Measuring viscosity

Hydramotion viscometers are designed to operate for long periods without requiring adjustment. From time to time, however, it may be necessary to modify certain operating parameters in order to take  account of variations in fluid characteristics.

General use

• Connect the viscometer transducer to the VP550.
• Apply power.
• Observe measured viscosity on the Main Display.

Periodic adjustment

• Ensure that the transducer is connected and the system is functioning normally.
• To access the User Options menus from the Main Display, press and hold down briefly, until “USER OPTIONS” appears on the display. The options will be displayed when the button is released.
• User Options are:

		reference
FLUID TEMPERATURE	temperature monitor	4.1
FLUID DENSITY	density monitor	4.2
TEMP CORRECTION	temperature correction parameter setting	4.1.1
AVERAGING FILTER	output averaging	4.3
TRANSIENT FILTER	output spike filter	4.4
VL SCALING	dynamic viscosity output scaling factors	4.5
VN SCALING	kinematic viscosity output scaling factors
PRESS CORRECTION	pressure correction parameter setting	4.6

• Throughout this manual, all examples are shown for dynamic viscosity. If kinematic viscosity has been selected, displays will show “VN” or “VNT” instead of “VL” or “VLT” respectively.

4.1 Temperature

The FLUID TEMPERATURE display shows:

SOURCE	temperature input selection
KEYPAD	keypad-entered temperature value
VP I/P	value obtained from external temperature sensor connected to the VP550
TRANSDUCER	temperature value measured by internal PRT
IN-USE	temperature value currently being used by the VP550

• Use SOURCE to select the desired temperature input.

4.1.1 Temperature Correction

• The TEMP CORRECTION menu shows:

VL	current “live” viscosity
VLT	temperature-corrected viscosity
TEMP	temperature value being used
REF-T	temperature to which viscosity should be corrected
TCF	fluid-specific temperature correction factor

• Viscosity corrected to a reference temperature is calculated as follows:

VT = temperature-corrected viscosity
VL = measured (“live”) viscosity
REF-T = reference temperature, °C
T = measured (“live”) temperature, °C
TCF = fluid-dependent constant (Temperature Correction Factor)

• Enter the desired reference temperature REF-T.
• Determine the Temperature Correction Factor TCF as described below and enter the value in the TCF line.

4.1.2 Calculation of Temperature Correction Factor (TCF)

• To calculate the TCF, determine the fluid viscosity at two different temperatures. The calculation is then:

where

V₁ = Viscosity at t₁ (°C)
V₂ = Viscosity at t₂ (°C)

• If possible, ensure that temperature values are chosen around the average operational temperature and the reference temperature.

4.1.3 Experimental determination of Temperature Correction Factor

• The Temperature Correction Factor TCF can be derived by trial and error. This can only be done if the corrected viscosity of the fluid is known to be stable over a measurement period, even though temperature changes are causing live viscosity to fluctuate.
• The objective is to find a value of TCF which causes the Corrected Viscosity reading to stabilise at a constant value even though Live Viscosity and temperature are changing.
• The procedure is:
  (1) Enter a value for TCF of about 5000. Is Corrected Viscosity becoming less variable?
  (2)Try increasing TCF to (for example) 5200. What is the effect on Corrected Viscosity?
  (3) If Corrected Viscosity appears to be less variable, increase TCF again, up to about 5500.
  (4) If Corrected Viscosity appears to be more variable, decrease TCF to around 4500.
  (5) Continue in this way until the Corrected Viscosity stabilises at a nearly constant value.

Note: it is important to recognise that this technique only works if the corrected viscosity is truly constant and is not legitimately changing.

4.2 Density

The FLUID DENSITY display shows:

SOURCE	density input selection
KEYPAD	keypad-entered density value
D LIVE	density value captured from an external density meter connected to the VP550
IN-USE	density value currently being used by the VP550

• The output signal from the transducer is actually the product of viscosity and density. In most cases changes in fluid density are negligible.
• To obtain an absolutely accurate viscosity reading:
  (1) enter the fluid density value in g/cm³ in KEYPAD, or
  (2) connect an external density meter providing a 4-20mA analogue
  density signal (see Section 1.3 for connection details and Section 3.3.2 for setup).
• Use SOURCE to select either keypad-entered (KEYPAD) or externally measured (VP I/P) density.
• The density value currently being used by the VP550 is shown in the IN-USE line.
• The factory-set density value is a nominal 1.0 g/cm³. The density of pure water at 20°C is 0.998 g/cm³.
• If kinematic viscosity is required, check that the IN-USE value is correct.

