Cable Tension Meters (or is it Tensiometers or Dynamometers?) for Barges, Rigs, Construction, and Pipeline Inspection.

We often get asked questions about the difference between tension meters, tensiometers, and dynamometers. There is good reason for the confusion because many different industries use the same words for different applications. For example, the word tensiometer, which we use to describe our primary cable tension/payout monitoring equipment, is also used to describe a soil moisture monitor, and dynamometer is a word that was widely used within North America for a device for measuring automotive horsepower.

At Measurement Technology NW we focus on measuring cable tension in many different applications (from 1,000 lbs-1,000,000 lbs) and we use the words tensiometer (dynamometer) and tension meter interchangeably. Even though the words mean much the same thing, our tensiometers can be provided with an extra sensor to measure payout and speed – two line control parameters that are not always considered when referencing a standard “tension meter”.

Tensiometers can be of two main styles; running line tensiometers (RLT’s) are designed to ride the line and provide measurements of tension, speed and/or payout as the line moves through the unit. Static tensiometers are devices that clamp onto to non-moving lines and only need to provide a measurement of line tension.

Our customers use MTNW tension monitoring devices in applications such as: offshore mooring, barge mooring, oceanographic payload monitoring, ROV launch and recovery system monitoring, tethered pipeline inspection (or pigging) monitoring, etc. Usually it is a safety requirement that drives the procurement of these tensiometers.

Here is a picture of a barge mooring monitoring project for which Ombak Marine Group in Kuala Lumpur has installed a group of MTNW’s tensiometers. These units are defined as tensiometers (or dynamometers) because we are helping them measure all three line parameters.

Our running line tensiometers are 3 sheave devices that detect tension by the deflection of the strain gauge transducer installed in the load pin that is in the center of the top sheave.

Here is another picture from an NPCC barge mooring monitoring project. NPCC, based in Dubai, ordered 8 running line tensiometers (or dynamometers) from us. You can see how we have installed this device in a compact area under a platform. It is tethered from above to avoid damage in a slack line event and tethered fore and aft to prevent rocking.

Whether you call it a tensiometer, a tension meter, or a dynamometer (and hopefully we’ve cleared away some of the confusion), we build the most rugged, accurate, and easily installed cable tension monitoring device in the industry.

Tags: cable tension, cable tension monitoring, tensiometers, tension measurement, tension meter

This entry was posted on Tuesday, January 12th, 2010 at 9:37 pm and is filed under Line Control Systems. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.

Winch Payout Calibration

After spending the last 15 years installing and commissioning winch monitoring systems, we’ve found that some of the most commonly asked questions are about calibration.

1. How do we calibrate the LCI-90 display for tension and payout/speed?
2. How often should we calibrate?

These are important questions because the calibration inherently affects the accuracy of the winch monitoring system.

The following are methods we have used to calibrate the payout/speed system using our LCI-90 displays:

First, once the payout has been calibrated the speed data is developed automatically as it is just a time stamp of the incoming pulses produced by the rotational sensors. As in most physical measurements with external sensors, there is a theoretical method and an empirical method to achieve the payout calibration. Let’s consider a single overboarding sheave with installed rotational sensors. The scalar calibration value used in the LCI-90 display is pulses per unit length.

The theoretical calculation is to compute the circumference of a diameter defined as one line diameter plus the root diameter of the sheave. Then divide the pulse per one sheave revolution by this circumference to get a pulse per unit length. Drop this value into the display.

Equation = {(line diameter + sheave root diameter) * π} / pulses per sheave revolution

However this is never “exactly accurate”. The better way to calibrate your payout/speed system is to run a known length of cable through the sheave and note the accumulated pulses.

With the LCI-90 we can set the display to a diagnostic screen and monitor the pulses entering the display. Through the front panel buttons reset the payout by pressing the RSET button twice quickly. Then run the known length of line through the sheave, it is best to maintain a little tension on the line as to keep it line from slipping on the sheave. When the line has passed through, note the number of pulses on the DIAG screen of the LCI-90. Then the calculation is very straight forward:

Equation = number of pulses / length of line passed through the sheaves.

The same line standard can be used to verify both of the procedures above. The above scenario was recently used aboard the RV Thompson to dial in the payout system on the trawl winch. They had an opportunity to compare their winch monitoring readings to a very accurate beacon and transponder unit provided by an ROV group. They discovered differences between the two systems. Once they implemented the empirical calibration technique mentioned above they were able to get both systems to match.

Calibration should be done annually or as often as you detect discrepancies.

We have provided a small spread sheet at the bottom of this post to help calculate the required calibration value.

We will tackle the tension calibration in a later blog post.

Please give us a call if you have questions.
Phone: 206.634.1308
Email: lci@mtnw-usa.com

Download Spreadsheet

Tags: calibration, tension measurement, winch payout, winch tension

This entry was posted on Thursday, October 1st, 2009 at 6:16 pm and is filed under Line Control Systems. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.