Specmanship 101

You're surfing the web for a low-cost accelerometer. Both IMI Sensors and its competitor offer essentially the same product with the same marketing, a low-cost, permanent-mount accelerometer. Despite similar marketing, one sensor has an 80 g acceleration range and the other has a 50 g acceleration range. It’s the same technology, same sensitivity (100 mV/g) and same supply voltage (18-30 VDC). So why would one sensor have an 80 g acceleration range and the other has a 50 g acceleration range?

Creative specmanship.

IMI Sensors has always had the policy of specifying product for worst case scenario while other companies do not.

The dynamic range is calculated as “supply (excitation) voltage minus bias voltage minus one volt”. You don’t have any control over the bias voltage nor the one extra volt. The one extra volt is consumed by the electronics.

The IMI Sensors product has a maximum bias voltage of 12 VDC and minimum supply (excitation) voltage of 18 VDC. In a worst case scenario, you have 5 volts to use for your signal (18-12-1 = 5 V). Hence, the IMI Sensors’ 100mV/g product has a 50 g acceleration range. Look at the specifications and it lists 50 g. See Figure 1.

The competitor’s sensor has a maximum bias voltage of 14 VDC and minimum supply voltage of 18 VDC. In a worst case scenario, you have 3 volts to use for your signal (18-14-1 = 3 V). Hence, the competitor’s 100mV/g product has a 30 g acceleration range. Look at their specifications and they will list 80 g! Why? The competitor assumes that:

• You probably will not send the sensor the minimum voltage.
• You probably will not get stuck with a 14 VDC bias voltage from the factory.
• You probably will not ask them about it.
• You probably will never notice.

Is acceleration range the only data point that competitors may manipulate?

No! In a competitive market, many companies will try to gain a competitive advantage by manipulating their specifications via creative specmanship. These efforts are not isolated to calculation of the acceleration range. Frequency response is another data point that is often manipulated because it seldom is tested.

As mentioned in Answer #1, IMI Sensors has always had the policy of specifying product for worst case scenario while other companies do not. The frequency response specification of each of IMI Sensors’ products can be confirmed by the product's calibration certificate. See Figure 2 below.

IMI Sensors ran a calibration on a competitor's sensor and found it to be out of spec. See Figure 3. Per the specifications, the sensitivity of the model was supposed to stay within 10% of the specified 100 mV/g sensitivity throughout the frequency span to 600,000 cpm. In actuality, the sensor had greater than 10% deviation before its specified high frequency limit. The deviation at 600,000 cpm was almost 80%, producing a 180 mV/g sensitivity.

If you have any questions on any sensor specification, please contact one of IMI Sensors’ Application Engineers. It is important to know what the specifications mean and when something just doesn’t add up. With IMI Sensors, your model is guaranteed to meet specifications. We give you the worst case scenario as we think it is a more honest and realistic approach.