dependent upon the vessel design (reactor, batch tank, inventory silo, etc.), support structure, piping attachments, lateral restraint system, vessel environment (temperature, traffic, nearby equipment), and proper selection of transducer accessories. In short, weigh system accuracy is inexorably tied to the degree of attention given to the mechanical details and vessel functionality.

High accuracy, process weighing systems exhibit system errors under 0.05% for buy-and-sell to 0.25%. Precision load cells or weigh modules with full temperature compensation must be used. To achieve this, the following mechanical requirements are imposed:
  • The weigh vessel must be fully supported by load cells/weigh modules. The number of load cells/weigh modules may vary from one (in tension) to eight (in compression). Generally, as the number of load cells de-creases, the vessel wall thickness and support structure stiffness must increase to carry the higher vessel support reactions lest vessel deformation cause calibration errors.
  • Mechanical restrictions from attached piping and lateral restraints should be avoided. Highly flexible piping attachments are recommended.
  • Hot gas or steam-heating schemes which produce variable buoyancy should be avoided. Consult factory for alternate solutions.
  • Low accuracy inventory weighing systems are those with a system error greater than 0.5%. General purpose cells/ modules are satisfactory for these systems. Mechanical considerations are relaxed considerably:
  • The weigh vessel need only be partially supported by load cells/weigh modules, usually one or two on any side or end of the vessel. This, however, requires the contents to be self-leveling and the vessel itself to be without partitions, so that the load fraction carried by the load cells/ modules is unchanging. (Vessels falling into these two categories must be fully supported, independent of the accuracy required.)
  • Modest mechanical restrictions may be tolerated, but nonlinear mechanical hang-ups or frictional interfaces must still be avoided.
Accuracy Versus Repeatability

Low-accuracy, storage silo partially
supported by KIS Weigh Modules.
Do not confuse system accuracy with repeatability! As long as the mechanical error in a given system is linear with deflection and independent of the environment (temperature, traffic, surrounding vessels, etc.), the inherent system repeatability will be greater than its accuracy. For example, BLH Nobel Transducer Indicators typically have an overall accuracy specification of 0.01% of reading, ±1 count (or better), of which repeatability is but a small fraction. BLH Nobel load transducers, meanwhile, typically display a repeatability of 0.01 to 0.02%. Thus, most BLH Nobel systems will be repeatable within 0.03% of full scale, independent of how the system is calibrated. For most batching operations, repeatability is essential, whereas accuracy (actual pounds used) is of secondary importance once the operating parameters have been established. Field calibration, when required, is generally done by electronic simulation. For buy-and-sell installations, where distribution is by weight, calibration and repeatability are essential; field calibration is always performed employing a dead weight method.

Definitions:
  • Accuracy - Ability of the system to perform weighing functions within an acceptable or desirable tolerance; usually stated as a percentage of either full-scale reading, or ±n count(s) referred to the total number of scale divisions.
  • Repeatability - The ability of the system to read the same value when the measured weight is applied repeatedly in the same manner with the same quantity under constant conditions.
Maximum accuracy and repeatability is obtained by placing high accuracy load cells/modules at all support points and connecting them to instrumentation that measures each cell/module individually.

For more information on any process weighing application, contact Arjay Automation. They can be reached by phone at (800) 761-1749, or visit them on the Web at https://arjaynet.com.