4.3 Averaging filter

This display shows:

VL-INST	instantaneous (unfiltered) “live” viscosity
VL-AVG	averaged viscosity
AVG-NO	number of readings to average

• A Hydramotion viscometer takes a reading approximately once every second. The main display always shows the average of the last n consecutive readings, where n is the value of AVG-NO.
• For no filtering, set AVG-NO to 1.
• To smooth noise or jitter in the output, adjust the value of AVG-NO, i.e. the number of consecutive readings to be averaged.
• The greater the number AVG-NO the heavier the filtering, but also the longer the response time of the system.
• Generally, for good signal stability without particularly compromising response, set AVG-NO to 6. The optimum setting depends on the actual measurement application.

• If AVG-NO is greater than 32, the average is calculated as follows:

where

n = AVG-NO
VLAV = latest average
V_av = previous average
VL = latest reading

4.4 Transient filter

This display shows:

VL-INST	instantaneous “live” viscosity (unfiltered)
VL	filtered live viscosity
TOLERANCE	“tolerance band” (limits of acceptance)
SHOCKS	number of readings rejected

• Use the transient filter to reject spurious and unwanted spikes in the viscosity signal. Because this filter has the capacity to reject viscosity readings, use with care.
• The filter rejects any change in signal that exceeds the acceptance limits set by the TOLERANCE parameter. The VP550 then uses the last “good” value prior to the filter band being exceeded (Figure 22).
• For example: if TOLERANCE = 20 and average viscosity = 100, the filter will reject any measurement greater than 120 or less than 80.
• Set TOLERANCE high enough to pass real changes in fluid viscosity but reject the much bigger jumps experienced with unwanted transients.
• For no rejection (to cancel the filter), set TOLERANCE to 0 (zero). This is also the factory default setting.

4.5 Viscosity scaling

The VL SCALING menu includes:

V-RAW	calculated dynamic viscosity (unscaled)
VL	current “live” dynamic viscosity (scaled)
SPAN	scaling factor
OFFSET	viscosity offset

The VN SCALING menu includes:

V-RAW	calculated kinematic viscosity (unscaled)
VL	current “live” dynamic viscosity (scaled)
SPAN	scaling factor
OFFSET	viscosity offset

• A Hydramotion viscometer measures the amount of energy lost from the vibration of the sensing element because of the viscosity of the surrounding fluid. The energy lost is called the “loss factor”, L. Dynamic viscosity V (= “V-RAW”) is calculated as follows:

• The factors P30, P31, P32 etc. are “calibration constants” specific to the individual sensor. They are empirically determined and stored in the transducer during calibration at the factory. For your reference they are set out on the Calibration Certificate supplied with the transducer and can be viewed by navigating to SETUP > TRANSDUCER CAL > CONSTANTS.
• Two adjustment factors are provided, a scaling factor SPAN and an offset OFFSET. The full equation for instantaneous live dynamic viscosity VL is:

4.5.1 Matching readings from another viscometer

• Use SPAN to match the Hydramotion viscometer readings with those from a reference viscometer.
• To calculate the appropriate value of SPAN, measure the viscosity of a fluid using both viscometers at the same temperature and take the ratio of the two readings, i.e.

NOTE: Changing SPAN will change the viscosity value for both the display and the 4-20mA analogue output.

4.5.2 Conversion to other viscosity units

• The viscometer is factory-calibrated in centipoise (cP) (equivalent to millipascal-seconds, mPa·s) unless otherwise specified at the ordering stage.
• To convert from centipoise or centistokes to other units of viscosity, navigate to SETUP > VP SETUP > UNITS DISPLAY > Main Unit and select the desired units (see Section 3.3.1). Do not use SPAN for this purpose.

4.5.3 Minor calibration adjustment

• To null a small non-zero reading of viscosity in air, adjust the value of OFFSET.

4.6 Pressure correction

The PRESS CORRECTION menu shows:

VL	current “live” viscosity
VLT	temperature-corrected viscosity
VLTP	temperature- and pressure-corrected viscosity
PRESS	measured pressure
REF-P	pressure to which viscosity should be corrected
PCF	fluid-specific pressure correction factor

• Viscosity corrected to a reference pressure is calculated as follows:

where
VP = pressure-corrected viscosity

VL = measured (“live”) viscosity

REF-P = reference pressure, bar

PRESS = measured (“live”) pressure, bar

PCF = fluid-dependent constant (Pressure Correction Factor)

• Enter the desired reference pressure REF-P.
• Determine the Pressure Correction Factor PCF as described below and enter the value in the PCF line.

4.6.1 Calculation of Pressure Correction Factor (PCF)

• To calculate PCF, determine the fluid viscosity at two different pressures.
  The calculation is then:

where

V₁ = Viscosity at pressure p₁ (bar)
V₂ = Viscosity at pressure p₂ (bar)

• If possible, ensure that pressure values are chosen around the reference pressure (p₁) and the average operational pressure (p₂).