Is my US-020 defective or am I doing something wrong?

Hello
I bought a HC-SR04 ultrasonic sensor and it seemed to work well enough but the range fell short of the distance advertised (450cm) so I bought a US-020 which claimed to have a range of 700 cm.

I am finding that the US-020 is inaccurate for distances over about 106 cm. If I use both sensors at the same time (with a short wait to avoid interference) the US-020 gives the distance as significantly less (it is always less over 106 cm), e.g.
HC-SR04 208.63 cm
US-020 155.92 cm
although they are similar for values under about 106cm
I get the same problem with my code (below)

      delayMicroseconds(2);
      digitalWrite(Trig_pin, HIGH);
      delayMicroseconds(10);
      digitalWrite(Trig_pin, LOW);
      duration = pulseIn(Echo_pin, HIGH);
      dist_in_cm = (duration*0.034029) / 2 ;

and when using sonar.ping() from the NewPing library.

I am very new to electronics and the Arduino and do not understand what sort of problem(s) could cause this. Should I be clamouring for a refund or doing something different? Neither sensor came with instructions - I bought them from ebay.

Thanks in advance for any help.

Here are links to products similar to my sensors
http://www.ebay.co.uk/itm/1pcs-Ultrasonic-Module-HC-SR04-Distance-Measuring-Transducer-Sensor-for-Arduino-/281056175067?pt=LH_DefaultDomain_0&hash=item417040e3db
http://www.ebay.co.uk/itm/1pcs-US-020-Ultrasonic-Module-Distance-Measuring-Transducer-Sensor-DC-5V-/370749406298?pt=LH_DefaultDomain_0&hash=item565262fc5a

What's the object your measuring the distance to? A wall? Do you have an absolutely clear line of sight to the measuring object within the opening angle of the sensor? Recall the physics behind your measurement.

pylon:
What's the object your measuring the distance to? A wall? Do you have an absolutely clear line of sight to the measuring object within the opening angle of the sensor? Recall the physics behind your measurement.

Hi
Thanks for the reply. There is an absolutely clear line of sight. I have tried the ceiling, the wall, the TV screen, a piece of card, the sofa ...
The objects surfaces are perpendicular to the two sensors' lines of sight.

In my experience the HC-SR04 delivers quite accurate distance measurements, but I use them only below 1m. The sensors (both) have an opening angle of 15 degrees. 4 meters away this is more than one meter, maybe you get some reflections. Keep in mind, these are cheap sensors and the advertised maximum distance will be the best result with a absolute planar wall and nothing else between the sensor and the wall within probably a 60 degree angle. What kind of measurements do you plan to make that you need to measure distances of more than a meter? These types of sensor are intended to be used on little roboters and the like where they seldom have to measure longer distances.

Thanks for your help Pylon.

As you say, the HC-SR04 is quite accurate. I have tried a few more experiments and I think that the US-020 must have a much wider angle than the 15 degrees stated or the 15 degrees is not 'pointing out' from the board in the way I expect.

I wanted to use the sensors to measure the distance to the four walls of an empty room (of a known size) at 90 degree intervals to be able to pinpoint the locations where the measuring point could be in the room using trig. I have a stepper motor that I was going to use to turn the sensor 90 degrees between measurements or I was going to use four sensors in a square pointing outwards. I say 'was' as I now realise that it is not going to work as I had not taken into account the 15 degree angle.

Ho hum ... thanks again.