Quality - Process Control

Question 1

What is the meaning of variability?

Answer 1

Variability is the tendency of something to change or a lack of a regular pattern.

Question 2

What is the meaning of variation?

Answer 2

Variation is a change or difference in the properties of something.

Question 3

What are the two important concepts, that make up a specification?

Answer 3

Target - this is the value of the process parameter (e.g., temperature) or product parameter (e.g. % abv) that we want to achieve. A target indicates the value we are aiming for with regards to that parameter.

Range – any manufacturing process will have a degree of variation

Question 4

What are the two key rules to gain control over a process?

Answer 4

Centre the process on the target value, this means the average output of the process is at the target level of the specification.

Reduce the range, this means reducing the variability away from the target level.

Question 5

What is measurement error?

Answer 5

This is not a mistake made by the personnel in charge of measurement, but the inherent variability of the measurement process.

Question 6

How do we reduce measurement error?

Answer 6

through regular calibration, as well as by checking the equipment accuracy by measuring known standards then checking the result

Question 7

What is the run chart used for?

Answer 7

The run chart is a graph used to study how a process changes over time. Data are plotted in time order.

With the target as the central line, then the ULS & LLS as the range

Question 8

What is the The aim of statistical process?

Answer 8

is to use simple tools to improve the process until it is well within specification. This is known as the ‘capability’ of the process.

Question 9

Why are control charts a powerful tool?

Answer 9

They can provide early warnings as to when a process might be going out of control. They reflect what is normal for a process.

The control chart is a graph used to study how a process changes over time.

Data is plotted in a time order

Upper warning Limits % Lower warning limits is normally about 1.5
UCL & LCL - normally two

Question 10

What is a control loop?

Answer 10

In a control loop, a process variable is measured, and a signal is sent to a controller. The controller then checks the measurement against a set point and if the measurement is different to the set point, the controller changes the signal sent to an actuator (a mechanical device which adjusts a valve or other mechanism).

actuator responds to the signal by adjusting an input into the process to cause the process to change towards the set point

Question 11

What are benefits of using control charts?

Answer 11

can help us to identify problems and devise action plans to correct these problems in order of priority.

As control charts are based on facts, they are designed to reflect the actual process condition, they are seen as unbiased data.

They are based on specifications and control limits, using historical data. This means that they reflect what is happening within and during the process.

They use the concept of process capability, so they directly relate to the needs of our customers

They present the data in a pictorial format (a picture paints a thousand words). They uncover trends, which assist in solving real problems; they also help to prioritise issues.

Question 12

What are the two solutions that we can use for the problem solving?

Answer 12

Immediate corrective action - that fixes the current problem

&

Preventative action that ensures the problem does not occur in the first place.

Question 13

What are typical instruments used with control loops?

Answer 13

Temperature: resistance temperature detectors (RTD), and thermocouple (TC) probes

Pressure: pressure transducers

Gravity: Vibrating u-tube densitometers

Volume: Differential pressure or mass flow meters

Flow rate: Rotameters, magnetic flow meters (magflows)

Mass/weight: Counter-balance weights, load cells, or mass flow meters

Conductivity: Probes (measure the potential difference between two sensors)

pH: pH meters (measure the potential difference between a salt solution inside the pH probe and the sample outside the probe)