Getting soft around the middle or, what is Modulus compensation?

Being the New Year, many of us take notice that we no longer have that fit and trim shape we once enjoyed. We’ve become a little soft around the middle. Well, materials have a similar problem with applied weight and temperature changes.

As temperature is changed, most structural metals also change stiffness, with the bulk of them getting weaker (less capable of carrying load) as the temperature increases. This is known as modulus change with temperature, and means that for a given static load (like an applied weight), when the temperature increases the deformation (strain) increases. For transducers intended for stable operation over a wide temperature range, like those used in legal for trade applications, modulus change with temperature is a performance specification that strain gages can help meet.

Change of modulus with temperature is normally specified in percent change per 100 degree temperature difference; for example, -1.0%/100°C. This means that for every 100°C increase in temperature, Young’s modulus will decrease by 1.0% from the room temperature (~24°C) value; and, will increase by the same percentage given a 100°C decrease in temperature from normal ambient conditions. Therefore, when placed under a steady load from an applied weight, then as the temperature increases, the strain in the material will also increase, which causes the output of the transducer to increase, even though the load hasn’t changed.

Ni-Cr foil (Micro-Measurements K-foil) has a natural reduction in strain sensitivity (gage factor) with increased temperature. As the temperature rises, the gage factor goes down. Taking advantage of this fortunate situation, Micro-Measurements can specially process K-foil to have a change in strain sensitivity that cancels the temperature-induced change in surface strains caused by the modulus change of the common metals used for transducer applications. These are called Modulus Compensation gages, and they are designated with an MC in the part description; for example, N2K-MC-S100P-350. A particular compensation is specified using the appropriate M-option. For example, N2K-MC-S100P-350/M1 specifies a strain sensitivity slope change of -2.70%/100°C, which is used for many structural stainless steels. There are standard compensations for aluminum alloys (/M2), many tool steels (/M3), and a mid-range compensation (/M4). And, of course, custom compensations are available.

Using a Micro-Measurements Modulus Compensation strain gage can help keep the output of your transducer constant under changing temperature conditions, which is kind of like being fit and trim again.

So, the next time you see that your data is looking a little fat after a temperature increase, contact one of the Applications Engineers at Micro-Measurements on 919.365.3800 or mm@vpgsensors.com  to discuss how to get those numbers looking better.

bwatson's picture

Bob Watson

Director of Engineering